linux-kernel - Re: [PATCH] sched/core: introduce CPUTIME_FORCEIDLE

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Message-ID: <pb7jf52x2qpofgttzz3fphkeiuxuamjbjqb64paw7dvvtv2sxd@mgcol2syra6z>
Date: Mon, 26 Feb 2024 16:28:24 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Cruz Zhao <CruzZhao@...ux.alibaba.com>
Cc: tj@...nel.org, lizefan.x@...edance.com, hannes@...xchg.org, 
	mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: introduce CPUTIME_FORCEIDLE_TASK

Hello.

On Mon, Feb 19, 2024 at 04:41:34PM +0800, Cruz Zhao <CruzZhao@...ux.alibaba.com> wrote:
> As core sched uses rq_clock() as clock source to account forceidle
> time, irq time will be accounted into forceidle time. However, in
> some scenarios, forceidle sum will be much larger than exec runtime,
> e.g., we observed that forceidle time of task calling futex_wake()
> is 50% larger than exec runtime, which is confusing.

And those 50% turned out to be all attributed to irq time (that's
suggested by your diagram)?

(Could you argue about that time with data from /proc/stat alone?)

> Interfaces:
>  - task level: /proc/$pid/sched, row core_forceidle_task_sum.
>  - cgroup level: /sys/fs/cgroup/$cg/cpu.stat, row
>      core_sched.force_idle_task_usec.

Hm, when you touch this, could you please also add a section into
Documentation/admin-guide/cgroup-v2.rst about these entries?

(Alternatively, explain in the commit message why those aren't supposed
to be documented.
Alternative altenratively, would mere documenting of
core_sched.force_idle_usec help to prevent the confusion that you called
out above?)

Also, I wonder if the rstat counting code shouldn't be hidden with
CONFIG_SCHED_DEBUG too? (IIUC, that's the same one required to see
analogous stats in /proc/$pid/sched.)

Regards,
Michal

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