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Date:   Wed, 24 Feb 2021 18:41:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>, 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>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Qais Yousef <qais.yousef@arm.com>
Subject: Re: [PATCH 0/7 v4] move update blocked load outside newidle_balance
Message-ID: <YDaPtc47NnB5BGEW@hirez.programming.kicks-ass.net>
References: <20210224133007.28644-1-vincent.guittot@linaro.org>
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 <CAKfTPtCwmt9HHDuN7tVhZiy7R3e5XHuExU-PVOb++40fYzu-2Q@mail.gmail.com>
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On Wed, Feb 24, 2021 at 04:57:15PM +0100, Vincent Guittot wrote:
> On Wed, 24 Feb 2021 at 16:54, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Feb 24, 2021 at 02:30:00PM +0100, Vincent Guittot wrote:
> > > Joel reported long preempt and irq off sequence in newidle_balance because
> > > of a large number of CPU cgroups in use and having to be updated. This
> > > patchset moves the update outside newidle_imblance. This enables to early
> > > abort during the updates in case of pending irq as an example.
> > >
> > > Instead of kicking a normal ILB that will wakes up CPU which is already
> > > idle, patch 6 triggers the update of statistics in the idle thread of
> > > the CPU before selecting and entering an idle state.
> >
> > I'm confused... update_blocked_averages(), which calls
> > __update_blocked_fair(), which is the one doing the cgroup iteration
> > thing, runs with rq->lock held, and thus will have IRQs disabled any
> > which way around we turn this thing.
> >
> > Or is the problem that we called nohz_idle_balance(), which does
> > update_nohz_stats() -> update_blocked_averages() for evey NOHZ cpu from
> > newidle balance, such that we get NR_NOHZ_CPUS * NR_CGROUPS IRQ latency?
> > Which is now reduced to just NR_CGROUPS ?
> 
> Yes we can now abort between each cpu update

OK, shall I add something like:

This reduces the IRQ latency from O(nr_cgroups * nr_nohz_cpus) to
O(nr_cgroups).

To the changelog of patch #1 ?