linux-kernel - Re: [PATCH v4] sched/freq: move call to cpufreq_update

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Date:   Fri, 15 Nov 2019 14:30:58 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Doug Smythies <dsmythies@...us.net>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sargun Dhillon <sargun@...gun.me>, Tejun Heo <tj@...nel.org>,
        Xie XiuQi <xiexiuqi@...wei.com>, xiezhipeng1@...wei.com,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

On Fri, 15 Nov 2019 at 14:02, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Nov 15, 2019 at 12:03:31PM +0100, Vincent Guittot wrote:
>
> > This patch does 2 things:
> > - fix the spurious call to cpufreq just before attaching a task
>
> Right, so that one doesn't concern me too much.
>
> > - make sure cpufreq is still called when cfs is 0 but not irq/rt or dl
>
> But per the rq->has_blocked_load logic we would mostly stop sending
> events once we reach all 0s.
>
> Now, most of those updates will be through _nohz_idle_balance() ->
> update_nohz_stats(), which are remote, which means intel_pstate is
> ignoring them anyway.
>
> Now the _nohz_idle_balance() -> update_blocked_averages() thing runs
> local, and that will update the one random idle CPU we picked to run
> nohz balance, but all others will be left where they were.
>
> So why does intel_pstate care... Esp. on SKL+ with per-core P state this
> is of dubious value.

Doug mentioned some periodic timers that were running on the CPUs

>
> Also, and maybe I should go read back, why do we care what the P state
> is when we're mostly in C states anyway? These are all idle CPUs,
> otherwise we wouldkn't be running update_blocked_averages() on them
> anyway.

AFAIU, there is not 100% idle but they have periodic timers that will
fire and run at higher P state


>
>
> Much confusion..