linux-kernel - Re: [RFC PATCH] cgroup: Track time in cgroup v2 freezer

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Message-ID: <gn6xiuqczaoiepdczg364cj46riiskvqwgvyaawbb3bpaybaw4@5iiohkyscrek>
Date: Tue, 17 Jun 2025 11:49:36 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Tiffany Yang <ynaffit@...gle.com>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	kernel-team@...roid.com, 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>, 
	Tejun Heo <tj@...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>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [RFC PATCH] cgroup: Track time in cgroup v2 freezer

Hello.

On Tue, Jun 03, 2025 at 10:43:05PM +0000, Tiffany Yang <ynaffit@...gle.com> wrote:
> The cgroup v2 freezer controller allows user processes to be dynamically
> added to and removed from an interruptible frozen state from
> userspace.

Beware of freezing by migration vs freezing by cgroup attribute change.
The latter is primary design of cgroup v2, the former is "only" for
consistency.

> This feature is helpful for application management, as it
> allows background tasks to be frozen to prevent them from being
> scheduled or otherwise contending with foreground tasks for resources.

> Still, applications are usually unaware of their having been placed in
> the freezer cgroup, so any watchdog timers they may have set will fire
> when they exit. To address this problem, I propose tracking the per-task
> frozen time and exposing it to userland via procfs.

But the watchdog fires rightfully when the application does not run,
doesn't it?
It should be responsibility of the "freezing agent" to prepare or notify
the application about expected latencies.

> but the main focus in this initial submission is establishing the
> right UAPI for this accounting information.

/proc/<pid>/cgroup_v2_freezer_time_frozen looks quite extraordinary with
other similar metrics, my first thought would be a field in
/proc/<pid>/stat (or track it per cgroup as Tejun suggests).

Could you please primarily explain why the application itself should
care about the frozen time (and not other causes of delay)?

Thanks,
Michal

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