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

[Peter Zijlstra]

On Thu, Nov 14, 2019 at 06:07:31PM +0100, Vincent Guittot 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.

Doesn't this just show that a dumb rate limit in the driver is broken?

> 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.

I don't think it does that; that is, iirc the return value of
___update_load_sum() is 1 every time a period lapses. So even if the avg
is 0 and doesn't change, it'll still return 1 on every period.

Which is what that dumb rate-limit thing wants of course. But I'm still
thinking that it's stupid to do. If nothing changes, don't generate
events.

If anything, update_blocked_avgerages() should look at
@done/others_have_blocked() to emit events for rt,dl,irq.

So why are we making the scheduler code more ugly instead of fixing that
driver?