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

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Date:   Fri, 15 Nov 2019 14:46:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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, Nov 15, 2019 at 02:30:58PM +0100, Vincent Guittot wrote:
> 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

If it is pure timers, I don't see how those CPUs end up calling
cpufreq_update_util().

Per the above argument, only the CPU that ran nohz balance gets an
update call, all the other CPUs that remain idle (or only serve IRQs)
never get the call.