linux-kernel - Re: [PATCH 10/39] sched: Factor out update_other_load_avgs() from __update_blocked

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Date: Wed, 26 Jun 2024 10:49:22 -1000
From: Tejun Heo <tj@...nel.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.org>, torvalds@...ux-foundation.org,
	mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	bristot@...hat.com, vschneid@...hat.com, ast@...nel.org,
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...nel.org,
	joshdon@...gle.com, brho@...gle.com, pjt@...gle.com,
	derkling@...gle.com, haoluo@...gle.com, dvernet@...a.com,
	dschatzberg@...a.com, dskarlat@...cmu.edu, riel@...riel.com,
	changwoo@...lia.com, himadrics@...ia.fr, memxor@...il.com,
	andrea.righi@...onical.com, joel@...lfernandes.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	kernel-team@...a.com
Subject: Re: [PATCH 10/39] sched: Factor out update_other_load_avgs() from
 __update_blocked_others()

Hello,

On Tue, Jun 25, 2024 at 11:13:54AM +0200, Vincent Guittot wrote:
> > Hmm.... I think I saw RT's schedutil signal stuck high constantly pushing up
> > the frequency. I might be mistaken tho. I'll check again.
> 
> This is used when selecting a frequency for fair tasks

When schedutil is used as the governor, sugov_get_util() provides the source
utilization information to determine the target frequency. sugov_get_util()
gets the CFS util metric from cpu_util_cfs_boost() and then runs it through
effective_cpu_util().

effective_cpu_util() does a bunch of things including adding cpu_util_irq(),
cpu_util_rt() and cpu_util_dl(), so if SCX doesn't decay these utilization
metrics, they never decay and schedutil ends up making decisions with stale
stuck-high numbers. I can easily confirm the behavior by sprinkling some
trace_printks and commenting out update_other_load_avgs() on the SCX side.

Thanks.

-- 
tejun