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Documentation: PM: Adjust freezing-of-tasks.rst to the freezer changes

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The core freezer logic has been modified by commit f5d39b0208
("freezer,sched: Rewrite core freezer logic"), so adjust the
documentation to reflect the new code. The main changes include:

 - Drop references to PF_FROZEN and PF_FREEZER_SKIP
 - Describe TASK_FROZEN, TASK_FREEZABLE and __TASK_FREEZABLE_UNSAFE
 - Replace system_freezing_cnt with freezer_active
 - Use a different example for the loop of a freezable kernel thread,
   since the old code is gone gone

Signed-off-by: Kevin Hao <haokexin@gmail.com>
[ rjw: Subject and changelog edits, doc text adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Kevin Hao 2023-12-12 22:00:43 +08:00 committed by Rafael J. Wysocki
parent 489c693bd0
commit 4bbf0b6a64
1 changed files with 46 additions and 35 deletions
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81
Documentation/power/freezing-of-tasks.rst
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@ -14,27 +14,28 @@ architectures).
II. How does it work?
=====================
There are three per-task flags used for that, PF_NOFREEZE, PF_FROZEN
and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have
PF_NOFREEZE unset (all user space processes and some kernel threads) are
regarded as 'freezable' and treated in a special way before the system enters a
suspend state as well as before a hibernation image is created (in what follows
we only consider hibernation, but the description also applies to suspend).
There is one per-task flag (PF_NOFREEZE) and three per-task states
(TASK_FROZEN, TASK_FREEZABLE and __TASK_FREEZABLE_UNSAFE) used for that.
The tasks that have PF_NOFREEZE unset (all user space tasks and some kernel
threads) are regarded as 'freezable' and treated in a special way before the
system enters a sleep state as well as before a hibernation image is created
(hibernation is directly covered by what follows, but the description applies
to system-wide suspend too).
Namely, as the first step of the hibernation procedure the function
freeze_processes() (defined in kernel/power/process.c) is called. A system-wide
variable system_freezing_cnt (as opposed to a per-task flag) is used to indicate
whether the system is to undergo a freezing operation. And freeze_processes()
sets this variable. After this, it executes try_to_freeze_tasks() that sends a
fake signal to all user space processes, and wakes up all the kernel threads.
All freezable tasks must react to that by calling try_to_freeze(), which
results in a call to __refrigerator() (defined in kernel/freezer.c), which sets
the task's PF_FROZEN flag, changes its state to TASK_UNINTERRUPTIBLE and makes
it loop until PF_FROZEN is cleared for it. Then, we say that the task is
'frozen' and therefore the set of functions handling this mechanism is referred
to as 'the freezer' (these functions are defined in kernel/power/process.c,
kernel/freezer.c & include/linux/freezer.h). User space processes are generally
frozen before kernel threads.
static key freezer_active (as opposed to a per-task flag or state) is used to
indicate whether the system is to undergo a freezing operation. And
freeze_processes() sets this static key. After this, it executes
try_to_freeze_tasks() that sends a fake signal to all user space processes, and
wakes up all the kernel threads. All freezable tasks must react to that by
calling try_to_freeze(), which results in a call to __refrigerator() (defined
in kernel/freezer.c), which changes the task's state to TASK_FROZEN, and makes
it loop until it is woken by an explicit TASK_FROZEN wakeup. Then, that task
is regarded as 'frozen' and so the set of functions handling this mechanism is
referred to as 'the freezer' (these functions are defined in
kernel/power/process.c, kernel/freezer.c & include/linux/freezer.h). User space
tasks are generally frozen before kernel threads.
__refrigerator() must not be called directly. Instead, use the
try_to_freeze() function (defined in include/linux/freezer.h), that checks
@ -43,31 +44,40 @@ if the task is to be frozen and makes the task enter __refrigerator().
For user space processes try_to_freeze() is called automatically from the
signal-handling code, but the freezable kernel threads need to call it
explicitly in suitable places or use the wait_event_freezable() or
wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
that combine interruptible sleep with checking if the task is to be frozen and
calling try_to_freeze(). The main loop of a freezable kernel thread may look
wait_event_freezable_timeout() macros (defined in include/linux/wait.h)
that put the task to sleep (TASK_INTERRUPTIBLE) or freeze it (TASK_FROZEN) if
freezer_active is set. The main loop of a freezable kernel thread may look
like the following one::
set_freezable();
do {
hub_events();
wait_event_freezable(khubd_wait,
!list_empty(&hub_event_list) ||
kthread_should_stop());
} while (!kthread_should_stop() || !list_empty(&hub_event_list));
(from drivers/usb/core/hub.c::hub_thread()).
while (true) {
struct task_struct *tsk = NULL;
If a freezable kernel thread fails to call try_to_freeze() after the freezer has
initiated a freezing operation, the freezing of tasks will fail and the entire
hibernation operation will be cancelled. For this reason, freezable kernel
threads must call try_to_freeze() somewhere or use one of the
wait_event_freezable(oom_reaper_wait, oom_reaper_list != NULL);
spin_lock_irq(&oom_reaper_lock);
if (oom_reaper_list != NULL) {
tsk = oom_reaper_list;
oom_reaper_list = tsk->oom_reaper_list;
}
spin_unlock_irq(&oom_reaper_lock);
if (tsk)
oom_reap_task(tsk);
}
(from mm/oom_kill.c::oom_reaper()).
If a freezable kernel thread is not put to the frozen state after the freezer
has initiated a freezing operation, the freezing of tasks will fail and the
entire system-wide transition will be cancelled. For this reason, freezable
kernel threads must call try_to_freeze() somewhere or use one of the
wait_event_freezable() and wait_event_freezable_timeout() macros.
After the system memory state has been restored from a hibernation image and
devices have been reinitialized, the function thaw_processes() is called in
order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that
have been frozen leave __refrigerator() and continue running.
order to wake up each frozen task. Then, the tasks that have been frozen leave
__refrigerator() and continue running.
Rationale behind the functions dealing with freezing and thawing of tasks
@ -96,7 +106,8 @@ III. Which kernel threads are freezable?
Kernel threads are not freezable by default. However, a kernel thread may clear
PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
directly is not allowed). From this point it is regarded as freezable
and must call try_to_freeze() in a suitable place.
and must call try_to_freeze() or variants of wait_event_freezable() in a
suitable place.
IV. Why do we do that?
======================