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SubjectRe: [PATCH v2 4/6] cpufreq: schedutil: update CFS util only if used
Hi Vikram,

On Thu, Jul 6, 2017 at 11:44 PM, Vikram Mulukutla
<markivx@codeaurora.org> wrote:
> On 2017-07-04 10:34, Patrick Bellasi wrote:
>>
>> Currently the utilization of the FAIR class is collected before locking
>> the policy. Although that should not be a big issue for most cases, we
>> also don't really know how much latency there can be between the
>> utilization reading and its usage.
>>
>> Let's get the FAIR utilization right before its usage to be better in
>> sync with the current status of a CPU.
>>
>> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> Cc: Viresh Kumar <viresh.kumar@linaro.org>
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-pm@vger.kernel.org
>> ---
>> 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 98704d8..df433f1 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -308,10 +308,9 @@ static void sugov_update_shared(struct
>> update_util_data *hook, u64 time,
>> if (unlikely(current == sg_policy->thread))
>> return;
>>
>> - sugov_get_util(&util, &max);
>> -
>> raw_spin_lock(&sg_policy->update_lock);
>>
>> + sugov_get_util(&util, &max);
>> sg_cpu->util = util;
>> sg_cpu->max = max;
>
>
> Given that the utilization update hooks are called with the per-cpu rq lock
> held (for all classes), I don't think PELT utilization can change throughout
> the lifetime of the cpufreq_update_{util,this_cpu} call? Even with Viresh's
> remote cpu callback series we'd still have to hold the rq lock across
> cpufreq_update_util.. what can change today is 'max'
> (arch_scale_cpu_capacity) when a cpufreq policy is shared, so the patch is
> still needed for that reason I think?
>

I didn't follow, Could you elaborate more why you think the patch
helps with the case where max changes while the per-cpu rq lock held?

thanks,

-Joel

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