linux-kernel - Re: [PATCH 07/10] sched,cfs: fix zero length timeslice calculation

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Date:   Fri, 05 Jul 2019 11:15:01 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>, Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 07/10] sched,cfs: fix zero length timeslice calculation

On Fri, 2019-07-05 at 16:51 +0200, Vincent Guittot wrote:
> On Fri, 28 Jun 2019 at 22:49, Rik van Riel <riel@...riel.com> wrote:
> > The way the time slice length is currently calculated, not only do
> > high
> > priority tasks get longer time slices than low priority tasks, but
> > due
> > to fixed point math, low priority tasks could end up with a zero
> > length
> > time slice. This can lead to cache thrashing and other
> > inefficiencies.
> > 
> > Simplify the logic a little bit, and cap the minimum time slice
> > length
> > to sysctl_sched_min_granularity.
> > 
> > Tasks that end up getting a time slice length too long for their
> > relative
> > priority will simply end up having their vruntime advanced much
> > faster than
> > other tasks, resulting in them receiving time slices less
> > frequently.
> > 
> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > ---
> >  kernel/sched/fair.c | 25 ++++++++-----------------
> >  1 file changed, 8 insertions(+), 17 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d48bff5118fc..8da2823401ca 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -671,22 +671,6 @@ static inline u64 calc_delta_fair(u64 delta,
> > struct sched_entity *se)
> >         return delta;
> >  }
> > 
> > -/*
> > - * The idea is to set a period in which each task runs once.
> > - *
> > - * When there are too many tasks (sched_nr_latency) we have to
> > stretch
> > - * this period because otherwise the slices get too small.
> > - *
> > - * p = (nr <= nl) ? l : l*nr/nl
> > - */
> > -static u64 __sched_period(unsigned long nr_running)
> > -{
> > -       if (unlikely(nr_running > sched_nr_latency))
> > -               return nr_running * sysctl_sched_min_granularity;
> > -       else
> > -               return sysctl_sched_latency;
> > -}
> > -
> >  /*
> >   * We calculate the wall-time slice from the period by taking a
> > part
> >   * proportional to the weight.
> > @@ -695,7 +679,7 @@ static u64 __sched_period(unsigned long
> > nr_running)
> >   */
> >  static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity
> > *se)
> >  {
> > -       u64 slice = __sched_period(cfs_rq->nr_running + !se-
> > >on_rq);
> > +       u64 slice = sysctl_sched_latency;
> 
> Is the change above and the remove of __sched_period() really needed
> for fixing the null time slice problem ?
> This change impacts how tasks will preempt each other and as a result
> the throughput. It should have it dedicated patch so we can evaluate
> its impact

Good point. I will split this up into two patches for v3.

Thank you.

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