linux-kernel - Re: [PATCH 9/9] sched,fair: Alternative sched

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Date:   Fri, 26 Mar 2021 19:30:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Josh Don <joshdon@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>, greg@...ah.com
Subject: Re: [PATCH 9/9] sched,fair: Alternative sched_slice()

On Fri, Mar 26, 2021 at 04:37:03PM +0100, Vincent Guittot wrote:
> On Fri, 26 Mar 2021 at 11:43, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > The current sched_slice() seems to have issues; there's two possible
> > things that could be improved:
> >
> >  - the 'nr_running' used for __sched_period() is daft when cgroups are
> >    considered. Using the RQ wide h_nr_running seems like a much more
> >    consistent number.
> >
> >  - (esp) cgroups can slice it real fine, which makes for easy
> >    over-scheduling, ensure min_gran is what the name says.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/sched/fair.c     |   15 ++++++++++++++-
> >  kernel/sched/features.h |    3 +++
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> >
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -680,7 +680,16 @@ static u64 __sched_period(unsigned long
> >   */
> >  static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  {
> > -       u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
> > +       unsigned int nr_running = cfs_rq->nr_running;
> > +       u64 slice;
> > +
> > +       if (sched_feat(ALT_PERIOD))
> > +               nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
> > +
> > +       slice = __sched_period(nr_running + !se->on_rq);
> > +
> > +       if (sched_feat(BASE_SLICE))
> > +               slice -= sysctl_sched_min_granularity;
> >
> >         for_each_sched_entity(se) {
> >                 struct load_weight *load;
> > @@ -697,6 +706,10 @@ static u64 sched_slice(struct cfs_rq *cf
> >                 }
> >                 slice = __calc_delta(slice, se->load.weight, load);
> >         }
> > +
> > +       if (sched_feat(BASE_SLICE))
> > +               slice += sysctl_sched_min_granularity;
> 
> Why not only doing a max of slice and sysctl_sched_min_granularity
> instead of scaling only the part above sysctl_sched_min_granularity ?
> 
> With your change, cases where the slices would have been in a good
> range already, will be modified as well

Can do I suppose. Not sure how I ended up with this.