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docs: rcu: convert some articles from html to ReST

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There are 4 RCU articles that are written on html format.

The way they are, they can't be part of the Linux Kernel
documentation body nor share the styles and pdf output.

So, convert them to ReST format.

This way, make htmldocs and make pdfdocs will produce a
documentation output that will be like the original ones, but
will be part of the Linux Kernel documentation body.

Part of the conversion was done with the help of pandoc, but
the result had some broken things that had to be manually
fixed.

Following are manual changes Mauro made when doing the automatic conversion:
Quoting from: https://lore.kernel.org/rcu/20190726154550.5eeae294@coco.lan/

> > At least the pandoc's version I used here has a bug: its conversion
> > from html to ReST on those files only start after a <body> tag - or
> > when the first quiz table starts. I only discovered that adding a
> > <body> at the beginning of the file solve this book at the last
> > conversions.
> >
> > So, for most html->ReST conversions, I manually converted the first
> > part of the document, basically stripping html paragraph tags and
> > by replacing highlights by the ReST syntax.
> >
> > Also, all the quiz tables seem to assume some javascript macro or
> > css style that would be hiding the answer part until the mouse moves
> > to it. Such macro/css was not there at the kernel tree. So, the quiz
> > answers have the same color as the background, making them invisible.
> > Even if we had such macro/css, this is not portable for pdf/LaTeX output
> > (and I'm not sure if this would work with ePub).
> >
> > So, I ended by manually doing the table conversion.
> >
> > Finally, I double-checked if the conversions ended ok, addressing any
> > issues that might have heppened.
> >
> > So, after both automatic conversion and manual fixes, I opened both the
> > html files produced by Sphinx and the original ones and compared them
> > line per line (except for the indexes, as Sphinx produces them
> > automatically), in order to see if all information from the original
> > files will be there on a format close to what we have on other ReST
> > files, fixing any pending issues if any.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Mauro Carvalho Chehab 2019-08-01 17:39:18 -04:00 committed by Paul E. McKenney
parent c07e6f36bc
commit ccc9971e21
11 changed files with 4978 additions and 6104 deletions
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head><title>A Tour Through TREE_RCU's Expedited Grace Periods</title>
<meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<h2>Introduction</h2>
This document describes RCU's expedited grace periods.
Unlike RCU's normal grace periods, which accept long latencies to attain
high efficiency and minimal disturbance, expedited grace periods accept
lower efficiency and significant disturbance to attain shorter latencies.
<p>
There are two flavors of RCU (RCU-preempt and RCU-sched), with an earlier
third RCU-bh flavor having been implemented in terms of the other two.
Each of the two implementations is covered in its own section.
<ol>
<li> <a href="#Expedited Grace Period Design">
Expedited Grace Period Design</a>
<li> <a href="#RCU-preempt Expedited Grace Periods">
RCU-preempt Expedited Grace Periods</a>
<li> <a href="#RCU-sched Expedited Grace Periods">
RCU-sched Expedited Grace Periods</a>
<li> <a href="#Expedited Grace Period and CPU Hotplug">
Expedited Grace Period and CPU Hotplug</a>
<li> <a href="#Expedited Grace Period Refinements">
Expedited Grace Period Refinements</a>
</ol>
<h2><a name="Expedited Grace Period Design">
Expedited Grace Period Design</a></h2>
<p>
The expedited RCU grace periods cannot be accused of being subtle,
given that they for all intents and purposes hammer every CPU that
has not yet provided a quiescent state for the current expedited
grace period.
The one saving grace is that the hammer has grown a bit smaller
over time: The old call to <tt>try_stop_cpus()</tt> has been
replaced with a set of calls to <tt>smp_call_function_single()</tt>,
each of which results in an IPI to the target CPU.
The corresponding handler function checks the CPU's state, motivating
a faster quiescent state where possible, and triggering a report
of that quiescent state.
As always for RCU, once everything has spent some time in a quiescent
state, the expedited grace period has completed.
<p>
The details of the <tt>smp_call_function_single()</tt> handler's
operation depend on the RCU flavor, as described in the following
sections.
<h2><a name="RCU-preempt Expedited Grace Periods">
RCU-preempt Expedited Grace Periods</a></h2>
<p>
<tt>CONFIG_PREEMPT=y</tt> kernels implement RCU-preempt.
The overall flow of the handling of a given CPU by an RCU-preempt
expedited grace period is shown in the following diagram:
<p><img src="ExpRCUFlow.svg" alt="ExpRCUFlow.svg" width="55%">
<p>
The solid arrows denote direct action, for example, a function call.
The dotted arrows denote indirect action, for example, an IPI
or a state that is reached after some time.
<p>
If a given CPU is offline or idle, <tt>synchronize_rcu_expedited()</tt>
will ignore it because idle and offline CPUs are already residing
in quiescent states.
Otherwise, the expedited grace period will use
<tt>smp_call_function_single()</tt> to send the CPU an IPI, which
is handled by <tt>rcu_exp_handler()</tt>.
<p>
However, because this is preemptible RCU, <tt>rcu_exp_handler()</tt>
can check to see if the CPU is currently running in an RCU read-side
critical section.
If not, the handler can immediately report a quiescent state.
Otherwise, it sets flags so that the outermost <tt>rcu_read_unlock()</tt>
invocation will provide the needed quiescent-state report.
This flag-setting avoids the previous forced preemption of all
CPUs that might have RCU read-side critical sections.
In addition, this flag-setting is done so as to avoid increasing
the overhead of the common-case fastpath through the scheduler.
<p>
Again because this is preemptible RCU, an RCU read-side critical section
can be preempted.
When that happens, RCU will enqueue the task, which will the continue to
block the current expedited grace period until it resumes and finds its
outermost <tt>rcu_read_unlock()</tt>.
The CPU will report a quiescent state just after enqueuing the task because
the CPU is no longer blocking the grace period.
It is instead the preempted task doing the blocking.
The list of blocked tasks is managed by <tt>rcu_preempt_ctxt_queue()</tt>,
which is called from <tt>rcu_preempt_note_context_switch()</tt>, which
in turn is called from <tt>rcu_note_context_switch()</tt>, which in
turn is called from the scheduler.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why not just have the expedited grace period check the
state of all the CPUs?
After all, that would avoid all those real-time-unfriendly IPIs.
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because we want the RCU read-side critical sections to run fast,
which means no memory barriers.
Therefore, it is not possible to safely check the state from some
other CPU.
And even if it was possible to safely check the state, it would
still be necessary to IPI the CPU to safely interact with the
upcoming <tt>rcu_read_unlock()</tt> invocation, which means that
the remote state testing would not help the worst-case
latency that real-time applications care about.
<p><font color="ffffff">One way to prevent your real-time
application from getting hit with these IPIs is to
build your kernel with <tt>CONFIG_NO_HZ_FULL=y</tt>.
RCU would then perceive the CPU running your application
as being idle, and it would be able to safely detect that
state without needing to IPI the CPU.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
Please note that this is just the overall flow:
Additional complications can arise due to races with CPUs going idle
or offline, among other things.
<h2><a name="RCU-sched Expedited Grace Periods">
RCU-sched Expedited Grace Periods</a></h2>
<p>
<tt>CONFIG_PREEMPT=n</tt> kernels implement RCU-sched.
The overall flow of the handling of a given CPU by an RCU-sched
expedited grace period is shown in the following diagram:
<p><img src="ExpSchedFlow.svg" alt="ExpSchedFlow.svg" width="55%">
<p>
As with RCU-preempt, RCU-sched's
<tt>synchronize_rcu_expedited()</tt> ignores offline and
idle CPUs, again because they are in remotely detectable
quiescent states.
However, because the
<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
leave no trace of their invocation, in general it is not possible to tell
whether or not the current CPU is in an RCU read-side critical section.
The best that RCU-sched's <tt>rcu_exp_handler()</tt> can do is to check
for idle, on the off-chance that the CPU went idle while the IPI
was in flight.
If the CPU is idle, then <tt>rcu_exp_handler()</tt> reports
the quiescent state.
<p> Otherwise, the handler forces a future context switch by setting the
NEED_RESCHED flag of the current task's thread flag and the CPU preempt
counter.
At the time of the context switch, the CPU reports the quiescent state.
Should the CPU go offline first, it will report the quiescent state
at that time.
<h2><a name="Expedited Grace Period and CPU Hotplug">
Expedited Grace Period and CPU Hotplug</a></h2>
<p>
The expedited nature of expedited grace periods require a much tighter
interaction with CPU hotplug operations than is required for normal
grace periods.
In addition, attempting to IPI offline CPUs will result in splats, but
failing to IPI online CPUs can result in too-short grace periods.
Neither option is acceptable in production kernels.
<p>
The interaction between expedited grace periods and CPU hotplug operations
is carried out at several levels:
<ol>
<li> The number of CPUs that have ever been online is tracked
by the <tt>rcu_state</tt> structure's <tt>-&gt;ncpus</tt>
field.
The <tt>rcu_state</tt> structure's <tt>-&gt;ncpus_snap</tt>
field tracks the number of CPUs that have ever been online
at the beginning of an RCU expedited grace period.
Note that this number never decreases, at least in the absence
of a time machine.
<li> The identities of the CPUs that have ever been online is
tracked by the <tt>rcu_node</tt> structure's
<tt>-&gt;expmaskinitnext</tt> field.
The <tt>rcu_node</tt> structure's <tt>-&gt;expmaskinit</tt>
field tracks the identities of the CPUs that were online
at least once at the beginning of the most recent RCU
expedited grace period.
The <tt>rcu_state</tt> structure's <tt>-&gt;ncpus</tt> and
<tt>-&gt;ncpus_snap</tt> fields are used to detect when
new CPUs have come online for the first time, that is,
when the <tt>rcu_node</tt> structure's <tt>-&gt;expmaskinitnext</tt>
field has changed since the beginning of the last RCU
expedited grace period, which triggers an update of each
<tt>rcu_node</tt> structure's <tt>-&gt;expmaskinit</tt>
field from its <tt>-&gt;expmaskinitnext</tt> field.
<li> Each <tt>rcu_node</tt> structure's <tt>-&gt;expmaskinit</tt>
field is used to initialize that structure's
<tt>-&gt;expmask</tt> at the beginning of each RCU
expedited grace period.
This means that only those CPUs that have been online at least
once will be considered for a given grace period.
<li> Any CPU that goes offline will clear its bit in its leaf
<tt>rcu_node</tt> structure's <tt>-&gt;qsmaskinitnext</tt>
field, so any CPU with that bit clear can safely be ignored.
However, it is possible for a CPU coming online or going offline
to have this bit set for some time while <tt>cpu_online</tt>
returns <tt>false</tt>.
<li> For each non-idle CPU that RCU believes is currently online, the grace
period invokes <tt>smp_call_function_single()</tt>.
If this succeeds, the CPU was fully online.
Failure indicates that the CPU is in the process of coming online
or going offline, in which case it is necessary to wait for a
short time period and try again.
The purpose of this wait (or series of waits, as the case may be)
is to permit a concurrent CPU-hotplug operation to complete.
<li> In the case of RCU-sched, one of the last acts of an outgoing CPU
is to invoke <tt>rcu_report_dead()</tt>, which
reports a quiescent state for that CPU.
However, this is likely paranoia-induced redundancy. <!-- @@@ -->
</ol>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why all the dancing around with multiple counters and masks
tracking CPUs that were once online?
Why not just have a single set of masks tracking the currently
online CPUs and be done with it?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Maintaining single set of masks tracking the online CPUs <i>sounds</i>
easier, at least until you try working out all the race conditions
between grace-period initialization and CPU-hotplug operations.
For example, suppose initialization is progressing down the
tree while a CPU-offline operation is progressing up the tree.
This situation can result in bits set at the top of the tree
that have no counterparts at the bottom of the tree.
Those bits will never be cleared, which will result in
grace-period hangs.
In short, that way lies madness, to say nothing of a great many
bugs, hangs, and deadlocks.
<p><font color="ffffff">
In contrast, the current multi-mask multi-counter scheme ensures
that grace-period initialization will always see consistent masks
up and down the tree, which brings significant simplifications
over the single-mask method.
<p><font color="ffffff">
This is an instance of
<a href="http://www.cs.columbia.edu/~library/TR-repository/reports/reports-1992/cucs-039-92.ps.gz"><font color="ffffff">
deferring work in order to avoid synchronization</a>.
Lazily recording CPU-hotplug events at the beginning of the next
grace period greatly simplifies maintenance of the CPU-tracking
bitmasks in the <tt>rcu_node</tt> tree.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h2><a name="Expedited Grace Period Refinements">
Expedited Grace Period Refinements</a></h2>
<ol>
<li> <a href="#Idle-CPU Checks">Idle-CPU checks</a>.
<li> <a href="#Batching via Sequence Counter">
Batching via sequence counter</a>.
