linux-kernel - Re: [PATCH 6/7 v3] sched/fair: trigger the update of blocked load on newly idle cpu

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Date:   Mon, 15 Feb 2021 16:02:12 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Qais Yousef <qais.yousef@....com>
Subject: Re: [PATCH 6/7 v3] sched/fair: trigger the update of blocked load on
 newly idle cpu

On Fri, 12 Feb 2021 at 20:19, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 12/02/21 15:17, Vincent Guittot wrote:
> > Instead of waking up a random and already idle CPU, we can take advantage
> > of this_cpu being about to enter idle to run the ILB and update the
> > blocked load.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c  | 24 +++++++++++++++++++++---
> >  kernel/sched/idle.c  |  6 ++++++
> >  kernel/sched/sched.h |  5 +++++
> >  3 files changed, 32 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5d285d93e433..cd0ea635225e 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10453,6 +10453,24 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> >       return true;
> >  }
> >
> > +/*
> > + * Check if we need to run the ILB for updating blocked load before entering
> > + * idle state.
> > + */
> > +void nohz_run_idle_balance(int cpu)
> > +{
> > +     unsigned int flags;
> > +
> > +     flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
> > +
> > +     if (flags && !need_resched()) {
> > +             struct rq *rq = cpu_rq(cpu);
> > +
> > +             rq->nohz_idle_balance = flags;
> > +             nohz_idle_balance(rq, CPU_IDLE);
> > +     }
>
> So this can now run a full fledged nohz_idle_balance() if NOHZ_BALANCE_MASK
> is set.

Yes.
>
> I don't think there is anything inherently wrong with it - the
> nohz_idle_balance() call resulting from the kick_ilb() IPI will just bail
> out due to the flags being cleared here. This wasn't immediately clear to
> me however.

In fact, I forgot to replace the WARN_ON in nohz_csd_func() by a
simple return as reported by kernel test robot / oliver.sang@...el.com

>
> > +}
> > +