linux-kernel - Re: [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

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Date:   Thu, 27 Jul 2017 11:56:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        smuckle.linux@...il.com, juri.lelli@....com,
        Morten.Rasmussen@....com, patrick.bellasi@....com,
        eas-dev@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

On Wed, Jul 26, 2017 at 02:52:32PM +0530, Viresh Kumar wrote:
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 45fcf21ad685..bb834747e49b 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -72,10 +72,15 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
>  
>  /************************ Governor internals ***********************/
>  
> -static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> +static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time,
> +				     int target_cpu)
>  {
>  	s64 delta_ns;
>  
> +	/* Don't allow remote callbacks */

So I think you can reduce confusion in general if we extend this comment
somewhat.

	/*
	 * Since cpufreq_update_util() is called with rq->lock held for
	 * the @target_cpu, our per-cpu data is fully serialized.
	 *
	 * However, drivers cannot in general deal with cross-cpu
	 * requests, so while get_next_freq() will work, our
	 * sugov_update_commit() -> cpufreq_driver_fast_switch()
	 * call will not.
	 *
	 * Hence stop here for remote requests, as calculating the
	 * frequency is pointless if we cannot in fact act on it.
	 */

> +	if (smp_processor_id() != target_cpu)
> +		return false;
> +
>  	if (sg_policy->work_in_progress)
>  		return false;
>