<li> <a href="#Funnel Locking and Wait/Wakeup">
Funnel locking and wait/wakeup</a>.
<li> <a href="#Use of Workqueues">Use of Workqueues</a>.
<li> <a href="#Stall Warnings">Stall warnings</a>.
<li> <a href="#Mid-Boot Operation">Mid-boot operation</a>.
</ol>
<h3><a name="Idle-CPU Checks">Idle-CPU Checks</a></h3>
<p>
Each expedited grace period checks for idle CPUs when initially forming
the mask of CPUs to be IPIed and again just before IPIing a CPU
(both checks are carried out by <tt>sync_rcu_exp_select_cpus()</tt>).
If the CPU is idle at any time between those two times, the CPU will
not be IPIed.
Instead, the task pushing the grace period forward will include the
idle CPUs in the mask passed to <tt>rcu_report_exp_cpu_mult()</tt>.
<p>
For RCU-sched, there is an additional check:
If the IPI has interrupted the idle loop, then
<tt>rcu_exp_handler()</tt> invokes <tt>rcu_report_exp_rdp()</tt>
to report the corresponding quiescent state.
<p>
For RCU-preempt, there is no specific check for idle in the
IPI handler (<tt>rcu_exp_handler()</tt>), but because
RCU read-side critical sections are not permitted within the
idle loop, if <tt>rcu_exp_handler()</tt> sees that the CPU is within
RCU read-side critical section, the CPU cannot possibly be idle.
Otherwise, <tt>rcu_exp_handler()</tt> invokes
<tt>rcu_report_exp_rdp()</tt> to report the corresponding quiescent
state, regardless of whether or not that quiescent state was due to
the CPU being idle.
<p>
In summary, RCU expedited grace periods check for idle when building
the bitmask of CPUs that must be IPIed, just before sending each IPI,
and (either explicitly or implicitly) within the IPI handler.
<h3><a name="Batching via Sequence Counter">
Batching via Sequence Counter</a></h3>
<p>
If each grace-period request was carried out separately, expedited
grace periods would have abysmal scalability and
problematic high-load characteristics.
Because each grace-period operation can serve an unlimited number of
updates, it is important to <i>batch</i> requests, so that a single
expedited grace-period operation will cover all requests in the
corresponding batch.
<p>
This batching is controlled by a sequence counter named
<tt>-&gt;expedited_sequence</tt> in the <tt>rcu_state</tt> structure.
This counter has an odd value when there is an expedited grace period
in progress and an even value otherwise, so that dividing the counter
value by two gives the number of completed grace periods.
During any given update request, the counter must transition from
even to odd and then back to even, thus indicating that a grace
period has elapsed.
Therefore, if the initial value of the counter is <tt>s</tt>,
the updater must wait until the counter reaches at least the
value <tt>(s+3)&amp;~0x1</tt>.
This counter is managed by the following access functions:
<ol>
<li> <tt>rcu_exp_gp_seq_start()</tt>, which marks the start of
an expedited grace period.
<li> <tt>rcu_exp_gp_seq_end()</tt>, which marks the end of an
expedited grace period.
<li> <tt>rcu_exp_gp_seq_snap()</tt>, which obtains a snapshot of
the counter.
<li> <tt>rcu_exp_gp_seq_done()</tt>, which returns <tt>true</tt>
if a full expedited grace period has elapsed since the
corresponding call to <tt>rcu_exp_gp_seq_snap()</tt>.
</ol>
<p>
Again, only one request in a given batch need actually carry out
a grace-period operation, which means there must be an efficient
way to identify which of many concurrent reqeusts will initiate
the grace period, and that there be an efficient way for the
remaining requests to wait for that grace period to complete.
However, that is the topic of the next section.
<h3><a name="Funnel Locking and Wait/Wakeup">
Funnel Locking and Wait/Wakeup</a></h3>
<p>
The natural way to sort out which of a batch of updaters will initiate
the expedited grace period is to use the <tt>rcu_node</tt> combining
tree, as implemented by the <tt>exp_funnel_lock()</tt> function.
The first updater corresponding to a given grace period arriving
at a given <tt>rcu_node</tt> structure records its desired grace-period
sequence number in the <tt>-&gt;exp_seq_rq</tt> field and moves up
to the next level in the tree.
Otherwise, if the <tt>-&gt;exp_seq_rq</tt> field already contains
the sequence number for the desired grace period or some later one,
the updater blocks on one of four wait queues in the
<tt>-&gt;exp_wq[]</tt> array, using the second-from-bottom
and third-from bottom bits as an index.
An <tt>-&gt;exp_lock</tt> field in the <tt>rcu_node</tt> structure
synchronizes access to these fields.
<p>
An empty <tt>rcu_node</tt> tree is shown in the following diagram,
with the white cells representing the <tt>-&gt;exp_seq_rq</tt> field
and the red cells representing the elements of the
<tt>-&gt;exp_wq[]</tt> array.
<p><img src="Funnel0.svg" alt="Funnel0.svg" width="75%">
<p>
The next diagram shows the situation after the arrival of Task&nbsp;A
and Task&nbsp;B at the leftmost and rightmost leaf <tt>rcu_node</tt>
structures, respectively.
The current value of the <tt>rcu_state</tt> structure's
<tt>-&gt;expedited_sequence</tt> field is zero, so adding three and
clearing the bottom bit results in the value two, which both tasks
record in the <tt>-&gt;exp_seq_rq</tt> field of their respective
<tt>rcu_node</tt> structures:
<p><img src="Funnel1.svg" alt="Funnel1.svg" width="75%">
<p>
Each of Tasks&nbsp;A and&nbsp;B will move up to the root
<tt>rcu_node</tt> structure.
Suppose that Task&nbsp;A wins, recording its desired grace-period sequence
number and resulting in the state shown below:
<p><img src="Funnel2.svg" alt="Funnel2.svg" width="75%">
<p>
Task&nbsp;A now advances to initiate a new grace period, while Task&nbsp;B
moves up to the root <tt>rcu_node</tt> structure, and, seeing that
its desired sequence number is already recorded, blocks on
<tt>-&gt;exp_wq[1]</tt>.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why <tt>-&gt;exp_wq[1]</tt>?
Given that the value of these tasks' desired sequence number is
two, so shouldn't they instead block on <tt>-&gt;exp_wq[2]</tt>?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
No.
<p><font color="ffffff">
Recall that the bottom bit of the desired sequence number indicates
whether or not a grace period is currently in progress.
It is therefore necessary to shift the sequence number right one
bit position to obtain the number of the grace period.
This results in <tt>-&gt;exp_wq[1]</tt>.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
If Tasks&nbsp;C and&nbsp;D also arrive at this point, they will compute the
same desired grace-period sequence number, and see that both leaf
<tt>rcu_node</tt> structures already have that value recorded.
They will therefore block on their respective <tt>rcu_node</tt>
structures' <tt>-&gt;exp_wq[1]</tt> fields, as shown below:
<p><img src="Funnel3.svg" alt="Funnel3.svg" width="75%">
<p>
Task&nbsp;A now acquires the <tt>rcu_state</tt> structure's
<tt>-&gt;exp_mutex</tt> and initiates the grace period, which
increments <tt>-&gt;expedited_sequence</tt>.
Therefore, if Tasks&nbsp;E and&nbsp;F arrive, they will compute
a desired sequence number of 4 and will record this value as
shown below:
<p><img src="Funnel4.svg" alt="Funnel4.svg" width="75%">
<p>
Tasks&nbsp;E and&nbsp;F will propagate up the <tt>rcu_node</tt>
combining tree, with Task&nbsp;F blocking on the root <tt>rcu_node</tt>
structure and Task&nbsp;E wait for Task&nbsp;A to finish so that
it can start the next grace period.
The resulting state is as shown below:
<p><img src="Funnel5.svg" alt="Funnel5.svg" width="75%">
<p>
Once the grace period completes, Task&nbsp;A
starts waking up the tasks waiting for this grace period to complete,
increments the <tt>-&gt;expedited_sequence</tt>,
acquires the <tt>-&gt;exp_wake_mutex</tt> and then releases the
<tt>-&gt;exp_mutex</tt>.
This results in the following state:
<p><img src="Funnel6.svg" alt="Funnel6.svg" width="75%">
<p>
Task&nbsp;E can then acquire <tt>-&gt;exp_mutex</tt> and increment
<tt>-&gt;expedited_sequence</tt> to the value three.
If new tasks&nbsp;G and&nbsp;H arrive and moves up the combining tree at the
same time, the state will be as follows:
<p><img src="Funnel7.svg" alt="Funnel7.svg" width="75%">
<p>
Note that three of the root <tt>rcu_node</tt> structure's
waitqueues are now occupied.
However, at some point, Task&nbsp;A will wake up the
tasks blocked on the <tt>-&gt;exp_wq</tt> waitqueues, resulting
in the following state:
<p><img src="Funnel8.svg" alt="Funnel8.svg" width="75%">
<p>
Execution will continue with Tasks&nbsp;E and&nbsp;H completing
their grace periods and carrying out their wakeups.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
What happens if Task&nbsp;A takes so long to do its wakeups
that Task&nbsp;E's grace period completes?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Then Task&nbsp;E will block on the <tt>-&gt;exp_wake_mutex</tt>,
which will also prevent it from releasing <tt>-&gt;exp_mutex</tt>,
which in turn will prevent the next grace period from starting.
This last is important in preventing overflow of the
<tt>-&gt;exp_wq[]</tt> array.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="Use of Workqueues">Use of Workqueues</a></h3>
<p>
In earlier implementations, the task requesting the expedited
grace period also drove it to completion.
This straightforward approach had the disadvantage of needing to
account for POSIX signals sent to user tasks,
so more recent implemementations use the Linux kernel's
<a href="https://www.kernel.org/doc/Documentation/core-api/workqueue.rst">workqueues</a>.
<p>
The requesting task still does counter snapshotting and funnel-lock
processing, but the task reaching the top of the funnel lock
does a <tt>schedule_work()</tt> (from <tt>_synchronize_rcu_expedited()</tt>
so that a workqueue kthread does the actual grace-period processing.
Because workqueue kthreads do not accept POSIX signals, grace-period-wait
processing need not allow for POSIX signals.
In addition, this approach allows wakeups for the previous expedited
grace period to be overlapped with processing for the next expedited
grace period.
Because there are only four sets of waitqueues, it is necessary to
ensure that the previous grace period's wakeups complete before the
next grace period's wakeups start.
This is handled by having the <tt>-&gt;exp_mutex</tt>
guard expedited grace-period processing and the
<tt>-&gt;exp_wake_mutex</tt> guard wakeups.
The key point is that the <tt>-&gt;exp_mutex</tt> is not released
until the first wakeup is complete, which means that the
<tt>-&gt;exp_wake_mutex</tt> has already been acquired at that point.
This approach ensures that the previous grace period's wakeups can
be carried out while the current grace period is in process, but
that these wakeups will complete before the next grace period starts.
This means that only three waitqueues are required, guaranteeing that
the four that are provided are sufficient.
<h3><a name="Stall Warnings">Stall Warnings</a></h3>
<p>
Expediting grace periods does nothing to speed things up when RCU
readers take too long, and therefore expedited grace periods check
for stalls just as normal grace periods do.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But why not just let the normal grace-period machinery
detect the stalls, given that a given reader must block
both normal and expedited grace periods?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because it is quite possible that at a given time there
is no normal grace period in progress, in which case the
normal grace period cannot emit a stall warning.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
The <tt>synchronize_sched_expedited_wait()</tt> function loops waiting
for the expedited grace period to end, but with a timeout set to the
current RCU CPU stall-warning time.
If this time is exceeded, any CPUs or <tt>rcu_node</tt> structures
blocking the current grace period are printed.
Each stall warning results in another pass through the loop, but the
second and subsequent passes use longer stall times.
<h3><a name="Mid-Boot Operation">Mid-boot operation</a></h3>
<p>
The use of workqueues has the advantage that the expedited
grace-period code need not worry about POSIX signals.
Unfortunately, it has the
corresponding disadvantage that workqueues cannot be used until
they are initialized, which does not happen until some time after
the scheduler spawns the first task.
Given that there are parts of the kernel that really do want to
execute grace periods during this mid-boot &ldquo;dead zone&rdquo;,
expedited grace periods must do something else during thie time.
<p>
What they do is to fall back to the old practice of requiring that the
requesting task drive the expedited grace period, as was the case
before the use of workqueues.
However, the requesting task is only required to drive the grace period
during the mid-boot dead zone.
Before mid-boot, a synchronous grace period is a no-op.
Some time after mid-boot, workqueues are used.
<p>
Non-expedited non-SRCU synchronous grace periods must also operate
normally during mid-boot.
This is handled by causing non-expedited grace periods to take the
expedited code path during mid-boot.
<p>
The current code assumes that there are no POSIX signals during
the mid-boot dead zone.
