linux-kernel - Re: [RFC PATCH 2/2] cpufreq/schedutil: Remove iowait boost

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Date: Mon, 25 Mar 2024 02:37:26 +0000
From: Qais Yousef <qyousef@...alina.io>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Christian Loehle <christian.loehle@....com>,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	juri.lelli@...hat.com, mingo@...hat.com, dietmar.eggemann@....com,
	vschneid@...hat.com, vincent.guittot@...aro.org,
	Johannes.Thumshirn@....com, adrian.hunter@...el.com,
	ulf.hansson@...aro.org, andres@...razel.de, asml.silence@...il.com,
	linux-pm@...r.kernel.org, linux-block@...r.kernel.org,
	io-uring@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] cpufreq/schedutil: Remove iowait boost

On 03/18/24 18:08, Rafael J. Wysocki wrote:
> On Mon, Mar 18, 2024 at 5:40 PM Christian Loehle
> <christian.loehle@....com> wrote:
> >
> > On 18/03/2024 14:07, Rafael J. Wysocki wrote:
> > > On Mon, Mar 4, 2024 at 9:17 PM Christian Loehle
> > > <christian.loehle@....com> wrote:
> > >>
> > >> The previous commit provides a new cpu_util_cfs_boost_io interface for
> > >> schedutil which uses the io boosted utilization of the per-task
> > >> tracking strategy. Schedutil iowait boosting is therefore no longer
> > >> necessary so remove it.
> > >
> > > I'm wondering about the cases when schedutil is used without EAS.
> > >
> > > Are they still going to be handled as before after this change?
> >
> > Well they should still get boosted (under the new conditions) and according
> > to my tests that does work.
> 
> OK
> 
> > Anything in particular you're worried about?
> 
> It is not particularly clear to me how exactly the boost is taken into
> account without EAS.
> 
> > So in terms of throughput I see similar results with EAS and CAS+sugov.
> > I'm happy including numbers in the cover letter for future versions, too.
> > So far my intuition was that nobody would care enough to include them
> > (as long as it generally still works).
> 
> Well, IMV clear understanding of the changes is more important.

I think the major thing we need to be careful about is the behavior when the
task is sleeping. I think the boosting will be removed when the task is
dequeued and I can bet there will be systems out there where the BLOCK softirq
being boosted when the task is sleeping will matter.

FWIW I do have an implementation for per-task iowait boost where I went a step
further and converted intel_pstate too and like Christian didn't notice
a regression. But I am not sure (rather don't think) I triggered this use case.
I can't tell when the systems truly have per-cpu cpufreq control or just appear
so and they are actually shared but not visible at linux level.