Re: [PATCH v7 16/22] sched: Defer wakeup in ttwu() for unschedulable frozen tasks

[Peter Zijlstra]

On Thu, May 27, 2021 at 04:10:16PM +0200, Peter Zijlstra wrote:
> On Tue, May 25, 2021 at 04:14:26PM +0100, Will Deacon wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 42e2aecf087c..6cb9677d635a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3529,6 +3529,19 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  	if (!(p->state & state))
> >  		goto unlock;
> >  
> > +#ifdef CONFIG_FREEZER
> > +	/*
> > +	 * If we're going to wake up a thread which may be frozen, then
> > +	 * we can only do so if we have an active CPU which is capable of
> > +	 * running it. This may not be the case when resuming from suspend,
> > +	 * as the secondary CPUs may not yet be back online. See __thaw_task()
> > +	 * for the actual wakeup.
> > +	 */
> > +	if (unlikely(frozen_or_skipped(p)) &&
> > +	    !cpumask_intersects(cpu_active_mask, task_cpu_possible_mask(p)))
> > +		goto unlock;
> > +#endif
> > +
> >  	trace_sched_waking(p);
> >  
> >  	/* We're going to change ->state: */
> 
> OK, I really hate this. This is slowing down the very hot wakeup path
> for the silly freezer that *never* happens. Let me try and figure out if
> there's another option.


How's something *completely* untested like this?

---
diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 0621c5f86c39..44ece41c3db3 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -24,7 +24,7 @@ extern unsigned int freeze_timeout_msecs;
  */
 static inline bool frozen(struct task_struct *p)
 {
-	return p->flags & PF_FROZEN;
+	return p->state == TASK_FROZEN;
 }
 
 extern bool freezing_slow_path(struct task_struct *p);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2982cfab1ae9..7e7775c5b742 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -95,7 +95,8 @@ struct task_group;
 #define TASK_WAKING			0x0200
 #define TASK_NOLOAD			0x0400
 #define TASK_NEW			0x0800
-#define TASK_STATE_MAX			0x1000
+#define TASK_FROZEN			0x1000
+#define TASK_STATE_MAX			0x2000
 
 /* Convenience macros for the sake of set_current_state: */
 #define TASK_KILLABLE			(TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
@@ -1579,7 +1580,6 @@ extern struct pid *cad_pid;
 #define PF_USED_MATH		0x00002000	/* If unset the fpu must be initialized before use */
 #define PF_USED_ASYNC		0x00004000	/* Used async_schedule*(), used by module init */
 #define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */
-#define PF_FROZEN		0x00010000	/* Frozen for system suspend */
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
diff --git a/kernel/freezer.c b/kernel/freezer.c
index dc520f01f99d..be6c86078510 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -63,18 +63,13 @@ bool __refrigerator(bool check_kthr_stop)
 	pr_debug("%s entered refrigerator\n", current->comm);
 
 	for (;;) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
-
 		spin_lock_irq(&freezer_lock);
-		current->flags |= PF_FROZEN;
-		if (!freezing(current) ||
-		    (check_kthr_stop && kthread_should_stop()))
-			current->flags &= ~PF_FROZEN;
+		if (freezing(current) && (check_kthr_stop && kthread_should_stop())) {
+			set_current_state(TASK_FROZEN);
+			was_frozen = true;
+		}
 		spin_unlock_irq(&freezer_lock);
 
-		if (!(current->flags & PF_FROZEN))
-			break;
-		was_frozen = true;
 		schedule();
 	}
 
@@ -149,7 +144,7 @@ void __thaw_task(struct task_struct *p)
 
 	spin_lock_irqsave(&freezer_lock, flags);
 	if (frozen(p))
-		wake_up_process(p);
+		wake_up_state(p, TASK_FROZEN);
 	spin_unlock_irqrestore(&freezer_lock, flags);
 }
 
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 396ebaebea3f..71a6509f8d4f 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -92,8 +92,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 	 * Ensure the task is not frozen.
 	 * Also, skip vfork and any other user process that freezer should skip.
 	 */
-	if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
-	    return;
+	if (unlikely((t->flags & PF_FREEZER_SKIP) || frozen(t)))
+		return;
 
 	/*
 	 * When a freshly created task is scheduled once, changes its state to
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 401f012349d1..1eedf6a044f3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5889,7 +5889,7 @@ static void __sched notrace __schedule(bool preempt)
 			prev->sched_contributes_to_load =
 				(prev_state & TASK_UNINTERRUPTIBLE) &&
 				!(prev_state & TASK_NOLOAD) &&
-				!(prev->flags & PF_FROZEN);
+				!(prev_state & TASK_FROZEN);
 
 			if (prev->sched_contributes_to_load)
 				rq->nr_uninterruptible++;