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Date:   Wed, 24 Feb 2021 16:53:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     mingo@redhat.com, juri.lelli@redhat.com, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, linux-kernel@vger.kernel.org,
        joel@joelfernandes.org, valentin.schneider@arm.com,
        fweisbec@gmail.com, tglx@linutronix.de, qais.yousef@arm.com
Subject: Re: [PATCH 0/7 v4] move update blocked load outside newidle_balance
Message-ID: <YDZ2kl2dpHUgmjTS@hirez.programming.kicks-ass.net>
References: <20210224133007.28644-1-vincent.guittot@linaro.org>
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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 ?