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doc: Update Tasks RCU and Tasks Rude RCU description in Requirements.rst

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This commit adds more detail to the Tasks RCU and Tasks Rude RCU
descriptions in Requirements.rst.  While in the area, add Tasks Trace
RCU to the Tasks-RCU table of contents.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
This commit is contained in:
Paul E. McKenney 2024-04-05 11:00:45 -07:00
parent 1613e604df
commit 293d901348
1 changed files with 16 additions and 0 deletions
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16
Documentation/RCU/Design/Requirements/Requirements.rst
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@ -2357,6 +2357,7 @@ section.
#. `Sched Flavor (Historical)`_
#. `Sleepable RCU`_
#. `Tasks RCU`_
#. `Tasks Trace RCU`_
Bottom-Half Flavor (Historical)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -2610,6 +2611,16 @@ critical sections that are delimited by voluntary context switches, that
is, calls to schedule(), cond_resched(), and
synchronize_rcu_tasks(). In addition, transitions to and from
userspace execution also delimit tasks-RCU read-side critical sections.
Idle tasks are ignored by Tasks RCU, and Tasks Rude RCU may be used to
interact with them.
Note well that involuntary context switches are *not* Tasks-RCU quiescent
states. After all, in preemptible kernels, a task executing code in a
trampoline might be preempted. In this case, the Tasks-RCU grace period
clearly cannot end until that task resumes and its execution leaves that
trampoline. This means, among other things, that cond_resched() does
not provide a Tasks RCU quiescent state. (Instead, use rcu_softirq_qs()
from softirq or rcu_tasks_classic_qs() otherwise.)
The tasks-RCU API is quite compact, consisting only of
call_rcu_tasks(), synchronize_rcu_tasks(), and
@ -2632,6 +2643,11 @@ moniker. And this operation is considered to be quite rude by real-time
workloads that don't want their ``nohz_full`` CPUs receiving IPIs and
by battery-powered systems that don't want their idle CPUs to be awakened.
Once kernel entry/exit and deep-idle functions have been properly tagged
``noinstr``, Tasks RCU can start paying attention to idle tasks (except
those that are idle from RCU's perspective) and then Tasks Rude RCU can
be removed from the kernel.
The tasks-rude-RCU API is also reader-marking-free and thus quite compact,
consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(),
and rcu_barrier_tasks_rude().