However, if an overwhelming need for POSIX signals somehow arises,
appropriate adjustments can be made to the expedited stall-warning code.
One such adjustment would reinstate the pre-workqueue stall-warning
checks, but only during the mid-boot dead zone.
<p>
With this refinement, synchronous grace periods can now be used from
task context pretty much any time during the life of the kernel.
That is, aside from some points in the suspend, hibernate, or shutdown
code path.
<h3><a name="Summary">
Summary</a></h3>
<p>
Expedited grace periods use a sequence-number approach to promote
batching, so that a single grace-period operation can serve numerous
requests.
A funnel lock is used to efficiently identify the one task out of
a concurrent group that will request the grace period.
All members of the group will block on waitqueues provided in
the <tt>rcu_node</tt> structure.
The actual grace-period processing is carried out by a workqueue.
<p>
CPU-hotplug operations are noted lazily in order to prevent the need
for tight synchronization between expedited grace periods and
CPU-hotplug operations.
The dyntick-idle counters are used to avoid sending IPIs to idle CPUs,
at least in the common case.
RCU-preempt and RCU-sched use different IPI handlers and different
code to respond to the state changes carried out by those handlers,
but otherwise use common code.
<p>
Quiescent states are tracked using the <tt>rcu_node</tt> tree,
and once all necessary quiescent states have been reported,
all tasks waiting on this expedited grace period are awakened.
A pair of mutexes are used to allow one grace period's wakeups
to proceed concurrently with the next grace period's processing.
<p>
This combination of mechanisms allows expedited grace periods to
run reasonably efficiently.
However, for non-time-critical tasks, normal grace periods should be
used instead because their longer duration permits much higher
degrees of batching, and thus much lower per-request overheads.
</body></html>

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=================================================
A Tour Through TREE_RCU's Expedited Grace Periods
=================================================
Introduction
============
This document describes RCU's expedited grace periods.
Unlike RCU's normal grace periods, which accept long latencies to attain
high efficiency and minimal disturbance, expedited grace periods accept
lower efficiency and significant disturbance to attain shorter latencies.
There are two flavors of RCU (RCU-preempt and RCU-sched), with an earlier
third RCU-bh flavor having been implemented in terms of the other two.
Each of the two implementations is covered in its own section.
Expedited Grace Period Design
=============================
The expedited RCU grace periods cannot be accused of being subtle,
given that they for all intents and purposes hammer every CPU that
has not yet provided a quiescent state for the current expedited
grace period.
The one saving grace is that the hammer has grown a bit smaller
over time: The old call to ``try_stop_cpus()`` has been
replaced with a set of calls to ``smp_call_function_single()``,
each of which results in an IPI to the target CPU.
The corresponding handler function checks the CPU's state, motivating
a faster quiescent state where possible, and triggering a report
of that quiescent state.
As always for RCU, once everything has spent some time in a quiescent
state, the expedited grace period has completed.
The details of the ``smp_call_function_single()`` handler's
operation depend on the RCU flavor, as described in the following
sections.
RCU-preempt Expedited Grace Periods
===================================
``CONFIG_PREEMPT=y`` kernels implement RCU-preempt.
The overall flow of the handling of a given CPU by an RCU-preempt
expedited grace period is shown in the following diagram:
.. kernel-figure:: ExpRCUFlow.svg
The solid arrows denote direct action, for example, a function call.
The dotted arrows denote indirect action, for example, an IPI
or a state that is reached after some time.
If a given CPU is offline or idle, ``synchronize_rcu_expedited()``
will ignore it because idle and offline CPUs are already residing
in quiescent states.
Otherwise, the expedited grace period will use
``smp_call_function_single()`` to send the CPU an IPI, which
is handled by ``rcu_exp_handler()``.
However, because this is preemptible RCU, ``rcu_exp_handler()``
can check to see if the CPU is currently running in an RCU read-side
critical section.
If not, the handler can immediately report a quiescent state.
Otherwise, it sets flags so that the outermost ``rcu_read_unlock()``
invocation will provide the needed quiescent-state report.
This flag-setting avoids the previous forced preemption of all
CPUs that might have RCU read-side critical sections.
In addition, this flag-setting is done so as to avoid increasing
the overhead of the common-case fastpath through the scheduler.
Again because this is preemptible RCU, an RCU read-side critical section
can be preempted.
When that happens, RCU will enqueue the task, which will the continue to
block the current expedited grace period until it resumes and finds its
outermost ``rcu_read_unlock()``.
The CPU will report a quiescent state just after enqueuing the task because
the CPU is no longer blocking the grace period.
It is instead the preempted task doing the blocking.
The list of blocked tasks is managed by ``rcu_preempt_ctxt_queue()``,
which is called from ``rcu_preempt_note_context_switch()``, which
in turn is called from ``rcu_note_context_switch()``, which in
turn is called from the scheduler.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| Why not just have the expedited grace period check the state of all |
| the CPUs? After all, that would avoid all those real-time-unfriendly |
| IPIs. |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| Because we want the RCU read-side critical sections to run fast, |
| which means no memory barriers. Therefore, it is not possible to |
| safely check the state from some other CPU. And even if it was |
| possible to safely check the state, it would still be necessary to |
| IPI the CPU to safely interact with the upcoming |
| ``rcu_read_unlock()`` invocation, which means that the remote state |
| testing would not help the worst-case latency that real-time |
| applications care about. |
| |
| One way to prevent your real-time application from getting hit with |
| these IPIs is to build your kernel with ``CONFIG_NO_HZ_FULL=y``. RCU |
| would then perceive the CPU running your application as being idle, |
| and it would be able to safely detect that state without needing to |
| IPI the CPU. |
+-----------------------------------------------------------------------+
Please note that this is just the overall flow: Additional complications
can arise due to races with CPUs going idle or offline, among other
things.
RCU-sched Expedited Grace Periods
---------------------------------
``CONFIG_PREEMPT=n`` kernels implement RCU-sched. The overall flow of
the handling of a given CPU by an RCU-sched expedited grace period is
shown in the following diagram:
.. kernel-figure:: ExpSchedFlow.svg
As with RCU-preempt, RCU-sched's ``synchronize_rcu_expedited()`` ignores
offline and idle CPUs, again because they are in remotely detectable
quiescent states. However, because the ``rcu_read_lock_sched()`` and
``rcu_read_unlock_sched()`` leave no trace of their invocation, in
general it is not possible to tell whether or not the current CPU is in
an RCU read-side critical section. The best that RCU-sched's
``rcu_exp_handler()`` can do is to check for idle, on the off-chance
that the CPU went idle while the IPI was in flight. If the CPU is idle,
then ``rcu_exp_handler()`` reports the quiescent state.
Otherwise, the handler forces a future context switch by setting the
NEED_RESCHED flag of the current task's thread flag and the CPU preempt
counter. At the time of the context switch, the CPU reports the
quiescent state. Should the CPU go offline first, it will report the
quiescent state at that time.
Expedited Grace Period and CPU Hotplug
--------------------------------------
The expedited nature of expedited grace periods require a much tighter
interaction with CPU hotplug operations than is required for normal
grace periods. In addition, attempting to IPI offline CPUs will result
in splats, but failing to IPI online CPUs can result in too-short grace
periods. Neither option is acceptable in production kernels.
The interaction between expedited grace periods and CPU hotplug
operations is carried out at several levels:
#. The number of CPUs that have ever been online is tracked by the
``rcu_state`` structure's ``->ncpus`` field. The ``rcu_state``
structure's ``->ncpus_snap`` field tracks the number of CPUs that
have ever been online at the beginning of an RCU expedited grace
period. Note that this number never decreases, at least in the
absence of a time machine.
#. The identities of the CPUs that have ever been online is tracked by
the ``rcu_node`` structure's ``->expmaskinitnext`` field. The
``rcu_node`` structure's ``->expmaskinit`` field tracks the
identities of the CPUs that were online at least once at the
beginning of the most recent RCU expedited grace period. The
``rcu_state`` structure's ``->ncpus`` and ``->ncpus_snap`` fields are
used to detect when new CPUs have come online for the first time,
that is, when the ``rcu_node`` structure's ``->expmaskinitnext``
field has changed since the beginning of the last RCU expedited grace
period, which triggers an update of each ``rcu_node`` structure's
``->expmaskinit`` field from its ``->expmaskinitnext`` field.
#. Each ``rcu_node`` structure's ``->expmaskinit`` field is used to
initialize that structure's ``->expmask`` at the beginning of each
RCU expedited grace period. This means that only those CPUs that have
been online at least once will be considered for a given grace
period.
#. Any CPU that goes offline will clear its bit in its leaf ``rcu_node``
structure's ``->qsmaskinitnext`` field, so any CPU with that bit
clear can safely be ignored. However, it is possible for a CPU coming
online or going offline to have this bit set for some time while
``cpu_online`` returns ``false``.
#. For each non-idle CPU that RCU believes is currently online, the
grace period invokes ``smp_call_function_single()``. If this
succeeds, the CPU was fully online. Failure indicates that the CPU is
in the process of coming online or going offline, in which case it is
necessary to wait for a short time period and try again. The purpose
of this wait (or series of waits, as the case may be) is to permit a
concurrent CPU-hotplug operation to complete.
#. In the case of RCU-sched, one of the last acts of an outgoing CPU is
to invoke ``rcu_report_dead()``, which reports a quiescent state for
that CPU. However, this is likely paranoia-induced redundancy.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| Why all the dancing around with multiple counters and masks tracking |
| CPUs that were once online? Why not just have a single set of masks |
| tracking the currently online CPUs and be done with it? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| Maintaining single set of masks tracking the online CPUs *sounds* |
| easier, at least until you try working out all the race conditions |
| between grace-period initialization and CPU-hotplug operations. For |
| example, suppose initialization is progressing down the tree while a |
| CPU-offline operation is progressing up the tree. This situation can |
| result in bits set at the top of the tree that have no counterparts |
| at the bottom of the tree. Those bits will never be cleared, which |
| will result in grace-period hangs. In short, that way lies madness, |
| to say nothing of a great many bugs, hangs, and deadlocks. |
| In contrast, the current multi-mask multi-counter scheme ensures that |
| grace-period initialization will always see consistent masks up and |
| down the tree, which brings significant simplifications over the |
| single-mask method. |
| |
| This is an instance of `deferring work in order to avoid |
| synchronization <http://www.cs.columbia.edu/~library/TR-repository/re |
| ports/reports-1992/cucs-039-92.ps.gz>`__. |
| Lazily recording CPU-hotplug events at the beginning of the next |
| grace period greatly simplifies maintenance of the CPU-tracking |
| bitmasks in the ``rcu_node`` tree. |
+-----------------------------------------------------------------------+
Expedited Grace Period Refinements
----------------------------------
Idle-CPU Checks
~~~~~~~~~~~~~~~
Each expedited grace period checks for idle CPUs when initially forming
the mask of CPUs to be IPIed and again just before IPIing a CPU (both
checks are carried out by ``sync_rcu_exp_select_cpus()``). If the CPU is
idle at any time between those two times, the CPU will not be IPIed.
Instead, the task pushing the grace period forward will include the idle
CPUs in the mask passed to ``rcu_report_exp_cpu_mult()``.
For RCU-sched, there is an additional check: If the IPI has interrupted
the idle loop, then ``rcu_exp_handler()`` invokes
``rcu_report_exp_rdp()`` to report the corresponding quiescent state.
For RCU-preempt, there is no specific check for idle in the IPI handler
(``rcu_exp_handler()``), but because RCU read-side critical sections are
not permitted within the idle loop, if ``rcu_exp_handler()`` sees that
the CPU is within RCU read-side critical section, the CPU cannot
possibly be idle. Otherwise, ``rcu_exp_handler()`` invokes
``rcu_report_exp_rdp()`` to report the corresponding quiescent state,
regardless of whether or not that quiescent state was due to the CPU
being idle.
In summary, RCU expedited grace periods check for idle when building the
bitmask of CPUs that must be IPIed, just before sending each IPI, and
(either explicitly or implicitly) within the IPI handler.
Batching via Sequence Counter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If each grace-period request was carried out separately, expedited grace
periods would have abysmal scalability and problematic high-load
characteristics. Because each grace-period operation can serve an
unlimited number of updates, it is important to *batch* requests, so
that a single expedited grace-period operation will cover all requests
in the corresponding batch.
This batching is controlled by a sequence counter named
``->expedited_sequence`` in the ``rcu_state`` structure. This counter
has an odd value when there is an expedited grace period in progress and
an even value otherwise, so that dividing the counter value by two gives
the number of completed grace periods. During any given update request,
the counter must transition from even to odd and then back to even, thus
indicating that a grace period has elapsed. Therefore, if the initial
value of the counter is ``s``, the updater must wait until the counter
reaches at least the value ``(s+3)&~0x1``. This counter is managed by
the following access functions:
#. ``rcu_exp_gp_seq_start()``, which marks the start of an expedited
grace period.
