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

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Message-ID: <ZuCzk2CsLGAliHzQ@slm.duckdns.org>
Date: Tue, 10 Sep 2024 11:01:07 -1000
From: Tejun Heo <tj@...nel.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: Joshua Hahn <joshua.hahnjy@...il.com>, 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, Michal.

On Mon, Sep 02, 2024 at 05:45:39PM +0200, Michal Koutný wrote:
> - 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.

I think it's as useful as system-wide nice metric is. It's not a versatile
metric but is widely available and understood and people use it. Maybe a
workload is split across a sub-hierarchy and they wanna collect how much
lowpri threads are consuming. cpu.stats is available without cpu control
being enabled and people use it as a way to just aggregate metrics across a
portion of the system.

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

I think there are benefits to mirroring system wide metrics, at least ones
as widely spread as nice.

Thanks.

-- 
tejun