linux-kernel - Re: [PATCH 0/7 v4] move update blocked load outside newidle

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Date:   Wed, 24 Feb 2021 16:53:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        joel@...lfernandes.org, valentin.schneider@....com,
        fweisbec@...il.com, tglx@...utronix.de, qais.yousef@....com
Subject: Re: [PATCH 0/7 v4] move update blocked load outside newidle_balance

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 ?