#. ``rcu_exp_gp_seq_end()``, which marks the end of an expedited grace
period.
#. ``rcu_exp_gp_seq_snap()``, which obtains a snapshot of the counter.
#. ``rcu_exp_gp_seq_done()``, which returns ``true`` if a full expedited
grace period has elapsed since the corresponding call to
``rcu_exp_gp_seq_snap()``.
Again, only one request in a given batch need actually carry out a
grace-period operation, which means there must be an efficient way to
identify which of many concurrent reqeusts will initiate the grace
period, and that there be an efficient way for the remaining requests to
wait for that grace period to complete. However, that is the topic of
the next section.
Funnel Locking and Wait/Wakeup
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The natural way to sort out which of a batch of updaters will initiate
the expedited grace period is to use the ``rcu_node`` combining tree, as
implemented by the ``exp_funnel_lock()`` function. The first updater
corresponding to a given grace period arriving at a given ``rcu_node``
structure records its desired grace-period sequence number in the
``->exp_seq_rq`` field and moves up to the next level in the tree.
Otherwise, if the ``->exp_seq_rq`` field already contains the sequence
number for the desired grace period or some later one, the updater
blocks on one of four wait queues in the ``->exp_wq[]`` array, using the
second-from-bottom and third-from bottom bits as an index. An
``->exp_lock`` field in the ``rcu_node`` structure synchronizes access
to these fields.
An empty ``rcu_node`` tree is shown in the following diagram, with the
white cells representing the ``->exp_seq_rq`` field and the red cells
representing the elements of the ``->exp_wq[]`` array.
.. kernel-figure:: Funnel0.svg
The next diagram shows the situation after the arrival of Task A and
Task B at the leftmost and rightmost leaf ``rcu_node`` structures,
respectively. The current value of the ``rcu_state`` structure's
``->expedited_sequence`` field is zero, so adding three and clearing the
bottom bit results in the value two, which both tasks record in the
``->exp_seq_rq`` field of their respective ``rcu_node`` structures:
.. kernel-figure:: Funnel1.svg
Each of Tasks A and B will move up to the root ``rcu_node`` structure.
Suppose that Task A wins, recording its desired grace-period sequence
number and resulting in the state shown below:
.. kernel-figure:: Funnel2.svg
Task A now advances to initiate a new grace period, while Task B moves
up to the root ``rcu_node`` structure, and, seeing that its desired
sequence number is already recorded, blocks on ``->exp_wq[1]``.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| Why ``->exp_wq[1]``? Given that the value of these tasks' desired |
| sequence number is two, so shouldn't they instead block on |
| ``->exp_wq[2]``? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| No. |
| Recall that the bottom bit of the desired sequence number indicates |
| whether or not a grace period is currently in progress. It is |
| therefore necessary to shift the sequence number right one bit |
| position to obtain the number of the grace period. This results in |
| ``->exp_wq[1]``. |
+-----------------------------------------------------------------------+
If Tasks C and D also arrive at this point, they will compute the same
desired grace-period sequence number, and see that both leaf
``rcu_node`` structures already have that value recorded. They will
therefore block on their respective ``rcu_node`` structures'
``->exp_wq[1]`` fields, as shown below:
.. kernel-figure:: Funnel3.svg
Task A now acquires the ``rcu_state`` structure's ``->exp_mutex`` and
initiates the grace period, which increments ``->expedited_sequence``.
Therefore, if Tasks E and F arrive, they will compute a desired sequence
number of 4 and will record this value as shown below:
.. kernel-figure:: Funnel4.svg
Tasks E and F will propagate up the ``rcu_node`` combining tree, with
Task F blocking on the root ``rcu_node`` structure and Task E wait for
Task A to finish so that it can start the next grace period. The
resulting state is as shown below:
.. kernel-figure:: Funnel5.svg
Once the grace period completes, Task A starts waking up the tasks
waiting for this grace period to complete, increments the
``->expedited_sequence``, acquires the ``->exp_wake_mutex`` and then
releases the ``->exp_mutex``. This results in the following state:
.. kernel-figure:: Funnel6.svg
Task E can then acquire ``->exp_mutex`` and increment
``->expedited_sequence`` to the value three. If new tasks G and H arrive
and moves up the combining tree at the same time, the state will be as
follows:
.. kernel-figure:: Funnel7.svg
Note that three of the root ``rcu_node`` structure's waitqueues are now
occupied. However, at some point, Task A will wake up the tasks blocked
on the ``->exp_wq`` waitqueues, resulting in the following state:
.. kernel-figure:: Funnel8.svg
Execution will continue with Tasks E and H completing their grace
periods and carrying out their wakeups.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| What happens if Task A takes so long to do its wakeups that Task E's |
| grace period completes? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| Then Task E will block on the ``->exp_wake_mutex``, which will also |
| prevent it from releasing ``->exp_mutex``, which in turn will prevent |
| the next grace period from starting. This last is important in |
| preventing overflow of the ``->exp_wq[]`` array. |
+-----------------------------------------------------------------------+
Use of Workqueues
~~~~~~~~~~~~~~~~~
In earlier implementations, the task requesting the expedited grace
period also drove it to completion. This straightforward approach had
the disadvantage of needing to account for POSIX signals sent to user
tasks, so more recent implemementations use the Linux kernel's
`workqueues <https://www.kernel.org/doc/Documentation/core-api/workqueue.rst>`__.
The requesting task still does counter snapshotting and funnel-lock
processing, but the task reaching the top of the funnel lock does a
``schedule_work()`` (from ``_synchronize_rcu_expedited()`` so that a
workqueue kthread does the actual grace-period processing. Because
workqueue kthreads do not accept POSIX signals, grace-period-wait
processing need not allow for POSIX signals. In addition, this approach
allows wakeups for the previous expedited grace period to be overlapped
with processing for the next expedited grace period. Because there are
only four sets of waitqueues, it is necessary to ensure that the
previous grace period's wakeups complete before the next grace period's
wakeups start. This is handled by having the ``->exp_mutex`` guard
expedited grace-period processing and the ``->exp_wake_mutex`` guard
wakeups. The key point is that the ``->exp_mutex`` is not released until
the first wakeup is complete, which means that the ``->exp_wake_mutex``
has already been acquired at that point. This approach ensures that the
previous grace period's wakeups can be carried out while the current
grace period is in process, but that these wakeups will complete before
the next grace period starts. This means that only three waitqueues are
required, guaranteeing that the four that are provided are sufficient.
Stall Warnings
~~~~~~~~~~~~~~
Expediting grace periods does nothing to speed things up when RCU
readers take too long, and therefore expedited grace periods check for
stalls just as normal grace periods do.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| But why not just let the normal grace-period machinery detect the |
| stalls, given that a given reader must block both normal and |
| expedited grace periods? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| Because it is quite possible that at a given time there is no normal |
| grace period in progress, in which case the normal grace period |
| cannot emit a stall warning. |
+-----------------------------------------------------------------------+
The ``synchronize_sched_expedited_wait()`` function loops waiting for
the expedited grace period to end, but with a timeout set to the current
RCU CPU stall-warning time. If this time is exceeded, any CPUs or
``rcu_node`` structures blocking the current grace period are printed.
Each stall warning results in another pass through the loop, but the
second and subsequent passes use longer stall times.
Mid-boot operation
~~~~~~~~~~~~~~~~~~
The use of workqueues has the advantage that the expedited grace-period
code need not worry about POSIX signals. Unfortunately, it has the
corresponding disadvantage that workqueues cannot be used until they are
initialized, which does not happen until some time after the scheduler
spawns the first task. Given that there are parts of the kernel that
really do want to execute grace periods during this mid-boot “dead
zone”, expedited grace periods must do something else during thie time.
What they do is to fall back to the old practice of requiring that the
requesting task drive the expedited grace period, as was the case before
the use of workqueues. However, the requesting task is only required to
drive the grace period during the mid-boot dead zone. Before mid-boot, a
synchronous grace period is a no-op. Some time after mid-boot,
workqueues are used.
Non-expedited non-SRCU synchronous grace periods must also operate
normally during mid-boot. This is handled by causing non-expedited grace
periods to take the expedited code path during mid-boot.
The current code assumes that there are no POSIX signals during the
mid-boot dead zone. However, if an overwhelming need for POSIX signals
somehow arises, appropriate adjustments can be made to the expedited
stall-warning code. One such adjustment would reinstate the
pre-workqueue stall-warning checks, but only during the mid-boot dead
zone.
With this refinement, synchronous grace periods can now be used from
task context pretty much any time during the life of the kernel. That
is, aside from some points in the suspend, hibernate, or shutdown code
path.
Summary
~~~~~~~
Expedited grace periods use a sequence-number approach to promote
batching, so that a single grace-period operation can serve numerous
requests. A funnel lock is used to efficiently identify the one task out
of a concurrent group that will request the grace period. All members of
the group will block on waitqueues provided in the ``rcu_node``
structure. The actual grace-period processing is carried out by a
workqueue.
CPU-hotplug operations are noted lazily in order to prevent the need for
tight synchronization between expedited grace periods and CPU-hotplug
operations. The dyntick-idle counters are used to avoid sending IPIs to
idle CPUs, at least in the common case. RCU-preempt and RCU-sched use
different IPI handlers and different code to respond to the state
changes carried out by those handlers, but otherwise use common code.
Quiescent states are tracked using the ``rcu_node`` tree, and once all
necessary quiescent states have been reported, all tasks waiting on this
expedited grace period are awakened. A pair of mutexes are used to allow
one grace period's wakeups to proceed concurrently with the next grace
period's processing.
This combination of mechanisms allows expedited grace periods to run
reasonably efficiently. However, for non-time-critical tasks, normal
grace periods should be used instead because their longer duration
permits much higher degrees of batching, and thus much lower per-request
overheads.

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<p>August 8, 2017</p>
<p>This article was contributed by Paul E.&nbsp;McKenney</p>
<h3>Introduction</h3>
<p>This document gives a rough visual overview of how Tree RCU's
grace-period memory ordering guarantee is provided.
<ol>
<li> <a href="#What Is Tree RCU's Grace Period Memory Ordering Guarantee?">
What Is Tree RCU's Grace Period Memory Ordering Guarantee?</a>
<li> <a href="#Tree RCU Grace Period Memory Ordering Building Blocks">
Tree RCU Grace Period Memory Ordering Building Blocks</a>
<li> <a href="#Tree RCU Grace Period Memory Ordering Components">
Tree RCU Grace Period Memory Ordering Components</a>
<li> <a href="#Putting It All Together">Putting It All Together</a>
</ol>
<h3><a name="What Is Tree RCU's Grace Period Memory Ordering Guarantee?">
What Is Tree RCU's Grace Period Memory Ordering Guarantee?</a></h3>
<p>RCU grace periods provide extremely strong memory-ordering guarantees
for non-idle non-offline code.
Any code that happens after the end of a given RCU grace period is guaranteed
to see the effects of all accesses prior to the beginning of that grace
period that are within RCU read-side critical sections.
Similarly, any code that happens before the beginning of a given RCU grace
period is guaranteed to see the effects of all accesses following the end
of that grace period that are within RCU read-side critical sections.
<p>Note well that RCU-sched read-side critical sections include any region
of code for which preemption is disabled.
Given that each individual machine instruction can be thought of as
an extremely small region of preemption-disabled code, one can think of
<tt>synchronize_rcu()</tt> as <tt>smp_mb()</tt> on steroids.
<p>RCU updaters use this guarantee by splitting their updates into
two phases, one of which is executed before the grace period and
the other of which is executed after the grace period.
In the most common use case, phase one removes an element from
a linked RCU-protected data structure, and phase two frees that element.
For this to work, any readers that have witnessed state prior to the
phase-one update (in the common case, removal) must not witness state
following the phase-two update (in the common case, freeing).
<p>The RCU implementation provides this guarantee using a network
of lock-based critical sections, memory barriers, and per-CPU
processing, as is described in the following sections.
<h3><a name="Tree RCU Grace Period Memory Ordering Building Blocks">
Tree RCU Grace Period Memory Ordering Building Blocks</a></h3>
<p>The workhorse for RCU's grace-period memory ordering is the
critical section for the <tt>rcu_node</tt> structure's
<tt>-&gt;lock</tt>.
These critical sections use helper functions for lock acquisition, including
<tt>raw_spin_lock_rcu_node()</tt>,
<tt>raw_spin_lock_irq_rcu_node()</tt>, and
<tt>raw_spin_lock_irqsave_rcu_node()</tt>.
Their lock-release counterparts are
<tt>raw_spin_unlock_rcu_node()</tt>,
<tt>raw_spin_unlock_irq_rcu_node()</tt>, and
<tt>raw_spin_unlock_irqrestore_rcu_node()</tt>,
respectively.
