linux-kernel - Re: [PATCH v2 2/2] sched

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Message-ID: <ZoxVZPCrWokjfmFY@slm.duckdns.org>
Date: Mon, 8 Jul 2024 11:08:52 -1000
From: Tejun Heo <tj@...nel.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: rafael@...nel.org, viresh.kumar@...aro.org, linux-pm@...r.kernel.org,
	void@...ifault.com, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, mingo@...hat.com, peterz@...radead.org,
	David Vernet <dvernet@...a.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v2 2/2] sched_ext: Add cpuperf support

Hello, Vincent.

On Mon, Jul 08, 2024 at 09:51:08PM +0200, Vincent Guittot wrote:
> > Unless we add a WARN_ON_ONCE, if it doesn't behave as expected, the end
> > result will most likely be cpufreq sometimes picking a higher freq than
> > requested, which won't be the easiest to notice. Would you be against adding
> > WARN_ON_ONCE(scx_switched_all && !util) too?
> 
> A WARN_ON_ONCE to detect misbehavior would be ok

I tried this and it's a bit problematic. Migrating out all the tasks do
bring the numbers pretty close to zero but the math doesn't work out exactly
and it often leaves 1 in the averages. While the fair class is in use, they
would decay quickly through __update_blocked_fair(); however, when all tasks
are switched to sched_ext, that function doesn't get called and the
remaining small value never decays.

Now, the value being really low, it doesn't really matter but it's an
unnecessary complication. I can make sched_ext keep calling
__update_blocked_fair() in addition to update_other_load_avgs() to decay
fair's averages but that seems a lot more complicated than having one
scx_switched_all() test.

Thanks.

-- 
tejun