Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins

From: Qais Yousef <qyousef@layalina.io>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Lukasz Luba <lukasz.luba@arm.com>,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [RFC PATCH 0/7] sched: cpufreq: Remove magic margins
Date: Sun, 10 Sep 2023 19:46:09 +0100	[thread overview]
Message-ID: <20230910184609.fiqsxu3gmhvz34jn@airbuntu> (raw)
In-Reply-To: <20230907132631.GF10955@noisy.programming.kicks-ass.net>

On 09/07/23 15:26, Peter Zijlstra wrote:
> On Wed, Sep 06, 2023 at 10:18:50PM +0100, Qais Yousef wrote:
> 
> > This is probably controversial statement. But I am not in favour of util_est.
> > I need to collect the data, but I think we're better with 16ms PELT HALFLIFE as
> > default instead. But I will need to do a separate investigation on that.
> 
> I think util_est makes perfect sense, where PELT has to fundamentally

My concern about it is that it has inherit bias towards PERF. In the soap of
tasks running in the system, not all of which care about perf. The key tasks
tend to be the minority, I'd say. Again, I need to do more investigations but
my worry mainly comes from that and what impact it could have on power.

In an ideal world where userspace is fully uclamp aware, we shouldn't need it.
The task can set uclamp_min to make sure it sees the right performance at wake
up.

And depends on the outcome of this discussion, we might need to introduce
something to help speed up/slow down migration more selectively. So it could
become redundant control.

> decay non-running / non-runnable tasks in order to provide a temporal
> average, DVFS might be best served with a termporal max filter.

Ah, I certainly don't think DVFS need PELT HALFLIFE to be controlled. I only
see it being useful on HMP systems, under-powered specifically, that really
need faster *migration* times. Maybe we can find a better way to control this.
I'll think about it.

Not sure about the temporal max. Isn't this a bias towards perf first too?

Thanks!

--
Qais Yousef