For completeness, a
<tt>raw_spin_trylock_rcu_node()</tt>
is also provided.
The key point is that the lock-acquisition functions, including
<tt>raw_spin_trylock_rcu_node()</tt>, all invoke
<tt>smp_mb__after_unlock_lock()</tt> immediately after successful
acquisition of the lock.
<p>Therefore, for any given <tt>rcu_node</tt> structure, any access
happening before one of the above lock-release functions will be seen
by all CPUs as happening before any access happening after a later
one of the above lock-acquisition functions.
Furthermore, any access happening before one of the
above lock-release function on any given CPU will be seen by all
CPUs as happening before any access happening after a later one
of the above lock-acquisition functions executing on that same CPU,
even if the lock-release and lock-acquisition functions are operating
on different <tt>rcu_node</tt> structures.
Tree RCU uses these two ordering guarantees to form an ordering
network among all CPUs that were in any way involved in the grace
period, including any CPUs that came online or went offline during
the grace period in question.
<p>The following litmus test exhibits the ordering effects of these
lock-acquisition and lock-release functions:
<pre>
1 int x, y, z;
2
3 void task0(void)
4 {
5 raw_spin_lock_rcu_node(rnp);
6 WRITE_ONCE(x, 1);
7 r1 = READ_ONCE(y);
8 raw_spin_unlock_rcu_node(rnp);
9 }
10
11 void task1(void)
12 {
13 raw_spin_lock_rcu_node(rnp);
14 WRITE_ONCE(y, 1);
15 r2 = READ_ONCE(z);
16 raw_spin_unlock_rcu_node(rnp);
17 }
18
19 void task2(void)
20 {
21 WRITE_ONCE(z, 1);
22 smp_mb();
23 r3 = READ_ONCE(x);
24 }
25
26 WARN_ON(r1 == 0 &amp;&amp; r2 == 0 &amp;&amp; r3 == 0);
</pre>
<p>The <tt>WARN_ON()</tt> is evaluated at &ldquo;the end of time&rdquo;,
after all changes have propagated throughout the system.
Without the <tt>smp_mb__after_unlock_lock()</tt> provided by the
acquisition functions, this <tt>WARN_ON()</tt> could trigger, for example
on PowerPC.
The <tt>smp_mb__after_unlock_lock()</tt> invocations prevent this
<tt>WARN_ON()</tt> from triggering.
<p>This approach must be extended to include idle CPUs, which need
RCU's grace-period memory ordering guarantee to extend to any
RCU read-side critical sections preceding and following the current
idle sojourn.
This case is handled by calls to the strongly ordered
<tt>atomic_add_return()</tt> read-modify-write atomic operation that
is invoked within <tt>rcu_dynticks_eqs_enter()</tt> at idle-entry
time and within <tt>rcu_dynticks_eqs_exit()</tt> at idle-exit time.
The grace-period kthread invokes <tt>rcu_dynticks_snap()</tt> and
<tt>rcu_dynticks_in_eqs_since()</tt> (both of which invoke
an <tt>atomic_add_return()</tt> of zero) to detect idle CPUs.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But what about CPUs that remain offline for the entire
grace period?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Such CPUs will be offline at the beginning of the grace period,
so the grace period won't expect quiescent states from them.
Races between grace-period start and CPU-hotplug operations
are mediated by the CPU's leaf <tt>rcu_node</tt> structure's
<tt>-&gt;lock</tt> as described above.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>The approach must be extended to handle one final case, that
of waking a task blocked in <tt>synchronize_rcu()</tt>.
This task might be affinitied to a CPU that is not yet aware that
the grace period has ended, and thus might not yet be subject to
the grace period's memory ordering.
Therefore, there is an <tt>smp_mb()</tt> after the return from
<tt>wait_for_completion()</tt> in the <tt>synchronize_rcu()</tt>
code path.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
What? Where???
I don't see any <tt>smp_mb()</tt> after the return from
<tt>wait_for_completion()</tt>!!!
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
That would be because I spotted the need for that
<tt>smp_mb()</tt> during the creation of this documentation,
and it is therefore unlikely to hit mainline before v4.14.
Kudos to Lance Roy, Will Deacon, Peter Zijlstra, and
Jonathan Cameron for asking questions that sensitized me
to the rather elaborate sequence of events that demonstrate
the need for this memory barrier.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>Tree RCU's grace--period memory-ordering guarantees rely most
heavily on the <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>
field, so much so that it is necessary to abbreviate this pattern
in the diagrams in the next section.
For example, consider the <tt>rcu_prepare_for_idle()</tt> function
shown below, which is one of several functions that enforce ordering
of newly arrived RCU callbacks against future grace periods:
<pre>
1 static void rcu_prepare_for_idle(void)
2 {
3 bool needwake;
4 struct rcu_data *rdp;
5 struct rcu_dynticks *rdtp = this_cpu_ptr(&amp;rcu_dynticks);
6 struct rcu_node *rnp;
7 struct rcu_state *rsp;
8 int tne;
9
10 if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
11 rcu_is_nocb_cpu(smp_processor_id()))
12 return;
13 tne = READ_ONCE(tick_nohz_active);
14 if (tne != rdtp-&gt;tick_nohz_enabled_snap) {
15 if (rcu_cpu_has_callbacks(NULL))
16 invoke_rcu_core();
17 rdtp-&gt;tick_nohz_enabled_snap = tne;
18 return;
19 }
20 if (!tne)
21 return;
22 if (rdtp-&gt;all_lazy &amp;&amp;
23 rdtp-&gt;nonlazy_posted != rdtp-&gt;nonlazy_posted_snap) {
24 rdtp-&gt;all_lazy = false;
25 rdtp-&gt;nonlazy_posted_snap = rdtp-&gt;nonlazy_posted;
26 invoke_rcu_core();
27 return;
28 }
29 if (rdtp-&gt;last_accelerate == jiffies)
30 return;
31 rdtp-&gt;last_accelerate = jiffies;
32 for_each_rcu_flavor(rsp) {
33 rdp = this_cpu_ptr(rsp-&gt;rda);
34 if (rcu_segcblist_pend_cbs(&amp;rdp-&gt;cblist))
35 continue;
36 rnp = rdp-&gt;mynode;
37 raw_spin_lock_rcu_node(rnp);
38 needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
39 raw_spin_unlock_rcu_node(rnp);
40 if (needwake)
41 rcu_gp_kthread_wake(rsp);
42 }
43 }
</pre>
<p>But the only part of <tt>rcu_prepare_for_idle()</tt> that really
matters for this discussion are lines&nbsp;37&ndash;39.
We will therefore abbreviate this function as follows:
</p><p><img src="rcu_node-lock.svg" alt="rcu_node-lock.svg">
<p>The box represents the <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>
critical section, with the double line on top representing the additional
<tt>smp_mb__after_unlock_lock()</tt>.
<h3><a name="Tree RCU Grace Period Memory Ordering Components">
Tree RCU Grace Period Memory Ordering Components</a></h3>
<p>Tree RCU's grace-period memory-ordering guarantee is provided by
a number of RCU components:
<ol>
<li> <a href="#Callback Registry">Callback Registry</a>
<li> <a href="#Grace-Period Initialization">Grace-Period Initialization</a>
<li> <a href="#Self-Reported Quiescent States">
Self-Reported Quiescent States</a>
<li> <a href="#Dynamic Tick Interface">Dynamic Tick Interface</a>
<li> <a href="#CPU-Hotplug Interface">CPU-Hotplug Interface</a>
<li> <a href="Forcing Quiescent States">Forcing Quiescent States</a>
<li> <a href="Grace-Period Cleanup">Grace-Period Cleanup</a>
<li> <a href="Callback Invocation">Callback Invocation</a>
</ol>
<p>Each of the following section looks at the corresponding component
in detail.
<h4><a name="Callback Registry">Callback Registry</a></h4>
<p>If RCU's grace-period guarantee is to mean anything at all, any
access that happens before a given invocation of <tt>call_rcu()</tt>
must also happen before the corresponding grace period.
The implementation of this portion of RCU's grace period guarantee
is shown in the following figure:
</p><p><img src="TreeRCU-callback-registry.svg" alt="TreeRCU-callback-registry.svg">
<p>Because <tt>call_rcu()</tt> normally acts only on CPU-local state,
it provides no ordering guarantees, either for itself or for
phase one of the update (which again will usually be removal of
an element from an RCU-protected data structure).
It simply enqueues the <tt>rcu_head</tt> structure on a per-CPU list,
which cannot become associated with a grace period until a later
call to <tt>rcu_accelerate_cbs()</tt>, as shown in the diagram above.
<p>One set of code paths shown on the left invokes
<tt>rcu_accelerate_cbs()</tt> via
<tt>note_gp_changes()</tt>, either directly from <tt>call_rcu()</tt> (if
the current CPU is inundated with queued <tt>rcu_head</tt> structures)
or more likely from an <tt>RCU_SOFTIRQ</tt> handler.
Another code path in the middle is taken only in kernels built with
<tt>CONFIG_RCU_FAST_NO_HZ=y</tt>, which invokes
<tt>rcu_accelerate_cbs()</tt> via <tt>rcu_prepare_for_idle()</tt>.
The final code path on the right is taken only in kernels built with
<tt>CONFIG_HOTPLUG_CPU=y</tt>, which invokes
<tt>rcu_accelerate_cbs()</tt> via
<tt>rcu_advance_cbs()</tt>, <tt>rcu_migrate_callbacks</tt>,
<tt>rcutree_migrate_callbacks()</tt>, and <tt>takedown_cpu()</tt>,
which in turn is invoked on a surviving CPU after the outgoing
CPU has been completely offlined.
<p>There are a few other code paths within grace-period processing
that opportunistically invoke <tt>rcu_accelerate_cbs()</tt>.
However, either way, all of the CPU's recently queued <tt>rcu_head</tt>
structures are associated with a future grace-period number under
the protection of the CPU's lead <tt>rcu_node</tt> structure's
<tt>-&gt;lock</tt>.
In all cases, there is full ordering against any prior critical section
for that same <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>, and
also full ordering against any of the current task's or CPU's prior critical
sections for any <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>.
<p>The next section will show how this ordering ensures that any
accesses prior to the <tt>call_rcu()</tt> (particularly including phase
one of the update)
happen before the start of the corresponding grace period.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But what about <tt>synchronize_rcu()</tt>?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
The <tt>synchronize_rcu()</tt> passes <tt>call_rcu()</tt>
to <tt>wait_rcu_gp()</tt>, which invokes it.
So either way, it eventually comes down to <tt>call_rcu()</tt>.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h4><a name="Grace-Period Initialization">Grace-Period Initialization</a></h4>
<p>Grace-period initialization is carried out by
the grace-period kernel thread, which makes several passes over the
<tt>rcu_node</tt> tree within the <tt>rcu_gp_init()</tt> function.
This means that showing the full flow of ordering through the
grace-period computation will require duplicating this tree.
If you find this confusing, please note that the state of the
<tt>rcu_node</tt> changes over time, just like Heraclitus's river.
However, to keep the <tt>rcu_node</tt> river tractable, the
grace-period kernel thread's traversals are presented in multiple
parts, starting in this section with the various phases of
grace-period initialization.
<p>The first ordering-related grace-period initialization action is to
advance the <tt>rcu_state</tt> structure's <tt>-&gt;gp_seq</tt>
grace-period-number counter, as shown below:
</p><p><img src="TreeRCU-gp-init-1.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
<p>The actual increment is carried out using <tt>smp_store_release()</tt>,
which helps reject false-positive RCU CPU stall detection.
Note that only the root <tt>rcu_node</tt> structure is touched.
<p>The first pass through the <tt>rcu_node</tt> tree updates bitmasks
based on CPUs having come online or gone offline since the start of
the previous grace period.
In the common case where the number of online CPUs for this <tt>rcu_node</tt>
structure has not transitioned to or from zero,
this pass will scan only the leaf <tt>rcu_node</tt> structures.
However, if the number of online CPUs for a given leaf <tt>rcu_node</tt>
structure has transitioned from zero,
<tt>rcu_init_new_rnp()</tt> will be invoked for the first incoming CPU.
Similarly, if the number of online CPUs for a given leaf <tt>rcu_node</tt>
structure has transitioned to zero,
<tt>rcu_cleanup_dead_rnp()</tt> will be invoked for the last outgoing CPU.
The diagram below shows the path of ordering if the leftmost
<tt>rcu_node</tt> structure onlines its first CPU and if the next
<tt>rcu_node</tt> structure has no online CPUs
(or, alternatively if the leftmost <tt>rcu_node</tt> structure offlines
its last CPU and if the next <tt>rcu_node</tt> structure has no online CPUs).
</p><p><img src="TreeRCU-gp-init-2.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
<p>The final <tt>rcu_gp_init()</tt> pass through the <tt>rcu_node</tt>
tree traverses breadth-first, setting each <tt>rcu_node</tt> structure's
<tt>-&gt;gp_seq</tt> field to the newly advanced value from the
<tt>rcu_state</tt> structure, as shown in the following diagram.
