Messages in this thread Patch in this message | | | Date | Thu, 5 Mar 2020 17:40:27 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Pinning down a blocked task to extract diagnostics |
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On Thu, Mar 05, 2020 at 07:36:38AM -0800, Paul E. McKenney wrote: > On Thu, Mar 05, 2020 at 09:28:45AM -0500, Steven Rostedt wrote: > > On Thu, 5 Mar 2020 06:22:45 -0800 > > "Paul E. McKenney" <paulmck@kernel.org> wrote: > > > > > On Thu, Mar 05, 2020 at 09:13:37AM +0100, Peter Zijlstra wrote:
[ . . . ]
> > > How about if I add something like this, located right by try_to_wake_up()? > > > > > > bool try_to_keep_sleeping(struct task_struct *t) > > > { > > > raw_spin_lock_irq(&t->pi_lock); > > > switch (t->state) { > > > case TASK_RUNNING: > > > case TASK_WAKING: > > > raw_spin_unlock_irq(&t->pi_lock); > > > return false; > > > > > > default: > > > if (t->on_rq) { > > > > Somehow I think there still needs to be a read barrier before the test to > > on_rq. > > This is nowhere near a fastpath, so if there is uncertainty it gets > the smp_rmb(). Or an smp_load_acquire() on t->state. > > > > raw_spin_unlock_irq(&t->pi_lock); > > > return false; > > > } > > > > > > /* OK to extract consistent diagnostic information. */ > > > return true; > > > } > > > /* NOTREACHED */ > > > } > > > > > > Then a use might look like this: > > > > > > if (try_to_keep_sleeping(t)) > > > /* Extract consistent diagnostic information. */ > > > raw_spin_unlock_irq(&t->pi_lock); > > > > Perhaps we should have a allow_awake(t) to match it? > > > > allow_awake(t); > > > > Where we have: > > > > static inline allow_awake(struct task_struct *t) > > { > > raw_spin_unlock_irq(&t->pi_lock); > > } > > Makes sense to me!
So how about like this?
Thanx, Paul
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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. + */ +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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