linux-kernel - Re: [PATCH 5/5] cpufreq: schedutil: do not update rate limit ts when freq is unchanged

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Date:	Thu, 19 May 2016 01:44:36 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 5/5] cpufreq: schedutil: do not update rate limit ts when
 freq is unchanged

On Mon, May 9, 2016 at 11:20 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
> The rate limit timestamp (last_freq_update_time) is currently advanced
> anytime schedutil re-evaluates the policy regardless of whether the CPU
> frequency is changed or not. This means that utilization updates which
> have no effect can cause much more significant utilization updates
> (which require a large increase or decrease in CPU frequency) to be
> delayed due to rate limiting.
>
> Instead only update the rate limiting timstamp when the requested
> target-supported frequency changes. The rate limit will now apply to
> the rate of CPU frequency changes rather than the rate of
> re-evaluations of the policy frequency.
>
> Signed-off-by: Steve Muckle <smuckle@...aro.org>

I'm sort of divided here to be honest.

> ---
>  kernel/sched/cpufreq_schedutil.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e185075fcb5c..4d2907c8a142 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -117,12 +117,11 @@ static void sugov_update_commit(struct sugov_cpu *sg_cpu, int cpu, u64 time,
>         struct sugov_policy *sg_policy = sg_cpu->sg_policy;
>         struct cpufreq_policy *policy = sg_policy->policy;
>
> -       sg_policy->last_freq_update_time = time;
> -
>         if (sg_policy->next_freq == next_freq) {
>                 trace_cpu_frequency(policy->cur, cpu);

You should at least rate limit the trace_cpu_frequency() thing here if
you don't want to advance the last update time I think, or you may
easily end up with the trace buffer flooded by irrelevant stuff.

>                 return;
>         }
> +       sg_policy->last_freq_update_time = time;
>         sg_policy->next_freq = next_freq;
>
>         if (sugov_queue_remote_callback(sg_policy, cpu))
> --
> 2.4.10
>