linux-kernel - Re: cpu.stat in core or cpu controller (was Re: [RFC PATCH v2] cgroup: Track time in cgroup v2 freezer)

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Message-ID: <2c723007-710f-4592-9fe2-7534eb47e74f@huaweicloud.com>
Date: Tue, 22 Jul 2025 17:01:50 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Tejun Heo <tj@...nel.org>
Cc: Michal Koutný <mkoutny@...e.com>,
 Tiffany Yang <ynaffit@...gle.com>, linux-kernel@...r.kernel.org,
 John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Stephen Boyd <sboyd@...nel.org>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Pavel Machek <pavel@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Chen Ridong <chenridong@...wei.com>, kernel-team@...roid.com,
 Jonathan Corbet <corbet@....net>, cgroups@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: cpu.stat in core or cpu controller (was Re: [RFC PATCH v2]
 cgroup: Track time in cgroup v2 freezer)

On 2025/7/20 0:27, Tejun Heo wrote:
> On Sat, Jul 19, 2025 at 10:01:07AM +0800, Chen Ridong wrote:
> ...
>> What I'm considering is moving the implementation of cpu.stat from cgroup_base_files to
>> cpu_cgrp_subsys—without changing the user-facing interface (filenames and content remain the same).
>> However, the interface would only appear if the CPU subsystem is enabled.
>>
>> Currently, cpu.stat and cpu.stat.local are visible in every cgroup, even when the CPU subsystem is
>> disabled. The only populated fields in such cases are:
>>
>> - usage_usec
>> - user_usec
>> - system_usec
>> - nice_usec
>>
>> I’m unsure whether this change would be acceptable?
> 
> I don't think so and don't really see what benefits moving the stats would
> bring. Why would we move these?
> 
> Thanks.
> 

Thank you for your attention. My intention is to better modularize the cgroup code by moving CPU
subsystem-specific statistics out of the core cgroup implementation (cgroup.c and rstat.c).

Specifically, this change would allow us to:

1.Remove these CPU-specific callbacks from the core:
  css_extra_stat_show()
  css_local_stat_show()
2. Clean up the 'is_self' logic in rstat.c.
3. Make the stat handling consistent across subsystems (currently cpu.stat is the only
subsystem-specific stat implemented in the core).

Best regards,
Ridong.