Re: [PATCH] sched/fair: Dequeue sched_delayed tasks when waking to a busy CPU

Mike Galbraith
2024-11-12T07:05:51Z

On Fri, 2024-11-08 at 01:24 +0100, Mike Galbraith wrote:
> On Thu, 2024-11-07 at 15:09 +0100, Peter Zijlstra wrote:
> > On Thu, Nov 07, 2024 at 03:02:36PM +0100, Mike Galbraith wrote:
> > > On Thu, 2024-11-07 at 10:46 +0100, Mike Galbraith wrote:
> > > > On Thu, 2024-11-07 at 05:03 +0100, Mike Galbraith wrote:
> > > > >
> > > > > I built that patch out of curiosity, and yeah, set_next_task_fair()
> > > > > finding a cfs_rq->curr ends play time pretty quickly.
> > > >
> > > > The below improved uptime, and trace_printk() says it's doing the
> > > > intended, so I suppose I'll add a feature and see what falls out.
> > >
> > > From netperf, I got.. number tabulation practice.  Three runs of each
> > > test with and without produced nothing but variance/noise.
> >
> > Make it go away then.
> >
> > If you could write a Changelog for you inspired bit and stick my cleaned
> > up version under it, I'd be much obliged.
>
> Salut, much obliged for eyeball relief.

Unfortunate change log place holder below aside, I think this patch may
need to be yanked as trading one not readily repeatable regression for
at least one that definitely is, and likely multiple others.

(adds knob)

tbench 8

NO_MIGRATE_DELAYED 3613.49 MB/sec
MIGRATE_DELAYED 3145.59 MB/sec
NO_DELAY_DEQUEUE 3355.42 MB/sec

First line is DELAY_DEQUEUE restoring pre-EEVDF tbench throughput as
I've mentioned it doing, but $subject promptly did away with that and
then some.

I thought I might be able to do away with the reservation like side
effect of DELAY_DEQUEUE by borrowing h_nr_delayed from...

sched/eevdf: More PELT vs DELAYED_DEQUEUE

...for cgroups free test config, but Q/D poke at idle_cpu() helped not
at all.

> ---snip---
>
> Phil Auld (Redhat) reported an fio benchmark regression having been found
> to have been caused by addition of the DELAY_DEQUEUE feature, suggested it
> may be related to wakees losing the ability to migrate, and confirmed that
> restoration of same indeed did restore previous performance.
>
> (de-uglified-a-lot-by)
>
> Reported-by: Phil Auld <pauld@redhat.com>
> Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
> Link: https://lore.kernel.org/lkml/20241101124715.GA689589@pauld.westford.csb/
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> ---
>  kernel/sched/core.c  |   48 +++++++++++++++++++++++++++++-------------------
>  kernel/sched/sched.h |    5 +++++
>  2 files changed, 34 insertions(+), 19 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3783,28 +3783,38 @@ ttwu_do_activate(struct rq *rq, struct t
>   */
>  static int ttwu_runnable(struct task_struct *p, int wake_flags)
>  {
> -       struct rq_flags rf;
> -       struct rq *rq;
> -       int ret = 0;
> -
> -       rq = __task_rq_lock(p, &rf);
> -       if (task_on_rq_queued(p)) {
> -               update_rq_clock(rq);
> -               if (p->se.sched_delayed)
> -                       enqueue_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_DELAYED);
> -               if (!task_on_cpu(rq, p)) {
> -                       /*
> -                        * When on_rq && !on_cpu the task is preempted, see if
> -                        * it should preempt the task that is current now.
> -                        */
> -                       wakeup_preempt(rq, p, wake_flags);
> +       CLASS(__task_rq_lock, rq_guard)(p);
> +       struct rq *rq = rq_guard.rq;
> +
> +       if (!task_on_rq_queued(p))
> +               return 0;
> +
> +       update_rq_clock(rq);
> +       if (p->se.sched_delayed) {
> +               int queue_flags = ENQUEUE_DELAYED | ENQUEUE_NOCLOCK;
> +
> +               /*
> +                * Since sched_delayed means we cannot be current anywhere,
> +                * dequeue it here and have it fall through to the
> +                * select_task_rq() case further along the ttwu() path.
> +                */
> +               if (rq->nr_running > 1 && p->nr_cpus_allowed > 1) {
> +                       dequeue_task(rq, p, DEQUEUE_SLEEP | queue_flags);
> +                       return 0;
>                 }
> -               ttwu_do_wakeup(p);
> -               ret = 1;
> +
> +               enqueue_task(rq, p, queue_flags);
> +       }
> +       if (!task_on_cpu(rq, p)) {
> +               /*
> +                * When on_rq && !on_cpu the task is preempted, see if
> +                * it should preempt the task that is current now.
> +                */
> +               wakeup_preempt(rq, p, wake_flags);
>         }
> -       __task_rq_unlock(rq, &rf);
> +       ttwu_do_wakeup(p);
>
> -       return ret;
> +       return 1;
>  }
>
>  #ifdef CONFIG_SMP
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1779,6 +1779,11 @@ task_rq_unlock(struct rq *rq, struct tas
>         raw_spin_unlock_irqrestore(&p->pi_lock, rf->flags);
>  }
>
> +DEFINE_LOCK_GUARD_1(__task_rq_lock, struct task_struct,
> +                   _T->rq = __task_rq_lock(_T->lock, &_T->rf),
> +                   __task_rq_unlock(_T->rq, &_T->rf),
> +                   struct rq *rq; struct rq_flags rf)
> +
>  DEFINE_LOCK_GUARD_1(task_rq_lock, struct task_struct,
>                     _T->rq = task_rq_lock(_T->lock, &_T->rf),
>                     task_rq_unlock(_T->rq, _T->lock, &_T->rf),
>
>