linux-kernel - Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs

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Message-ID: <1846666.Va7CsUJLIx@aspire.rjw.lan>
Date:   Mon, 20 Mar 2017 13:35:12 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs

On Monday, March 20, 2017 11:36:45 AM Peter Zijlstra wrote:
> On Sun, Mar 19, 2017 at 02:34:32PM +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > 
> > The PELT metric used by the schedutil governor underestimates the
> > CPU utilization in some cases.  The reason for that may be time spent
> > in interrupt handlers and similar which is not accounted for by PELT.
> > 
> > That can be easily demonstrated by running kernel compilation on
> > a Sandy Bridge Intel processor, running turbostat in parallel with
> > it and looking at the values written to the MSR_IA32_PERF_CTL
> > register.  Namely, the expected result would be that when all CPUs
> > were 100% busy, all of them would be requested to run in the maximum
> > P-state, but observation shows that this clearly isn't the case.
> > The CPUs run in the maximum P-state for a while and then are
> > requested to run slower and go back to the maximum P-state after
> > a while again.  That causes the actual frequency of the processor to
> > visibly oscillate below the sustainable maximum in a jittery fashion
> > which clearly is not desirable.
> > 
> > To work around this issue use the observation that, from the
> > schedutil governor's perspective, CPUs that are never idle should
> > always run at the maximum frequency and make that happen.
> > 
> > To that end, add a counter of idle calls to struct sugov_cpu and
> > modify cpuidle_idle_call() to increment that counter every time it
> > is about to put the given CPU into an idle state.  Next, make the
> > schedutil governor look at that counter for the current CPU every
> > time before it is about to start heavy computations.  If the counter
> > has not changed for over SUGOV_BUSY_THRESHOLD time (equal to 50 ms),
> > the CPU has not been idle for at least that long and the governor
> > will choose the maximum frequency for it without looking at the PELT
> > metric at all.
> 
> Why the time limit?

One iteration appeared to be a bit too aggressive, but honestly I think
I need to check again if this thing is regarded as viable at all.