linux-kernel - Re: [RFC PATCH v2 4/7] sched/fair: Use CFS util_avg

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Date: Fri, 15 Mar 2024 13:31:06 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Hongyan Xia <hongyan.xia2@....com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Juri Lelli <juri.lelli@...hat.com>, Steven Rostedt <rostedt@...dmis.org>,
 Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
 Daniel Bristot de Oliveira <bristot@...hat.com>,
 Valentin Schneider <vschneid@...hat.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>
Cc: Qais Yousef <qyousef@...alina.io>,
 Morten Rasmussen <morten.rasmussen@....com>,
 Lukasz Luba <lukasz.luba@....com>,
 Christian Loehle <christian.loehle@....com>, linux-kernel@...r.kernel.org,
 David Dai <davidai@...gle.com>, Saravana Kannan <saravanak@...gle.com>,
 linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH v2 4/7] sched/fair: Use CFS util_avg_uclamp for
 utilization and frequency

On 01/02/2024 14:12, Hongyan Xia wrote:

[...]

> @@ -7685,11 +7697,13 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  static unsigned long
>  cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost)
>  {
> -	struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
> -	unsigned long util = READ_ONCE(cfs_rq->avg.util_avg);
> +	struct rq *rq = cpu_rq(cpu);
> +	struct cfs_rq *cfs_rq = &rq->cfs;
> +	unsigned long util = root_cfs_util(rq);
> +	bool capped = uclamp_rq_is_capped(rq);

I try to rerun your tests in your 2 ipynbs (cover letter) but this let's
the sum aggr stack go sideways ...

if 'sched_uclamp_used' then uclamp_rq_is_capped() will call
cpu_util_cfs()->cpu_util() which then calls uclamp_rq_is_capped()
recursively resulting in a stack overflow.

Do you have a fix for that you can share? For the time I remove the call
to uclamp_rq_is_capped() in cpu_util().

[...]