Re: Pinning down a blocked task to extract diagnostics

["Paul E. McKenney" <paulmck@kernel.org>]

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

------------------------------------------------------------------------

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.
  *