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

[Vikram Mulukutla]

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?

Thanks,
Vikram

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