linux-kernel - Re: [PATCH] sched/fair: Rate limit calls to update_blocked

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Date:   Mon, 25 Jan 2021 14:23:51 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Dietmar Eggeman <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages()
 for NOHZ

On Fri, 22 Jan 2021 at 19:39, Qais Yousef <qais.yousef@....com> wrote:
>
> On 01/22/21 17:56, Vincent Guittot wrote:
> > > ---
> > >  kernel/sched/fair.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 04a3ce20da67..fe2dc0024db5 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -8381,7 +8381,7 @@ static bool update_nohz_stats(struct rq *rq, bool force)
> > >         if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
> > >                 return false;
> > >
> > > -       if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick))
> > > +       if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick + (HZ/20)))
> >
> > This condition is there to make sure to update blocked load at most
> > once a tick in order to filter newly idle case otherwise the rate
> > limit is already done by load balance interval
> > This hard coded (HZ/20) looks really like an ugly hack
>
> This was meant as an RFC patch to discuss the problem really.
>
> Joel is seeing update_blocked_averages() taking ~100us. Half of it seems in
> processing __update_blocked_fair() and the other half in sugov_update_shared().
> So roughly 50us each. Note that each function is calling an iterator in

Can I assume that a freq change happens if sugov_update_shared() takes 50us ?
which would mean that the update was useful at the end ?

> return. Correct me if my numbers are wrong Joel.
>
> Running on a little core on low frequency these numbers don't look too odd.
> So I'm not seeing how we can speed these functions up.
>
> But since update_sg_lb_stats() will end up with multiple calls to
> update_blocked_averages() in one go, this latency adds up quickly.
>
> One noticeable factor in Joel's system is the presence of a lot of cgroups.
> Which is essentially what makes __update_blocked_fair() expensive, and it seems
> to always return something has decayed so we end up with a call to
> sugov_update_shared() in every call.
>
> I think we should limit the expensive call to update_blocked_averages() but

At the opposite, some will complain that block values  stay stall to
high value and prevent any useful adjustment.

Also update_blocked average is already rate limited with idle and busy
load_balance

Seems like the problem raised by Joel is the number of newly idle load balance

> I honestly don't know what would be the right way to do it :-/
>
> Or maybe there's another better alternative too..
>
> Thanks
>
> --
> Qais Yousef
>
> >
> > >                 return true;
> > >
> > >         update_blocked_averages(cpu);
> > > --
> > > 2.30.0.280.ga3ce27912f-goog
> > >