</p><p><img src="TreeRCU-gp-init-3.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
<p>This change will also cause each CPU's next call to
<tt>__note_gp_changes()</tt>
to notice that a new grace period has started, as described in the next
section.
But because the grace-period kthread started the grace period at the
root (with the advancing of the <tt>rcu_state</tt> structure's
<tt>-&gt;gp_seq</tt> field) before setting each leaf <tt>rcu_node</tt>
structure's <tt>-&gt;gp_seq</tt> field, each CPU's observation of
the start of the grace period will happen after the actual start
of the grace period.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But what about the CPU that started the grace period?
Why wouldn't it see the start of the grace period right when
it started that grace period?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In some deep philosophical and overly anthromorphized
sense, yes, the CPU starting the grace period is immediately
aware of having done so.
However, if we instead assume that RCU is not self-aware,
then even the CPU starting the grace period does not really
become aware of the start of this grace period until its
first call to <tt>__note_gp_changes()</tt>.
On the other hand, this CPU potentially gets early notification
because it invokes <tt>__note_gp_changes()</tt> during its
last <tt>rcu_gp_init()</tt> pass through its leaf
<tt>rcu_node</tt> structure.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h4><a name="Self-Reported Quiescent States">
Self-Reported Quiescent States</a></h4>
<p>When all entities that might block the grace period have reported
quiescent states (or as described in a later section, had quiescent
states reported on their behalf), the grace period can end.
Online non-idle CPUs report their own quiescent states, as shown
in the following diagram:
</p><p><img src="TreeRCU-qs.svg" alt="TreeRCU-qs.svg" width="75%">
<p>This is for the last CPU to report a quiescent state, which signals
the end of the grace period.
Earlier quiescent states would push up the <tt>rcu_node</tt> tree
only until they encountered an <tt>rcu_node</tt> structure that
is waiting for additional quiescent states.
However, ordering is nevertheless preserved because some later quiescent
state will acquire that <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>.
<p>Any number of events can lead up to a CPU invoking
<tt>note_gp_changes</tt> (or alternatively, directly invoking
<tt>__note_gp_changes()</tt>), at which point that CPU will notice
the start of a new grace period while holding its leaf
<tt>rcu_node</tt> lock.
Therefore, all execution shown in this diagram happens after the
start of the grace period.
In addition, this CPU will consider any RCU read-side critical
section that started before the invocation of <tt>__note_gp_changes()</tt>
to have started before the grace period, and thus a critical
section that the grace period must wait on.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But a RCU read-side critical section might have started
after the beginning of the grace period
(the advancing of <tt>-&gt;gp_seq</tt> from earlier), so why should
the grace period wait on such a critical section?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
It is indeed not necessary for the grace period to wait on such
a critical section.
However, it is permissible to wait on it.
And it is furthermore important to wait on it, as this
lazy approach is far more scalable than a &ldquo;big bang&rdquo;
all-at-once grace-period start could possibly be.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>If the CPU does a context switch, a quiescent state will be
noted by <tt>rcu_node_context_switch()</tt> on the left.
On the other hand, if the CPU takes a scheduler-clock interrupt
while executing in usermode, a quiescent state will be noted by
<tt>rcu_sched_clock_irq()</tt> on the right.
Either way, the passage through a quiescent state will be noted
in a per-CPU variable.
<p>The next time an <tt>RCU_SOFTIRQ</tt> handler executes on
this CPU (for example, after the next scheduler-clock
interrupt), <tt>rcu_core()</tt> will invoke
<tt>rcu_check_quiescent_state()</tt>, which will notice the
recorded quiescent state, and invoke
<tt>rcu_report_qs_rdp()</tt>.
If <tt>rcu_report_qs_rdp()</tt> verifies that the quiescent state
really does apply to the current grace period, it invokes
<tt>rcu_report_rnp()</tt> which traverses up the <tt>rcu_node</tt>
tree as shown at the bottom of the diagram, clearing bits from
each <tt>rcu_node</tt> structure's <tt>-&gt;qsmask</tt> field,
and propagating up the tree when the result is zero.
<p>Note that traversal passes upwards out of a given <tt>rcu_node</tt>
structure only if the current CPU is reporting the last quiescent
state for the subtree headed by that <tt>rcu_node</tt> structure.
A key point is that if a CPU's traversal stops at a given <tt>rcu_node</tt>
structure, then there will be a later traversal by another CPU
(or perhaps the same one) that proceeds upwards
from that point, and the <tt>rcu_node</tt> <tt>-&gt;lock</tt>
guarantees that the first CPU's quiescent state happens before the
remainder of the second CPU's traversal.
Applying this line of thought repeatedly shows that all CPUs'
quiescent states happen before the last CPU traverses through
the root <tt>rcu_node</tt> structure, the &ldquo;last CPU&rdquo;
being the one that clears the last bit in the root <tt>rcu_node</tt>
structure's <tt>-&gt;qsmask</tt> field.
<h4><a name="Dynamic Tick Interface">Dynamic Tick Interface</a></h4>
<p>Due to energy-efficiency considerations, RCU is forbidden from
disturbing idle CPUs.
CPUs are therefore required to notify RCU when entering or leaving idle
state, which they do via fully ordered value-returning atomic operations
on a per-CPU variable.
The ordering effects are as shown below:
</p><p><img src="TreeRCU-dyntick.svg" alt="TreeRCU-dyntick.svg" width="50%">
<p>The RCU grace-period kernel thread samples the per-CPU idleness
variable while holding the corresponding CPU's leaf <tt>rcu_node</tt>
structure's <tt>-&gt;lock</tt>.
This means that any RCU read-side critical sections that precede the
idle period (the oval near the top of the diagram above) will happen
before the end of the current grace period.
Similarly, the beginning of the current grace period will happen before
any RCU read-side critical sections that follow the
idle period (the oval near the bottom of the diagram above).
<p>Plumbing this into the full grace-period execution is described
<a href="#Forcing Quiescent States">below</a>.
<h4><a name="CPU-Hotplug Interface">CPU-Hotplug Interface</a></h4>
<p>RCU is also forbidden from disturbing offline CPUs, which might well
be powered off and removed from the system completely.
CPUs are therefore required to notify RCU of their comings and goings
as part of the corresponding CPU hotplug operations.
The ordering effects are shown below:
</p><p><img src="TreeRCU-hotplug.svg" alt="TreeRCU-hotplug.svg" width="50%">
<p>Because CPU hotplug operations are much less frequent than idle transitions,
they are heavier weight, and thus acquire the CPU's leaf <tt>rcu_node</tt>
structure's <tt>-&gt;lock</tt> and update this structure's
<tt>-&gt;qsmaskinitnext</tt>.
The RCU grace-period kernel thread samples this mask to detect CPUs
having gone offline since the beginning of this grace period.
<p>Plumbing this into the full grace-period execution is described
<a href="#Forcing Quiescent States">below</a>.
<h4><a name="Forcing Quiescent States">Forcing Quiescent States</a></h4>
<p>As noted above, idle and offline CPUs cannot report their own
quiescent states, and therefore the grace-period kernel thread
must do the reporting on their behalf.
This process is called &ldquo;forcing quiescent states&rdquo;, it is
repeated every few jiffies, and its ordering effects are shown below:
</p><p><img src="TreeRCU-gp-fqs.svg" alt="TreeRCU-gp-fqs.svg" width="100%">
<p>Each pass of quiescent state forcing is guaranteed to traverse the
leaf <tt>rcu_node</tt> structures, and if there are no new quiescent
states due to recently idled and/or offlined CPUs, then only the
leaves are traversed.
However, if there is a newly offlined CPU as illustrated on the left
or a newly idled CPU as illustrated on the right, the corresponding
quiescent state will be driven up towards the root.
As with self-reported quiescent states, the upwards driving stops
once it reaches an <tt>rcu_node</tt> structure that has quiescent
states outstanding from other CPUs.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
The leftmost drive to root stopped before it reached
the root <tt>rcu_node</tt> structure, which means that
there are still CPUs subordinate to that structure on
which the current grace period is waiting.
Given that, how is it possible that the rightmost drive
to root ended the grace period?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Good analysis!
It is in fact impossible in the absence of bugs in RCU.
But this diagram is complex enough as it is, so simplicity
overrode accuracy.
You can think of it as poetic license, or you can think of
it as misdirection that is resolved in the
<a href="#Putting It All Together">stitched-together diagram</a>.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h4><a name="Grace-Period Cleanup">Grace-Period Cleanup</a></h4>
<p>Grace-period cleanup first scans the <tt>rcu_node</tt> tree
breadth-first advancing all the <tt>-&gt;gp_seq</tt> fields, then it
advances the <tt>rcu_state</tt> structure's <tt>-&gt;gp_seq</tt> field.
The ordering effects are shown below:
</p><p><img src="TreeRCU-gp-cleanup.svg" alt="TreeRCU-gp-cleanup.svg" width="75%">
<p>As indicated by the oval at the bottom of the diagram, once
grace-period cleanup is complete, the next grace period can begin.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But when precisely does the grace period end?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
There is no useful single point at which the grace period
can be said to end.
The earliest reasonable candidate is as soon as the last
CPU has reported its quiescent state, but it may be some
milliseconds before RCU becomes aware of this.
The latest reasonable candidate is once the <tt>rcu_state</tt>
structure's <tt>-&gt;gp_seq</tt> field has been updated,
but it is quite possible that some CPUs have already completed
phase two of their updates by that time.
In short, if you are going to work with RCU, you need to
learn to embrace uncertainty.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h4><a name="Callback Invocation">Callback Invocation</a></h4>
<p>Once a given CPU's leaf <tt>rcu_node</tt> structure's
<tt>-&gt;gp_seq</tt> field has been updated, that CPU can begin
invoking its RCU callbacks that were waiting for this grace period
to end.
These callbacks are identified by <tt>rcu_advance_cbs()</tt>,
which is usually invoked by <tt>__note_gp_changes()</tt>.
As shown in the diagram below, this invocation can be triggered by
the scheduling-clock interrupt (<tt>rcu_sched_clock_irq()</tt> on
the left) or by idle entry (<tt>rcu_cleanup_after_idle()</tt> on
the right, but only for kernels build with
<tt>CONFIG_RCU_FAST_NO_HZ=y</tt>).
Either way, <tt>RCU_SOFTIRQ</tt> is raised, which results in
<tt>rcu_do_batch()</tt> invoking the callbacks, which in turn
allows those callbacks to carry out (either directly or indirectly
via wakeup) the needed phase-two processing for each update.
</p><p><img src="TreeRCU-callback-invocation.svg" alt="TreeRCU-callback-invocation.svg" width="60%">
<p>Please note that callback invocation can also be prompted by any
number of corner-case code paths, for example, when a CPU notes that
it has excessive numbers of callbacks queued.
In all cases, the CPU acquires its leaf <tt>rcu_node</tt> structure's
<tt>-&gt;lock</tt> before invoking callbacks, which preserves the
required ordering against the newly completed grace period.
<p>However, if the callback function communicates to other CPUs,
for example, doing a wakeup, then it is that function's responsibility
to maintain ordering.
For example, if the callback function wakes up a task that runs on
some other CPU, proper ordering must in place in both the callback
function and the task being awakened.
To see why this is important, consider the top half of the
<a href="#Grace-Period Cleanup">grace-period cleanup</a> diagram.
The callback might be running on a CPU corresponding to the leftmost
leaf <tt>rcu_node</tt> structure, and awaken a task that is to run on
a CPU corresponding to the rightmost leaf <tt>rcu_node</tt> structure,
and the grace-period kernel thread might not yet have reached the
rightmost leaf.
In this case, the grace period's memory ordering might not yet have
reached that CPU, so again the callback function and the awakened
task must supply proper ordering.
<h3><a name="Putting It All Together">Putting It All Together</a></h3>
<p>A stitched-together diagram is
<a href="Tree-RCU-Diagram.html">here</a>.
<h3><a name="Legal Statement">
Legal Statement</a></h3>
<p>This work represents the view of the author and does not necessarily
represent the view of IBM.
</p><p>Linux is a registered trademark of Linus Torvalds.
</p><p>Other company, product, and service names may be trademarks or
service marks of others.
</body></html>

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======================================================
A Tour Through TREE_RCU's Grace-Period Memory Ordering
======================================================
August 8, 2017
This article was contributed by Paul E.&nbsp;McKenney
Introduction
============
This document gives a rough visual overview of how Tree RCU's
grace-period memory ordering guarantee is provided.
What Is Tree RCU's Grace Period Memory Ordering Guarantee?
==========================================================
RCU grace periods provide extremely strong memory-ordering guarantees
for non-idle non-offline code.
Any code that happens after the end of a given RCU grace period is guaranteed
to see the effects of all accesses prior to the beginning of that grace
period that are within RCU read-side critical sections.
