linux-kernel - Re: [PATCH v4] sched/freq: move call to cpufreq_update

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Message-ID: <20191115095447.GU4114@hirez.programming.kicks-ass.net>
Date:   Fri, 15 Nov 2019 10:54:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        dietmar.eggemann@....com, juri.lelli@...hat.com,
        rostedt@...dmis.org, mgorman@...e.de, dsmythies@...us.net,
        linux-pm@...r.kernel.org, torvalds@...ux-foundation.org,
        tglx@...utronix.de, sargun@...gun.me, tj@...nel.org,
        xiexiuqi@...wei.com, xiezhipeng1@...wei.com,
        srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

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?