linux-kernel - Re: [PATCH 2/6] sched/fair: remove unused parameter of update_nohz

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Date:   Tue, 9 Feb 2021 18:38:26 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Qais Yousef <qais.yousef@....com>
Subject: Re: [PATCH 2/6] sched/fair: remove unused parameter of update_nohz_stats

On Tue, 9 Feb 2021 at 14:45, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 05/02/2021 12:48, Vincent Guittot wrote:
> > idle load balance is the only user of update_nohz_stats and doesn't use
> > force parameter. Remove it
>
> Wasn't the 'force=true' from ilb eclipsing the jiffy resolution rate
> limiting '!time_after(jiffies, rq->last_blocked_load_update_tick)' of
> update_blocked_averages()?
>
> So IMHO this has the (maybe intended) side effect that (formerly forced
> updates) are now rate limited on one jiffy resolution too.

Calls to _nohz_idle_balance were already rate limited by load balance
interval and nohz.next_blocked.
This tick rate limit has been originally added for newidle_balance
case but there were some corner cases for _nohz_idle_balance that
could benefit of this too

>
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bfe1e235fe01..60b8c1c68ab9 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8352,7 +8352,7 @@ group_type group_classify(unsigned int imbalance_pct,
> >       return group_has_spare;
> >  }
> >
> > -static bool update_nohz_stats(struct rq *rq, bool force)
> > +static bool update_nohz_stats(struct rq *rq)
> >  {
> >  #ifdef CONFIG_NO_HZ_COMMON
> >       unsigned int cpu = rq->cpu;
> > @@ -8363,7 +8363,7 @@ 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 (!time_after(jiffies, rq->last_blocked_load_update_tick))
> >               return true;
> >
> >       update_blocked_averages(cpu);
> > @@ -10404,7 +10404,7 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
> >
> >               rq = cpu_rq(balance_cpu);
> >
> > -             has_blocked_load |= update_nohz_stats(rq, true);
> > +             has_blocked_load |= update_nohz_stats(rq);
> >
> >               /*
> >                * If time for next balance is due,
> >
>