Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210224133007.28644-1-vincent.guittot@linaro.org> <YDZ2kl2dpHUgmjTS@hirez.programming.kicks-ass.net>
In-Reply-To: <YDZ2kl2dpHUgmjTS@hirez.programming.kicks-ass.net>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 24 Feb 2021 16:57:15 +0100
Message-ID: <CAKfTPtCwmt9HHDuN7tVhZiy7R3e5XHuExU-PVOb++40fYzu-2Q@mail.gmail.com>
Subject: Re: [PATCH 0/7 v4] move update blocked load outside newidle_balance
To:     Peter Zijlstra <peterz@infradead.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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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