linux-kernel - Re: [PATCH v2 0/2] Exposing nice CPU usage to userspace

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Message-ID: <mhugflcvgtv2iwyz2i63dkteur3xhnqn5qi37gzwc7mkt4sfn7@32kenf3bnhzf>
Date: Mon, 2 Sep 2024 17:45:39 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: tj@...nel.org, cgroups@...r.kernel.org, hannes@...xchg.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, lizefan.x@...edance.com, 
	shuah@...nel.org
Subject: Re: [PATCH v2 0/2] Exposing nice CPU usage to userspace

Hello Joshua.

On Fri, Aug 30, 2024 at 07:19:37AM GMT, Joshua Hahn <joshua.hahnjy@...il.com> wrote:
> Exposing this metric will allow users to accurately probe the niced CPU
> metric for each workload, and make more informed decisions when
> directing higher priority tasks.

I'm afraid I can't still appreciate exposing this value:

- It makes (some) sense only on leave cgroups (where variously nice'd
  tasks are competing against each other). Not so much on inner node
  cgroups (where it's a mere sum but sibling cgroups could have different
  weights, so the absolute times would contribute differently).

- When all tasks have nice > 0 (or nice <= 0), it loses any information
  it could have had.

(Thus I don't know whether to commit to exposing that value via cgroups.)

I wonder, wouldn't your use case be equally served by some
post-processing [1] of /sys/kernel/debug/sched/debug info which is
already available?

Regards,
Michal

[1]
Your metric is supposed to represent
	Σ_i^tasks ∫_t is_nice(i, t) dt

If I try to address the second remark by looking at
	Σ_i^tasks ∫_t nice(i, t) dt

and that resembles (nice=0 ~ weight=1024)
	Σ_i^tasks ∫_t weight(i, t) dt

swap sum and int
	∫_t Σ_i^tasks weight(i, t) dt

where
	Σ_i^tasks weight(i, t) 

can be taken from
	/sys/kernel/debug/sched/debug:cfs_rq[0].load_avg

above is only for CPU nr=0. So processing would mean sampling that file
over all CPUs and time.

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