linux-kernel - Re: [PATCH 19/24] sched/eevdf: Fixup PELT vs DELAYED

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Message-ID: <CAKfTPtBD3iyR3XSssQDAU=vkPs70tZLukvbwPYpnSv63ra_-Ew@mail.gmail.com>
Date: Wed, 14 Aug 2024 14:59:00 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Valentin Schneider <vschneid@...hat.com>, mingo@...hat.com, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, linux-kernel@...r.kernel.org, kprateek.nayak@....com, 
	wuyun.abel@...edance.com, youssefesmat@...omium.org, tglx@...utronix.de, 
	efault@....de
Subject: Re: [PATCH 19/24] sched/eevdf: Fixup PELT vs DELAYED_DEQUEUE

On Wed, 14 Aug 2024 at 00:18, Peter Zijlstra <peterz@...radead.org> 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@...radead.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 :/
>
> > >  }
> > >
> > >  /*
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -816,6 +816,9 @@ static inline void se_update_runnable(st
> > >
> > >  static inline long se_runnable(struct sched_entity *se)
> > >  {
> > > +   if (se->sched_delayed)
> > > +           return false;
> > > +
> >
> > Per __update_load_avg_se(), delayed-dequeue entities are still ->on_rq, so
> > their load signal will increase. Do we want a similar helper for the @load
> > input of ___update_load_sum()?
>
> So the whole reason to keep then enqueued is so that they can continue
> to compete for vruntime, and vruntime is load based. So it would be very
> weird to remove them from load.

We only use the weight to update vruntime, not the load. The load is
used to balance tasks between cpus and if we keep a "delayed" dequeued
task in the load, we will artificially inflate the load_avg on this rq

Shouldn't we track separately the sum of the weight of delayed dequeue
to apply it only on vruntime update ?
>