Re: [PATCH 19/24] sched/eevdf: Fixup PELT vs DELAYED_DEQUEUE

Peter Zijlstra
2024-08-14T07:28:13Z

On Wed, Aug 14, 2024 at 09:25:48AM +0200, Peter Zijlstra wrote:
> On Wed, Aug 14, 2024 at 12:18:07AM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 13, 2024 at 02:43:56PM +0200, Valentin Schneider wrote:
> > > On 27/07/24 12:27, Peter Zijlstra wrote:
> > > > Note that tasks that are kept on the runqueue to burn off negative
> > > > lag, are not in fact runnable anymore, they'll get dequeued the moment
> > > > they get picked.
> > > >
> > > > As such, don't count this time towards runnable.
> > > >
> > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > > ---
> > > > kernel/sched/fair.c | 2 ++
> > > > kernel/sched/sched.h | 6 ++++++
> > > > 2 files changed, 8 insertions(+)
> > > >
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -5388,6 +5388,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
> > > > if (cfs_rq->next == se)
> > > > cfs_rq->next = NULL;
> > > > se->sched_delayed = 1;
> > > > + update_load_avg(cfs_rq, se, 0);
> > >
> > > Shouldn't this be before setting ->sched_delayed? accumulate_sum() should
> > > see the time delta as spent being runnable.
> > >
> > > > return false;
> > > > }
> > > > }
> > > > @@ -6814,6 +6815,7 @@ requeue_delayed_entity(struct sched_enti
> > > > }
> > > >
> > > > se->sched_delayed = 0;
> > > > + update_load_avg(cfs_rq, se, 0);
> > >
> > > Ditto on the ordering
> >
> > Bah, so I remember thinking about it and then I obviously go and do it
> > the exact wrong way around eh? Let me double check this tomorrow morning
> > with the brain slightly more awake :/
>
> OK, so I went over it again and I ended up with the below diff -- which
> assuming I didn't make a giant mess of things *again*, I should go fold
> back into various other patches ...
>
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1b15dbfb1ce5..fa8907f2c716 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5461,14 +5461,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> {
> bool sleep = flags & DEQUEUE_SLEEP;
>
> + update_curr(cfs_rq);
> +
> if (flags & DEQUEUE_DELAYED) {
> - /*
> - * DEQUEUE_DELAYED is typically called from pick_next_entity()
> - * at which point we've already done update_curr() and do not
> - * want to do so again.
> - */
> SCHED_WARN_ON(!se->sched_delayed);
> - se->sched_delayed = 0;
> } else {
> bool delay = sleep;
> /*

Because repeated update_curr() is harmless (I think I was thinking it
would move the clock, but it doesn't do that), but a missed
update_curr() makes a mess.

> @@ -5479,14 +5475,13 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> delay = false;
>
> SCHED_WARN_ON(delay && se->sched_delayed);
> - update_curr(cfs_rq);
>
> if (sched_feat(DELAY_DEQUEUE) && delay &&
> !entity_eligible(cfs_rq, se)) {
> if (cfs_rq->next == se)
> cfs_rq->next = NULL;
> - se->sched_delayed = 1;
> update_load_avg(cfs_rq, se, 0);
> + se->sched_delayed = 1;
> return false;
> }
> }

As you said, update to now, then mark delayed.

> @@ -5536,6 +5531,12 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
> update_min_vruntime(cfs_rq);
>
> + if (flags & DEQUEUE_DELAYED) {
> + se->sched_delayed = 0;
> + if (sched_feat(DELAY_ZERO) && se->vlag > 0)
> + se->vlag = 0;
> + }
> +
> if (cfs_rq->nr_running == 0)
> update_idle_cfs_rq_clock_pelt(cfs_rq);
>
> @@ -5611,11 +5612,6 @@ pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
> struct sched_entity *se = pick_eevdf(cfs_rq);
> if (se->sched_delayed) {
> dequeue_entities(rq, se, DEQUEUE_SLEEP | DEQUEUE_DELAYED);
> - SCHED_WARN_ON(se->sched_delayed);
> - SCHED_WARN_ON(se->on_rq);
> - if (sched_feat(DELAY_ZERO) && se->vlag > 0)
> - se->vlag = 0;
> -
> return NULL;
> }
> return se;

Because if we have to clear delayed at the end up dequeue, we might as
well do all of the fixups at that point.

> @@ -6906,7 +6902,7 @@ requeue_delayed_entity(struct sched_entity *se)
> struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
> /*
> - * se->sched_delayed should imply both: se->on_rq == 1.
> + * se->sched_delayed should imply: se->on_rq == 1.
> * Because a delayed entity is one that is still on
> * the runqueue competing until elegibility.
> */

edit fail, somewhere along history.

> @@ -6927,8 +6923,8 @@ requeue_delayed_entity(struct sched_entity *se)
> }
> }
>
> - se->sched_delayed = 0;
> update_load_avg(cfs_rq, se, 0);
> + se->sched_delayed = 0;
> }

What you said..