linux-kernel - Re: [PATCH V2 2/2] cpufreq: schedutil: Always process remote callback with slow switching

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Message-ID: <2514808.4A5PPQc5Ab@aspire.rjw.lan>
Date:   Tue, 22 Aug 2017 15:27:10 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri.Lelli@....com, joelaf@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] cpufreq: schedutil: Always process remote callback with slow switching

On Monday, August 14, 2017 11:20:16 AM CEST Viresh Kumar wrote:
> The frequency update from the utilization update handlers can be divided
> into two parts:
> 
> (A) Finding the next frequency
> (B) Updating the frequency
> 
> While any CPU can do (A), (B) can be restricted to a group of CPUs only,
> depending on the current platform.
> 
> For platforms where fast cpufreq switching is possible, both (A) and (B)
> are always done from the same CPU and that CPU should be capable of
> changing the frequency of the target CPU.
> 
> But for platforms where fast cpufreq switching isn't possible, after
> doing (A) we wake up a kthread which will eventually do (B). This
> kthread is already bound to the right set of CPUs, i.e. only those which
> can change the frequency of CPUs of a cpufreq policy. And so any CPU
> can actually do (A) in this case, as the frequency is updated from the
> right set of CPUs only.
> 
> Check cpufreq_can_do_remote_dvfs() only for the fast switching case.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> V2: s/policy/sg_policy->policy/, missed updating the commit with local
> updates earlier, noticed that just now.
> 
>  kernel/sched/cpufreq_schedutil.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 504d0752f8f2..9209d83ecdcf 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -84,13 +84,18 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  	 *
>  	 * However, drivers cannot in general deal with cross-cpu
>  	 * requests, so while get_next_freq() will work, our
> -	 * sugov_update_commit() call may not.
> +	 * sugov_update_commit() call may not for the fast switching platforms.
>  	 *
>  	 * Hence stop here for remote requests if they aren't supported
>  	 * by the hardware, as calculating the frequency is pointless if
>  	 * we cannot in fact act on it.
> +	 *
> +	 * For the slow switching platforms, the kthread is always scheduled on
> +	 * the right set of CPUs and any CPU can find the next frequency and
> +	 * schedule the kthread.
>  	 */
> -	if (!cpufreq_can_do_remote_dvfs(sg_policy->policy))
> +	if (sg_policy->policy->fast_switch_enabled &&
> +	    !cpufreq_can_do_remote_dvfs(sg_policy->policy))
>  		return false;
>  
>  	if (sg_policy->work_in_progress)
> 

Applied, thanks!