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 23:15:52 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	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 Thu, May 19, 2016 at 9:46 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
> On Thu, May 19, 2016 at 01:44:36AM +0200, Rafael J. Wysocki wrote:
>> 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.
>
> It is true that this means we'll do more frequency re-evaluations, they
> will occur until an actual frequency change is requested.
>
> But the way it stands now, with a system's typical background activity
> there are so many minor events that it is very common for throttling to
> be in effect, causing major events to be ignored.
>>
>> > ---
>> >  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.
>
> Going back to the reason this tracepoint exists, is it known why
> powertop thinks the CPU is idle when this tracepoint is removed? Maybe
> it's possible to get rid of this tracepoint altogether.

I'm not sure ATM.  It seems to go by the time stamps and declare idle
if it doesn't see updates for long enough.

I was hoping to be able to make cpufreq stats usable for the fast
switch case, but that appears to mean some major surgery in there.

But anyway this change again seems to be an optimization that might be
done later to me.

I guess there are many things that might be optimized in schedutil,
but I'd prefer to address one item at a time, maybe going after the
ones that appear most relevant first?