Re: [PATCH v2 4/6] cpufreq: schedutil: update CFS util only if used

[Joel Fernandes]

Hi Vikram,

On Thu, Jul 6, 2017 at 11:44 PM, Vikram Mulukutla
<markivx@xxxxxxxxxxxxxx> 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@xxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Cc: linux-pm@xxxxxxxxxxxxxxx
>> ---
>>  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