Messages in this thread Patch in this message | | | Date | Fri, 6 Mar 2020 13:01:31 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Pinning down a blocked task to extract diagnostics |
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On Fri, Mar 06, 2020 at 11:56:11AM -0500, Steven Rostedt wrote: > On Thu, 5 Mar 2020 17:40:27 -0800 > "Paul E. McKenney" <paulmck@kernel.org> wrote: > > > commit e2821ae6c6a6adaabc89ccd9babf4375a78e0626 > > Author: Paul E. McKenney <paulmck@kernel.org> > > Date: Thu Mar 5 16:53:58 2020 -0800 > > > > sched/core: Add functions to prevent sleepers from awakening > > > > In some cases, it is necessary to examine a consistent version of a > > sleeping process's state, in other words, it is necessary to keep > > that process in sleeping state. This commit therefore provides a > > try_to_keep_sleeping() function that acquires ->pi_lock to prevent > > wakeups from proceeding, returning true if the function is still asleep, > > and otherwise releasing ->pi_lock and returning false. > > > > This commit also provides an allow_awake() function (as suggested by > > by Steven Rostedt) that reverses the effect of a successful call to > > try_to_keep_sleeping(), allowing the process to once again be awakened. > > > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > > [ paulmck: Apply feedback from Peter Zijlstra and Steven Rostedt. ] > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Juri Lelli <juri.lelli@redhat.com> > > Cc: Vincent Guittot <vincent.guittot@linaro.org> > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> > > Cc: Steven Rostedt <rostedt@goodmis.org> > > Cc: Ben Segall <bsegall@google.com> > > Cc: Mel Gorman <mgorman@suse.de> > > > > diff --git a/include/linux/wait.h b/include/linux/wait.h > > index 3283c8d..aefea4a 100644 > > --- a/include/linux/wait.h > > +++ b/include/linux/wait.h > > @@ -1148,4 +1148,7 @@ int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, i > > (wait)->flags = 0; \ > > } while (0) > > > > +bool try_to_keep_sleeping(struct task_struct *p); > > +void allow_awake(struct task_struct *p); > > + > > #endif /* _LINUX_WAIT_H */ > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index fc1dfc0..b665ff7 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -2654,6 +2654,48 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > > } > > > > /** > > + * try_to_keep_sleeping - Attempt to force task to remain off runqueues > > + * @p: The process to remain asleep. > > + * > > + * Acquires the process's ->pi_lock and checks state. If the process > > + * is still blocked, returns @true and leave ->pi_lock held, otherwise > > + * releases ->pi_locked and returns @false. > > I would add a comment here that this is paired with allow_awake(). As well > as a "Returns" statement. > > * Returns: > * false if the task is awake, then no lock is taken. > * true if the task is sleeping, and then task's pi_lock will be held. > * allow_awake() must be used to release the pi_lock and let > * task @p awake again.
Good point, added.
> > + */ > > +bool try_to_keep_sleeping(struct task_struct *p) > > +{ > > + lockdep_assert_irqs_enabled(); > > + raw_spin_lock_irq(&p->pi_lock); > > + switch (p->state) { > > + case TASK_RUNNING: > > + case TASK_WAKING: > > + raw_spin_unlock_irq(&p->pi_lock); > > + return false; > > + > > + default: > > + smp_rmb(); /* See comments in try_to_wake_up(). */ > > I remember Peter asking to add a comment in try_to_wake_up() stating that > this is used, so that if that code is changed, this code may also need to > be updated.
Adding a reference to try_to_keep_sleeping() in try_to_wake_up()'s smp_rmb() comment, as shown below?
Thanx, Paul
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commit 57008d8adaa1e93acedea23e4858b9831e0dd075 Author: Paul E. McKenney <paulmck@kernel.org> Date: Thu Mar 5 16:53:58 2020 -0800
sched/core: Add functions to prevent sleepers from awakening In some cases, it is necessary to examine a consistent version of a sleeping process's state, in other words, it is necessary to keep that process in sleeping state. This commit therefore provides a try_to_keep_sleeping() function that acquires ->pi_lock to prevent wakeups from proceeding, returning true if the function is still asleep, and otherwise releasing ->pi_lock and returning false. This commit also provides an allow_awake() function (as suggested by by Steven Rostedt) that reverses the effect of a successful call to try_to_keep_sleeping(), allowing the process to once again be awakened. Signed-off-by: Paul E. McKenney <paulmck@kernel.org> [ paulmck: Apply feedback from Peter Zijlstra and Steven Rostedt. ] Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Juri Lelli <juri.lelli@redhat.com> Cc: Vincent Guittot <vincent.guittot@linaro.org> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Ben Segall <bsegall@google.com> Cc: Mel Gorman <mgorman@suse.de>
diff --git a/include/linux/wait.h b/include/linux/wait.h index 3283c8d..aefea4a 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -1148,4 +1148,7 @@ int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, i (wait)->flags = 0; \ } while (0) +bool try_to_keep_sleeping(struct task_struct *p); +void allow_awake(struct task_struct *p); + #endif /* _LINUX_WAIT_H */ diff --git a/kernel/sched/core.c b/kernel/sched/core.c index fc1dfc0..a935a3d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2580,6 +2580,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) * * Pairs with the LOCK+smp_mb__after_spinlock() on rq->lock in * __schedule(). See the comment for smp_mb__after_spinlock(). + * + * A similar smp_rmb() lives in try_to_keep_sleeping(). */ smp_rmb(); if (p->on_rq && ttwu_remote(p, wake_flags)) @@ -2654,6 +2656,54 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) } /** + * try_to_keep_sleeping - Attempt to force task to remain off runqueues + * @p: The process to remain asleep. + * + * Acquires the process's ->pi_lock and checks state. If the process + * is still blocked, returns @true and leave ->pi_lock held, otherwise + * releases ->pi_lock and returns @false. + * + * Returns: + * @false if the task is awake, in which case no lock is held. + * @true if the task is sleeping, in which case the process's + * ->pi_lock will be held. Use allow_awake() to release + * this lock and thus allow process @p to awaken. + */ +bool try_to_keep_sleeping(struct task_struct *p) +{ + lockdep_assert_irqs_enabled(); + raw_spin_lock_irq(&p->pi_lock); + switch (p->state) { + case TASK_RUNNING: + case TASK_WAKING: + raw_spin_unlock_irq(&p->pi_lock); + return false; + + default: + smp_rmb(); /* See comments in try_to_wake_up(). */ + if (p->on_rq) { + raw_spin_unlock_irq(&p->pi_lock); + return false; + } + return true; /* Process is now stuck in blocked state. */ + } + /* NOTREACHED */ +} + +/** + * allow_awake - Allow a kept-sleeping process to awaken + * @p: Process to be allowed to awaken. + * + * Given that @p was passed to an earlier call to try_to_keep_sleeping + * that returned @true, hence preventing @p from waking up, allow @p + * to once again be awakened. + */ +void allow_awake(struct task_struct *p) +{ + raw_spin_unlock_irq(&p->pi_lock); +} + +/** * wake_up_process - Wake up a specific process * @p: The process to be woken up. *
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