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SubjectRe: [RFC/RFT][PATCH] cpufreq: schedutil: Reduce frequencies slower
On 30-03-17, 23:36, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> The schedutil governor reduces frequencies too fast in some
> situations which cases undesirable performance drops to
> appear.
>
> To address that issue, make schedutil reduce the frequency slower by
> setting it to the average of the value chosen during the previous
> iteration of governor computations and the new one coming from its
> frequency selection formula.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=194963
> Reported-by: John <john.ettedgui@gmail.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>
> This addresses a practical issue, but one in the "responsiveness" or
> "interactivity" category which is quite hard to represent quantitatively.
>
> As reported by John in BZ194963, schedutil does not ramp up P-states quickly
> enough which causes audio issues to appear in his gaming setup. At least it
> evidently is worse than ondemand in this respect and the patch below helps.
>
> The patch essentially repeats the trick added some time ago to the load-based
> P-state selection algorithm in intel_pstate, which allowed us to make it viable
> for performance-oriented users, and which is to reduce frequencies at a slower
> pace.
>
> The reason why I chose the average is because it is computationally cheap
> and pretty much the max reasonable slowdown and the idea is that in case
> there's something about to run that we don't know about yet, it is better to
> stay at a higher level for a while more to avoid having to get up from the floor
> every time.
>
> But technically speaking it is a filter. :-)
>
> So among other things I'm wondering if that leads to substantial increases in
> energy consumption anywhere.
>
> Thanks,
> Rafael
>
> ---
> kernel/sched/cpufreq_schedutil.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -101,6 +101,9 @@ static void sugov_update_commit(struct s
> if (sg_policy->next_freq == next_freq)
> return;
>

Maybe add a comment over here on why we are adding this workaround .

> + if (sg_policy->next_freq > next_freq)
> + next_freq = (sg_policy->next_freq + next_freq) >> 1;
> +
> sg_policy->next_freq = next_freq;
> sg_policy->last_freq_update_time = time;

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

--
viresh

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