linux-kernel - Re: [PATCH v2 1/2] x86,sched: Add support for frequency invariance

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Message-ID: <20191002182754.GD4643@worktop.programming.kicks-ass.net>
Date:   Wed, 2 Oct 2019 20:27:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Giovanni Gherdovich <ggherdovich@...e.cz>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
        Len Brown <lenb@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>, x86@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Paul Turner <pjt@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Quentin Perret <qperret@...rret.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH v2 1/2] x86,sched: Add support for frequency invariance

On Wed, Oct 02, 2019 at 02:29:25PM +0200, Giovanni Gherdovich wrote:
> +void x86_arch_scale_freq_tick_enable(void)
> +{
> +	tick_disable = false;
> +}
> +
> +static void reset_scale_freq(void *arg)
> +{
> +	this_cpu_write(arch_cpu_freq, SCHED_CAPACITY_SCALE);
> +}
> +
> +void x86_arch_scale_freq_tick_disable(void)
> +{
> +	on_each_cpu(reset_scale_freq, NULL, 1);
> +	tick_disable = true;

I'm thikning this ought to be the other way around, otherwise we can get
a tick loosing the 1024 we just wrote in arch_cpu_freq.

> +}

You've lost the prev_{a,m}perf update, so the first tick after enable
will see 'funny' values.