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

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Date:   Wed, 17 Feb 2021 11:51:02 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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 15/02/21 16:02, Vincent Guittot wrote:
> On Fri, 12 Feb 2021 at 20:19, Valentin Schneider
> <valentin.schneider@....com> wrote:
>> 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
>

Can't that actually be a problem? kick_ilb() says:

         * Access to rq::nohz_csd is serialized by NOHZ_KICK_MASK; he who sets
         * the first flag owns it; cleared by nohz_csd_func().

So if you have:

  kick_ilb() -> kicks CPU42

And then said CPU42 goes through, before nohz_csd_func(),:

  do_idle() -> nohz_run_idle_balance()

you could have yet another CPU do:

  kick_ilb() -> kicks CPU42

which would break rq->nohz_csd serialization.

>>
>> > +}
>> > +