Re: [PATCH v2] sched/freq: move call to cpufreq_update_util

[Vincent Guittot]

On Wed, 13 Nov 2019 at 11:50, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>
> On 12.11.19 16:05, Vincent Guittot wrote:
> > Le Tuesday 12 Nov 2019 Ã 15:48:13 (+0100), Vincent Guittot a Ãcrit :
> >> 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_blocked_averages()
> >>   update_cfs_rq_load_avg()
> >>     cfs_rq_util_change -- > trig frequency update
> >>   attach_entity_load_avg()
> >>     cfs_rq_util_change -- > trig frequency update
>
> This looks like attach_entity_load_avg() is called from
> update_blocked_averages(). Do you refer to the attach_entity_load_avg()
> call from attach_entity_cfs_rq() or update_load_avg() here? I assume the
> former.

ah... typo mistake, i wanted to write update_load_avg
update_blocked_averages()
  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.
> >>
> >> 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,cpufreq}_util_change() into these 2 functions. It's also
>
> s/cpufreq_util_change()/cpufreq_update_util() ?

yes

>
> [...]
>
> >> I have just rebased the patch on latest tip/sched/core and made it a proper
> >> patchset after Doug reported that the problem has diseappeared according to
> >> his 1st results but tests results are not all based on the same v5.4-rcX
> >> and with menu instead of teo governor.
>
> I had some minor tweaks to do putting this on a0e813f26ebc ("sched/core:
> Further clarify sched_class::set_next_task()") ? I saw the '[tip:
> sched/urgent] sched/pelt: Fix update of blocked PELT ordering' tip-bot
> msg this morning though.

yes, a0e813f26ebc was item 1 and this patch is item 2 on top

>
> [...]
>
> >> @@ -7493,9 +7495,9 @@ static void update_blocked_averages(int cpu)
> >>       * that RT, DL and IRQ signals have been updated before updating CFS.
> >>       */
> >>      curr_class = rq->curr->sched_class;
> >> -    update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> >> -    update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> >> -    update_irq_load_avg(rq, 0);
> >> +    decayed |= update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> >> +    decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> >> +    decayed |= update_irq_load_avg(rq, 0);
>
> Why not 'decayed  = update_cfs_rq_load_avg()' like in the
> !CONFIG_FAIR_GROUP_SCHED case?

Because it is handled by the update_load_avg() in
for_each_leaf_cfs_rq_safe() loop

This means that we can have 2 calls to cpufreq_update_util in
update_blocked_average() but at least the values will be up to date in
both calls unlike previously.

I'm going to prepare an additional patch to remove this useless call.
I have also seen some possible further optimization that i need to
study a bit more before preparing a patch


>
> [...]