Similarly, any code that happens before the beginning of a given RCU grace
period is guaranteed to see the effects of all accesses following the end
of that grace period that are within RCU read-side critical sections.
Note well that RCU-sched read-side critical sections include any region
of code for which preemption is disabled.
Given that each individual machine instruction can be thought of as
an extremely small region of preemption-disabled code, one can think of
``synchronize_rcu()`` as ``smp_mb()`` on steroids.
RCU updaters use this guarantee by splitting their updates into
two phases, one of which is executed before the grace period and
the other of which is executed after the grace period.
In the most common use case, phase one removes an element from
a linked RCU-protected data structure, and phase two frees that element.
For this to work, any readers that have witnessed state prior to the
phase-one update (in the common case, removal) must not witness state
following the phase-two update (in the common case, freeing).
The RCU implementation provides this guarantee using a network
of lock-based critical sections, memory barriers, and per-CPU
processing, as is described in the following sections.
Tree RCU Grace Period Memory Ordering Building Blocks
=====================================================
The workhorse for RCU's grace-period memory ordering is the
critical section for the ``rcu_node`` structure's
``-&gt;lock``. These critical sections use helper functions for lock
acquisition, including ``raw_spin_lock_rcu_node()``,
``raw_spin_lock_irq_rcu_node()``, and ``raw_spin_lock_irqsave_rcu_node()``.
Their lock-release counterparts are ``raw_spin_unlock_rcu_node()``,
``raw_spin_unlock_irq_rcu_node()``, and
``raw_spin_unlock_irqrestore_rcu_node()``, respectively.
For completeness, a ``raw_spin_trylock_rcu_node()`` is also provided.
The key point is that the lock-acquisition functions, including
``raw_spin_trylock_rcu_node()``, all invoke ``smp_mb__after_unlock_lock()``
immediately after successful acquisition of the lock.
Therefore, for any given ``rcu_node`` structure, any access
happening before one of the above lock-release functions will be seen
by all CPUs as happening before any access happening after a later
one of the above lock-acquisition functions.
Furthermore, any access happening before one of the
above lock-release function on any given CPU will be seen by all
CPUs as happening before any access happening after a later one
of the above lock-acquisition functions executing on that same CPU,
even if the lock-release and lock-acquisition functions are operating
on different ``rcu_node`` structures.
Tree RCU uses these two ordering guarantees to form an ordering
network among all CPUs that were in any way involved in the grace
period, including any CPUs that came online or went offline during
the grace period in question.
The following litmus test exhibits the ordering effects of these
lock-acquisition and lock-release functions::
1 int x, y, z;
2
3 void task0(void)
4 {
5 raw_spin_lock_rcu_node(rnp);
6 WRITE_ONCE(x, 1);
7 r1 = READ_ONCE(y);
8 raw_spin_unlock_rcu_node(rnp);
9 }
10
11 void task1(void)
12 {
13 raw_spin_lock_rcu_node(rnp);
14 WRITE_ONCE(y, 1);
15 r2 = READ_ONCE(z);
16 raw_spin_unlock_rcu_node(rnp);
17 }
18
19 void task2(void)
20 {
21 WRITE_ONCE(z, 1);
22 smp_mb();
23 r3 = READ_ONCE(x);
24 }
25
26 WARN_ON(r1 == 0 &amp;&amp; r2 == 0 &amp;&amp; r3 == 0);
The ``WARN_ON()`` is evaluated at &ldquo;the end of time&rdquo;,
after all changes have propagated throughout the system.
Without the ``smp_mb__after_unlock_lock()`` provided by the
acquisition functions, this ``WARN_ON()`` could trigger, for example
on PowerPC.
The ``smp_mb__after_unlock_lock()`` invocations prevent this
``WARN_ON()`` from triggering.
This approach must be extended to include idle CPUs, which need
RCU's grace-period memory ordering guarantee to extend to any
RCU read-side critical sections preceding and following the current
idle sojourn.
This case is handled by calls to the strongly ordered
``atomic_add_return()`` read-modify-write atomic operation that
is invoked within ``rcu_dynticks_eqs_enter()`` at idle-entry
time and within ``rcu_dynticks_eqs_exit()`` at idle-exit time.
The grace-period kthread invokes ``rcu_dynticks_snap()`` and
``rcu_dynticks_in_eqs_since()`` (both of which invoke
an ``atomic_add_return()`` of zero) to detect idle CPUs.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| But what about CPUs that remain offline for the entire grace period? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| Such CPUs will be offline at the beginning of the grace period, so |
| the grace period won't expect quiescent states from them. Races |
| between grace-period start and CPU-hotplug operations are mediated |
| by the CPU's leaf ``rcu_node`` structure's ``->lock`` as described |
| above. |
+-----------------------------------------------------------------------+
The approach must be extended to handle one final case, that of waking a
task blocked in ``synchronize_rcu()``. This task might be affinitied to
a CPU that is not yet aware that the grace period has ended, and thus
might not yet be subject to the grace period's memory ordering.
Therefore, there is an ``smp_mb()`` after the return from
``wait_for_completion()`` in the ``synchronize_rcu()`` code path.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| What? Where??? I don't see any ``smp_mb()`` after the return from |
| ``wait_for_completion()``!!! |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| That would be because I spotted the need for that ``smp_mb()`` during |
| the creation of this documentation, and it is therefore unlikely to |
| hit mainline before v4.14. Kudos to Lance Roy, Will Deacon, Peter |
| Zijlstra, and Jonathan Cameron for asking questions that sensitized |
| me to the rather elaborate sequence of events that demonstrate the |
| need for this memory barrier. |
+-----------------------------------------------------------------------+
Tree RCU's grace--period memory-ordering guarantees rely most heavily on
the ``rcu_node`` structure's ``->lock`` field, so much so that it is
necessary to abbreviate this pattern in the diagrams in the next
section. For example, consider the ``rcu_prepare_for_idle()`` function
shown below, which is one of several functions that enforce ordering of
newly arrived RCU callbacks against future grace periods:
::
1 static void rcu_prepare_for_idle(void)
2 {
3 bool needwake;
4 struct rcu_data *rdp;
5 struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
6 struct rcu_node *rnp;
7 struct rcu_state *rsp;
8 int tne;
9
10 if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
11 rcu_is_nocb_cpu(smp_processor_id()))
12 return;
13 tne = READ_ONCE(tick_nohz_active);
14 if (tne != rdtp->tick_nohz_enabled_snap) {
15 if (rcu_cpu_has_callbacks(NULL))
16 invoke_rcu_core();
17 rdtp->tick_nohz_enabled_snap = tne;
18 return;
19 }
20 if (!tne)
21 return;
22 if (rdtp->all_lazy &&
23 rdtp->nonlazy_posted != rdtp->nonlazy_posted_snap) {
24 rdtp->all_lazy = false;
25 rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted;
26 invoke_rcu_core();
27 return;
28 }
29 if (rdtp->last_accelerate == jiffies)
30 return;
31 rdtp->last_accelerate = jiffies;
32 for_each_rcu_flavor(rsp) {
33 rdp = this_cpu_ptr(rsp->rda);
34 if (rcu_segcblist_pend_cbs(&rdp->cblist))
35 continue;
36 rnp = rdp->mynode;
37 raw_spin_lock_rcu_node(rnp);
38 needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
39 raw_spin_unlock_rcu_node(rnp);
40 if (needwake)
41 rcu_gp_kthread_wake(rsp);
42 }
43 }
But the only part of ``rcu_prepare_for_idle()`` that really matters for
this discussion are lines 37–39. We will therefore abbreviate this
function as follows:
.. kernel-figure:: rcu_node-lock.svg
The box represents the ``rcu_node`` structure's ``->lock`` critical
section, with the double line on top representing the additional
``smp_mb__after_unlock_lock()``.
Tree RCU Grace Period Memory Ordering Components
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tree RCU's grace-period memory-ordering guarantee is provided by a
number of RCU components:
#. `Callback Registry <#Callback%20Registry>`__
#. `Grace-Period Initialization <#Grace-Period%20Initialization>`__
#. `Self-Reported Quiescent
States <#Self-Reported%20Quiescent%20States>`__
#. `Dynamic Tick Interface <#Dynamic%20Tick%20Interface>`__
#. `CPU-Hotplug Interface <#CPU-Hotplug%20Interface>`__
#. `Forcing Quiescent States <Forcing%20Quiescent%20States>`__
#. `Grace-Period Cleanup <Grace-Period%20Cleanup>`__
#. `Callback Invocation <Callback%20Invocation>`__
Each of the following section looks at the corresponding component in
detail.
Callback Registry
^^^^^^^^^^^^^^^^^
If RCU's grace-period guarantee is to mean anything at all, any access
that happens before a given invocation of ``call_rcu()`` must also
happen before the corresponding grace period. The implementation of this
portion of RCU's grace period guarantee is shown in the following
figure:
.. kernel-figure:: TreeRCU-callback-registry.svg
Because ``call_rcu()`` normally acts only on CPU-local state, it
provides no ordering guarantees, either for itself or for phase one of
the update (which again will usually be removal of an element from an
RCU-protected data structure). It simply enqueues the ``rcu_head``
structure on a per-CPU list, which cannot become associated with a grace
period until a later call to ``rcu_accelerate_cbs()``, as shown in the
diagram above.
One set of code paths shown on the left invokes ``rcu_accelerate_cbs()``
via ``note_gp_changes()``, either directly from ``call_rcu()`` (if the
current CPU is inundated with queued ``rcu_head`` structures) or more
likely from an ``RCU_SOFTIRQ`` handler. Another code path in the middle
is taken only in kernels built with ``CONFIG_RCU_FAST_NO_HZ=y``, which
invokes ``rcu_accelerate_cbs()`` via ``rcu_prepare_for_idle()``. The
final code path on the right is taken only in kernels built with
``CONFIG_HOTPLUG_CPU=y``, which invokes ``rcu_accelerate_cbs()`` via
``rcu_advance_cbs()``, ``rcu_migrate_callbacks``,
``rcutree_migrate_callbacks()``, and ``takedown_cpu()``, which in turn
is invoked on a surviving CPU after the outgoing CPU has been completely
offlined.
There are a few other code paths within grace-period processing that
opportunistically invoke ``rcu_accelerate_cbs()``. However, either way,
all of the CPU's recently queued ``rcu_head`` structures are associated
with a future grace-period number under the protection of the CPU's lead
``rcu_node`` structure's ``->lock``. In all cases, there is full
ordering against any prior critical section for that same ``rcu_node``
structure's ``->lock``, and also full ordering against any of the
current task's or CPU's prior critical sections for any ``rcu_node``
structure's ``->lock``.
The next section will show how this ordering ensures that any accesses
prior to the ``call_rcu()`` (particularly including phase one of the
update) happen before the start of the corresponding grace period.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| But what about ``synchronize_rcu()``? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| The ``synchronize_rcu()`` passes ``call_rcu()`` to ``wait_rcu_gp()``, |
| which invokes it. So either way, it eventually comes down to |
| ``call_rcu()``. |
+-----------------------------------------------------------------------+
Grace-Period Initialization
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Grace-period initialization is carried out by the grace-period kernel
thread, which makes several passes over the ``rcu_node`` tree within the
``rcu_gp_init()`` function. This means that showing the full flow of
ordering through the grace-period computation will require duplicating
this tree. If you find this confusing, please note that the state of the
``rcu_node`` changes over time, just like Heraclitus's river. However,
to keep the ``rcu_node`` river tractable, the grace-period kernel
thread's traversals are presented in multiple parts, starting in this
section with the various phases of grace-period initialization.
The first ordering-related grace-period initialization action is to
advance the ``rcu_state`` structure's ``->gp_seq`` grace-period-number
counter, as shown below:
.. kernel-figure:: TreeRCU-gp-init-1.svg
The actual increment is carried out using ``smp_store_release()``, which
helps reject false-positive RCU CPU stall detection. Note that only the
root ``rcu_node`` structure is touched.
The first pass through the ``rcu_node`` tree updates bitmasks based on
CPUs having come online or gone offline since the start of the previous
grace period. In the common case where the number of online CPUs for
this ``rcu_node`` structure has not transitioned to or from zero, this
pass will scan only the leaf ``rcu_node`` structures. However, if the
number of online CPUs for a given leaf ``rcu_node`` structure has
transitioned from zero, ``rcu_init_new_rnp()`` will be invoked for the
first incoming CPU. Similarly, if the number of online CPUs for a given
leaf ``rcu_node`` structure has transitioned to zero,
``rcu_cleanup_dead_rnp()`` will be invoked for the last outgoing CPU.
The diagram below shows the path of ordering if the leftmost
``rcu_node`` structure onlines its first CPU and if the next
``rcu_node`` structure has no online CPUs (or, alternatively if the
leftmost ``rcu_node`` structure offlines its last CPU and if the next
``rcu_node`` structure has no online CPUs).
