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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Qais Yousef <qais.yousef@arm.com>
Subject: Re: [PATCH 6/7 v3] sched/fair: trigger the update of blocked load on newly idle cpu
In-Reply-To: <CAKfTPtDpTEUdRcgJ3dwvD6Fz3D02E46xNtv9BpfFmkjCV1shyA@mail.gmail.com>
References: <20210212141744.24284-1-vincent.guittot@linaro.org> <20210212141744.24284-7-vincent.guittot@linaro.org> <jhj5z2xt7oy.mognet@arm.com> <CAKfTPtDpTEUdRcgJ3dwvD6Fz3D02E46xNtv9BpfFmkjCV1shyA@mail.gmail.com>
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Date:   Wed, 17 Feb 2021 11:51:02 +0000
Message-ID: <jhjmtw2syjd.mognet@arm.com>
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On 15/02/21 16:02, Vincent Guittot wrote:
> On Fri, 12 Feb 2021 at 20:19, Valentin Schneider
> <valentin.schneider@arm.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@intel.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.

>>
>> > +}
>> > +