Date: Thu, 3 Mar 2016 19:26:24 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux PM list <linux-pm@vger.kernel.org>,
        Juri Lelli <juri.lelli@arm.com>,
        Steve Muckle <steve.muckle@linaro.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>
Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
 utilization data
Message-ID: <20160303182624.GU6356@twins.programming.kicks-ass.net>
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On Thu, Mar 03, 2016 at 05:28:55PM +0000, Dietmar Eggemann wrote:
> > +void arch_scale_freq_tick(void)
> > +{
> > +	u64 aperf, mperf;
> > +	u64 acnt, mcnt;
> > +
> > +	if (!static_cpu_has(X86_FEATURE_APERFMPERF))
> > +		return;
> > +
> > +	aperf = rdmsrl(MSR_IA32_APERF);
> > +	mperf = rdmsrl(MSR_IA32_APERF);
> > +
> > +	acnt = aperf - this_cpu_read(arch_prev_aperf);
> > +	mcnt = mperf - this_cpu_read(arch_prev_mperf);
> > +
> > +	this_cpu_write(arch_prev_aperf, aperf);
> > +	this_cpu_write(arch_prev_mperf, mperf);
> > +
> > +	this_cpu_write(arch_cpu_freq, div64_u64(acnt * SCHED_CAPACITY_SCALE, mcnt));
> 
> Wasn't there the problem that this ratio goes to zero if the cpu is idle
> in the old power estimation approach on x86?

Yeah, there was something funky.

SDM says they only count in C0 (ie. !idle), so it _should_ work.