.. kernel-figure:: TreeRCU-gp-init-1.svg
The final ``rcu_gp_init()`` pass through the ``rcu_node`` tree traverses
breadth-first, setting each ``rcu_node`` structure's ``->gp_seq`` field
to the newly advanced value from the ``rcu_state`` structure, as shown
in the following diagram.
.. kernel-figure:: TreeRCU-gp-init-1.svg
This change will also cause each CPU's next call to
``__note_gp_changes()`` to notice that a new grace period has started,
as described in the next section. But because the grace-period kthread
started the grace period at the root (with the advancing of the
``rcu_state`` structure's ``->gp_seq`` field) before setting each leaf
``rcu_node`` structure's ``->gp_seq`` field, each CPU's observation of
the start of the grace period will happen after the actual start of the
grace period.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| But what about the CPU that started the grace period? Why wouldn't it |
| see the start of the grace period right when it started that grace |
| period? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| In some deep philosophical and overly anthromorphized sense, yes, the |
| CPU starting the grace period is immediately aware of having done so. |
| However, if we instead assume that RCU is not self-aware, then even |
| the CPU starting the grace period does not really become aware of the |
| start of this grace period until its first call to |
| ``__note_gp_changes()``. On the other hand, this CPU potentially gets |
| early notification because it invokes ``__note_gp_changes()`` during |
| its last ``rcu_gp_init()`` pass through its leaf ``rcu_node`` |
| structure. |
+-----------------------------------------------------------------------+
Self-Reported Quiescent States
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
When all entities that might block the grace period have reported
quiescent states (or as described in a later section, had quiescent
states reported on their behalf), the grace period can end. Online
non-idle CPUs report their own quiescent states, as shown in the
following diagram:
.. kernel-figure:: TreeRCU-qs.svg
This is for the last CPU to report a quiescent state, which signals the
end of the grace period. Earlier quiescent states would push up the
``rcu_node`` tree only until they encountered an ``rcu_node`` structure
that is waiting for additional quiescent states. However, ordering is
nevertheless preserved because some later quiescent state will acquire
that ``rcu_node`` structure's ``->lock``.
Any number of events can lead up to a CPU invoking ``note_gp_changes``
(or alternatively, directly invoking ``__note_gp_changes()``), at which
point that CPU will notice the start of a new grace period while holding
its leaf ``rcu_node`` lock. Therefore, all execution shown in this
diagram happens after the start of the grace period. In addition, this
CPU will consider any RCU read-side critical section that started before
the invocation of ``__note_gp_changes()`` to have started before the
grace period, and thus a critical section that the grace period must
wait on.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| But a RCU read-side critical section might have started after the |
| beginning of the grace period (the advancing of ``->gp_seq`` from |
| earlier), so why should the grace period wait on such a critical |
| section? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| It is indeed not necessary for the grace period to wait on such a |
| critical section. However, it is permissible to wait on it. And it is |
| furthermore important to wait on it, as this lazy approach is far |
| more scalable than a “big bang” all-at-once grace-period start could |
| possibly be. |
+-----------------------------------------------------------------------+
If the CPU does a context switch, a quiescent state will be noted by
``rcu_node_context_switch()`` on the left. On the other hand, if the CPU
takes a scheduler-clock interrupt while executing in usermode, a
quiescent state will be noted by ``rcu_sched_clock_irq()`` on the right.
Either way, the passage through a quiescent state will be noted in a
per-CPU variable.
The next time an ``RCU_SOFTIRQ`` handler executes on this CPU (for
example, after the next scheduler-clock interrupt), ``rcu_core()`` will
invoke ``rcu_check_quiescent_state()``, which will notice the recorded
quiescent state, and invoke ``rcu_report_qs_rdp()``. If
``rcu_report_qs_rdp()`` verifies that the quiescent state really does
apply to the current grace period, it invokes ``rcu_report_rnp()`` which
traverses up the ``rcu_node`` tree as shown at the bottom of the
diagram, clearing bits from each ``rcu_node`` structure's ``->qsmask``
field, and propagating up the tree when the result is zero.
Note that traversal passes upwards out of a given ``rcu_node`` structure
only if the current CPU is reporting the last quiescent state for the
subtree headed by that ``rcu_node`` structure. A key point is that if a
CPU's traversal stops at a given ``rcu_node`` structure, then there will
be a later traversal by another CPU (or perhaps the same one) that
proceeds upwards from that point, and the ``rcu_node`` ``->lock``
guarantees that the first CPU's quiescent state happens before the
remainder of the second CPU's traversal. Applying this line of thought
repeatedly shows that all CPUs' quiescent states happen before the last
CPU traverses through the root ``rcu_node`` structure, the “last CPU”
being the one that clears the last bit in the root ``rcu_node``
structure's ``->qsmask`` field.
Dynamic Tick Interface
^^^^^^^^^^^^^^^^^^^^^^
Due to energy-efficiency considerations, RCU is forbidden from
disturbing idle CPUs. CPUs are therefore required to notify RCU when
entering or leaving idle state, which they do via fully ordered
value-returning atomic operations on a per-CPU variable. The ordering
effects are as shown below:
.. kernel-figure:: TreeRCU-dyntick.svg
The RCU grace-period kernel thread samples the per-CPU idleness variable
while holding the corresponding CPU's leaf ``rcu_node`` structure's
``->lock``. This means that any RCU read-side critical sections that
precede the idle period (the oval near the top of the diagram above)
will happen before the end of the current grace period. Similarly, the
beginning of the current grace period will happen before any RCU
read-side critical sections that follow the idle period (the oval near
the bottom of the diagram above).
Plumbing this into the full grace-period execution is described
`below <#Forcing%20Quiescent%20States>`__.
CPU-Hotplug Interface
^^^^^^^^^^^^^^^^^^^^^
RCU is also forbidden from disturbing offline CPUs, which might well be
powered off and removed from the system completely. CPUs are therefore
required to notify RCU of their comings and goings as part of the
corresponding CPU hotplug operations. The ordering effects are shown
below:
.. kernel-figure:: TreeRCU-hotplug.svg
Because CPU hotplug operations are much less frequent than idle
transitions, they are heavier weight, and thus acquire the CPU's leaf
``rcu_node`` structure's ``->lock`` and update this structure's
``->qsmaskinitnext``. The RCU grace-period kernel thread samples this
mask to detect CPUs having gone offline since the beginning of this
grace period.
Plumbing this into the full grace-period execution is described
`below <#Forcing%20Quiescent%20States>`__.
Forcing Quiescent States
^^^^^^^^^^^^^^^^^^^^^^^^
As noted above, idle and offline CPUs cannot report their own quiescent
states, and therefore the grace-period kernel thread must do the
reporting on their behalf. This process is called “forcing quiescent
states”, it is repeated every few jiffies, and its ordering effects are
shown below:
.. kernel-figure:: TreeRCU-gp-fqs.svg
Each pass of quiescent state forcing is guaranteed to traverse the leaf
``rcu_node`` structures, and if there are no new quiescent states due to
recently idled and/or offlined CPUs, then only the leaves are traversed.
However, if there is a newly offlined CPU as illustrated on the left or
a newly idled CPU as illustrated on the right, the corresponding
quiescent state will be driven up towards the root. As with
self-reported quiescent states, the upwards driving stops once it
reaches an ``rcu_node`` structure that has quiescent states outstanding
from other CPUs.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| The leftmost drive to root stopped before it reached the root |
| ``rcu_node`` structure, which means that there are still CPUs |
| subordinate to that structure on which the current grace period is |
| waiting. Given that, how is it possible that the rightmost drive to |
| root ended the grace period? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| Good analysis! It is in fact impossible in the absence of bugs in |
| RCU. But this diagram is complex enough as it is, so simplicity |
| overrode accuracy. You can think of it as poetic license, or you can |
| think of it as misdirection that is resolved in the |
| `stitched-together diagram <#Putting%20It%20All%20Together>`__. |
+-----------------------------------------------------------------------+
Grace-Period Cleanup
^^^^^^^^^^^^^^^^^^^^
Grace-period cleanup first scans the ``rcu_node`` tree breadth-first
advancing all the ``->gp_seq`` fields, then it advances the
``rcu_state`` structure's ``->gp_seq`` field. The ordering effects are
shown below:
.. kernel-figure:: TreeRCU-gp-cleanup.svg
As indicated by the oval at the bottom of the diagram, once grace-period
cleanup is complete, the next grace period can begin.
+-----------------------------------------------------------------------+
| **Quick Quiz**: |
+-----------------------------------------------------------------------+
| But when precisely does the grace period end? |
+-----------------------------------------------------------------------+
| **Answer**: |
+-----------------------------------------------------------------------+
| There is no useful single point at which the grace period can be said |
| to end. The earliest reasonable candidate is as soon as the last CPU |
| has reported its quiescent state, but it may be some milliseconds |
| before RCU becomes aware of this. The latest reasonable candidate is |
| once the ``rcu_state`` structure's ``->gp_seq`` field has been |
| updated, but it is quite possible that some CPUs have already |
| completed phase two of their updates by that time. In short, if you |
| are going to work with RCU, you need to learn to embrace uncertainty. |
+-----------------------------------------------------------------------+
Callback Invocation
^^^^^^^^^^^^^^^^^^^
Once a given CPU's leaf ``rcu_node`` structure's ``->gp_seq`` field has
been updated, that CPU can begin invoking its RCU callbacks that were
waiting for this grace period to end. These callbacks are identified by
``rcu_advance_cbs()``, which is usually invoked by
``__note_gp_changes()``. As shown in the diagram below, this invocation
can be triggered by the scheduling-clock interrupt
(``rcu_sched_clock_irq()`` on the left) or by idle entry
(``rcu_cleanup_after_idle()`` on the right, but only for kernels build
with ``CONFIG_RCU_FAST_NO_HZ=y``). Either way, ``RCU_SOFTIRQ`` is
raised, which results in ``rcu_do_batch()`` invoking the callbacks,
which in turn allows those callbacks to carry out (either directly or
indirectly via wakeup) the needed phase-two processing for each update.
.. kernel-figure:: TreeRCU-callback-invocation.svg
Please note that callback invocation can also be prompted by any number
of corner-case code paths, for example, when a CPU notes that it has
excessive numbers of callbacks queued. In all cases, the CPU acquires
its leaf ``rcu_node`` structure's ``->lock`` before invoking callbacks,
which preserves the required ordering against the newly completed grace
period.
However, if the callback function communicates to other CPUs, for
example, doing a wakeup, then it is that function's responsibility to
maintain ordering. For example, if the callback function wakes up a task
that runs on some other CPU, proper ordering must in place in both the
callback function and the task being awakened. To see why this is
important, consider the top half of the `grace-period
cleanup <#Grace-Period%20Cleanup>`__ diagram. The callback might be
running on a CPU corresponding to the leftmost leaf ``rcu_node``
structure, and awaken a task that is to run on a CPU corresponding to
the rightmost leaf ``rcu_node`` structure, and the grace-period kernel
thread might not yet have reached the rightmost leaf. In this case, the
grace period's memory ordering might not yet have reached that CPU, so
again the callback function and the awakened task must supply proper
ordering.
Putting It All Together
~~~~~~~~~~~~~~~~~~~~~~~
A stitched-together diagram is here:
.. kernel-figure:: TreeRCU-gp.svg
Legal Statement
~~~~~~~~~~~~~~~
This work represents the view of the author and does not necessarily
represent the view of IBM.
Linux is a registered trademark of Linus Torvalds.
Other company, product, and service names may be trademarks or service
marks of others.

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5
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@ -11,6 +11,11 @@ RCU concepts
listRCU
UP
Design/Memory-Ordering/Tree-RCU-Memory-Ordering
Design/Expedited-Grace-Periods/Expedited-Grace-Periods
Design/Requirements/Requirements
Design/Data-Structures/Data-Structures
.. only:: subproject and html
Indices

4
Documentation/RCU/whatisRCU.txt
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@ -302,7 +302,7 @@ rcu_dereference()
must prohibit. The rcu_dereference_protected() variant takes
a lockdep expression to indicate which locks must be acquired
by the caller. If the indicated protection is not provided,
a lockdep splat is emitted. See RCU/Design/Requirements/Requirements.html
a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst
and the API's code comments for more details and example usage.
The following diagram shows how each API communicates among the
@ -630,7 +630,7 @@ been able to write-acquire the lock otherwise. The smp_mb__after_spinlock()
promotes synchronize_rcu() to a full memory barrier in compliance with
the "Memory-Barrier Guarantees" listed in:
Documentation/RCU/Design/Requirements/Requirements.html.
Documentation/RCU/Design/Requirements/Requirements.rst
It is possible to nest rcu_read_lock(), since reader-writer locks may
be recursively acquired. Note also that rcu_read_lock() is immune