Re: [PATCH v2] freezer,sched: Use saved_state to reduce some spurious wakeups

[Peter Zijlstra]

On Mon, Sep 04, 2023 at 08:59:03PM -0700, Elliot Berman wrote:
> 
> 
> On 9/4/2023 2:23 PM, Peter Zijlstra wrote:
> > On Wed, Aug 30, 2023 at 10:42:39AM -0700, Elliot Berman wrote:
> > 
> > > Avoid the spurious wakeups by saving the state of TASK_FREEZABLE tasks.
> > > If the task was running before entering TASK_FROZEN state
> > > (__refrigerator()) or if the task received a wake up for the saved
> > > state, then the task is woken on thaw. saved_state from PREEMPT_RT locks
> > > can be re-used because freezer would not stomp on the rtlock wait flow:
> > > TASK_RTLOCK_WAIT isn't considered freezable.
> > 
> > You don't actually assert that anywhere I think, so the moment someone
> > makes that happen you crash and burn.
> > 
> 
> I can certainly add an assertion on the freezer side.

I think the assertion we have in ttwu_state_match() might be sufficient.

> > Also:
> > 
> > > -#ifdef CONFIG_PREEMPT_RT
> > > +#if IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_FREEZER)
> > 
> > That makes wakeup more horrible for everyone :/
> 
> I don't think the hot wakeup path is significantly impacted because the
> added checks come after the hot path is already not taken.

Perhaps we should start off by doing the below, instead of making it
more complicated instead. I suppose you're right about the overhead, but
run a hackbench just to make sure or something.


diff --git a/include/linux/sched.h b/include/linux/sched.h
index 77f01ac385f7..649ddb9adf0d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -749,11 +749,7 @@ struct task_struct {
 	struct thread_info		thread_info;
 #endif
 	unsigned int			__state;
-
-#ifdef CONFIG_PREEMPT_RT
-	/* saved state for "spinlock sleepers" */
 	unsigned int			saved_state;
-#endif
 
 	/*
 	 * This begins the randomizable portion of task_struct. Only
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2299a5cfbfb9..b566821614e1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2239,31 +2239,21 @@ int __task_state_match(struct task_struct *p, unsigned int state)
 	if (READ_ONCE(p->__state) & state)
 		return 1;
 
-#ifdef CONFIG_PREEMPT_RT
 	if (READ_ONCE(p->saved_state) & state)
 		return -1;
-#endif
+
 	return 0;
 }
 
 static __always_inline
 int task_state_match(struct task_struct *p, unsigned int state)
 {
-#ifdef CONFIG_PREEMPT_RT
-	int match;
-
 	/*
 	 * Serialize against current_save_and_set_rtlock_wait_state() and
 	 * current_restore_rtlock_saved_state().
 	 */
-	raw_spin_lock_irq(&p->pi_lock);
-	match = __task_state_match(p, state);
-	raw_spin_unlock_irq(&p->pi_lock);
-
-	return match;
-#else
+	guard(spin_lock_irq)(&p->pi_lock);
 	return __task_state_match(p, state);
-#endif
 }
 
 /*
@@ -4056,7 +4046,6 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
 
 	*success = !!(match = __task_state_match(p, state));
 
-#ifdef CONFIG_PREEMPT_RT
 	/*
 	 * Saved state preserves the task state across blocking on
 	 * an RT lock.  If the state matches, set p::saved_state to
@@ -4072,7 +4061,7 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
 	 */
 	if (match < 0)
 		p->saved_state = TASK_RUNNING;
-#endif
+
 	return match > 0;
 }