linux-kernel - Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS

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Message-ID: <20180201181634.GG2269@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Feb 2018 19:16:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@...s.arm.com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK

On Mon, Jan 29, 2018 at 07:31:07PM +0000, Valentin Schneider wrote:

> But for load update via _nohz_idle_balance(), we iterate through all of the
> nohz CPUS and unconditionally call update_blocked_averages(). This could be
> avoided by remembering which CPUs have stale load before going idle.
> Initially I thought that was what nohz.stats_state was for, but it isn't.
> With Vincent's patches it's only ever set to either 0 or 1, but we could use
> it as a CPU mask, and use it to skip nohz CPUs that don't have stale load in
> _nohz_idle_balance() (when NOHZ_STATS_KICK).

Yes, you'd need to allocate a second cpumask, worse you need atomic
bitops to set and clear bits there.

That all _might_ be worth it... dunno.