Re: [PATCH 12/24] sched/fair: Prepare exit/cleanup paths for delayed_dequeue

Peter Zijlstra
2024-08-14T05:53:44Z

On Wed, Aug 14, 2024 at 12:07:57AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 13, 2024 at 11:54:21PM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 13, 2024 at 02:43:47PM +0200, Valentin Schneider wrote:
> > > On 27/07/24 12:27, Peter Zijlstra wrote:
> > > > @@ -12817,10 +12830,26 @@ static void attach_task_cfs_rq(struct ta
> > > > static void switched_from_fair(struct rq *rq, struct task_struct *p)
> > > > {
> > > > detach_task_cfs_rq(p);
> > > > + /*
> > > > + * Since this is called after changing class, this isn't quite right.
> > > > + * Specifically, this causes the task to get queued in the target class
> > > > + * and experience a 'spurious' wakeup.
> > > > + *
> > > > + * However, since 'spurious' wakeups are harmless, this shouldn't be a
> > > > + * problem.
> > > > + */
> > > > + p->se.sched_delayed = 0;
> > > > + /*
> > > > + * While here, also clear the vlag, it makes little sense to carry that
> > > > + * over the excursion into the new class.
> > > > + */
> > > > + p->se.vlag = 0;
> > >
> > > RQ lock is held, the task can't be current if it's ->sched_delayed; is a
> > > dequeue_task() not possible at this point? Or just not worth it?
> >
> > Hurmph, I really can't remember why I did it like this :-(
>
> Obviously I remember it right after hitting send...
>
> We've just done:
>
> dequeue_task();
> p->sched_class = some_other_class;
> enqueue_task();
>
> IOW, we're enqueued as some other class at this point. There is no way
> we can fix it up at this point.

With just a little more sleep than last night, perhaps you're right
after all. Yes we're on a different class, but we can *still* dequeue it
again.

That is, something like the below ... I'll stick it on and see if
anything falls over.

---
kernel/sched/fair.c | 22 +++++++++-------------
1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 714826d97ef2..53c8f3ccfd0c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13105,20 +13105,16 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
{
detach_task_cfs_rq(p);
/*
- * Since this is called after changing class, this isn't quite right.
- * Specifically, this causes the task to get queued in the target class
- * and experience a 'spurious' wakeup.
- *
- * However, since 'spurious' wakeups are harmless, this shouldn't be a
- * problem.
- */
- p->se.sched_delayed = 0;
- /*
- * While here, also clear the vlag, it makes little sense to carry that
- * over the excursion into the new class.
+ * Since this is called after changing class, this is a little weird
+ * and we cannot use DEQUEUE_DELAYED.
*/
- p->se.vlag = 0;
- p->se.rel_deadline = 0;
+ if (p->se.sched_delayed) {
+ dequeue_task(DEQUEUE_NOCLOCK, DEQUEUE_SLEEP);
+ p->se.sched_delayed = 0;
+ p->se.rel_deadline = 0;
+ if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
+ p->se.vlag = 0;
+ }
}

static void switched_to_fair(struct rq *rq, struct task_struct *p)