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Date: Thu, 1 Feb 2018 19:16:34 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@foss.arm.com>,
        Ingo Molnar <mingo@kernel.org>,
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        Brendan Jackman <brendan.jackman@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK
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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.