Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

[Rafael J. Wysocki]

On Thu, Nov 14, 2019 at 6:07 PM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> update_cfs_rq_load_avg() calls cfs_rq_util_change() everytime pelt decays,
> which might be inefficient when cpufreq driver has rate limitation.
>
> When a task is attached on a CPU, we have call path:
>
> update_load_avg()
>   update_cfs_rq_load_avg()
>     cfs_rq_util_change -- > trig frequency update
>   attach_entity_load_avg()
>     cfs_rq_util_change -- > trig frequency update
>
> The 1st frequency update will not take into account the utilization of the
> newly attached task and the 2nd one might be discard because of rate
> limitation of the cpufreq driver.

Kind of on a second thought, it shouldn't matter for governors other
than schedutil that the new task's utilization is not taken into
account by the first update, because they measure utilization by
themselves.

> update_cfs_rq_load_avg() is only called by update_blocked_averages()
> and update_load_avg() so we can move the call to
> cfs_rq_util_change/cpufreq_update_util() into these 2 functions. It's also
> interesting to notice that update_load_avg() already calls directly
> cfs_rq_util_change() for !SMP case.
>
> This changes will also ensure that cpufreq_update_util() is called even
> when there is no more CFS rq in the leaf_cfs_rq_list to update but only
> irq, rt or dl pelt signals.

So this change appears to be the relevant one for non-schedutil governors.

Now, there is a rate limit in schedutil too, see
sugov_should_update_freq(), but I'm not sure if that matters in the
context of this patch.