linux-kernel - Re: [RFC PATCH v3 1/2] cgroup: cgroup.freeze.stat.local time accounting

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Message-ID: <oel2dyzlyvzggpwxprhwbd7n2ye2bst32izjnzywjdilo4nena@p7bcxgy6lqyq>
Date: Thu, 14 Aug 2025 17:54:44 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Tiffany Yang <ynaffit@...gle.com>
Cc: 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>, 
	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>, kernel-team@...roid.com, 
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/2] cgroup: cgroup.freeze.stat.local time
 accounting

Hello.

On Mon, Aug 04, 2025 at 08:29:41PM -0700, Tiffany Yang <ynaffit@...gle.com> wrote:
> +  cgroup.freeze.stat.local
> +	A read-only flat-keyed file which exists in non-root cgroups.
> +	The following entry is defined:
> +
> +	  freeze_time_total
> +		Cumulative time that this cgroup has spent between freezing and
> +		thawing, regardless of whether by self or ancestor groups.
> +		NB: (not) reaching "frozen" state is not accounted here.
> +
> +		Using the following ASCII representation of a cgroup's freezer
> +		state, ::
> +
> +			       1    _____
> +			frozen 0 __/     \__
> +			          ab    cd
> +
> +		.. Originally contributed by Michal Koutný <mkoutny@...e.com>
> +
> +		the duration being measured is the span between a and c.

This is so little "artwork" that a mere mention in commit message is OK
;-)

> +static int cgroup_freeze_local_stat_show(struct seq_file *seq, void *v)
> +{
> +	struct cgroup *cgrp = seq_css(seq)->cgroup;
> +	unsigned int sequence;
> +	u64 freeze_time;
> +
> +	do {
> +		sequence = read_seqcount_begin(&cgrp->freezer.freeze_seq);
> +		freeze_time = cgrp->freezer.freeze_time_total_ns;
> +		/* Add in current freezer interval if the task is now frozen */

Nit: cgrp is frozen, not a task here

> @@ -179,10 +179,16 @@ static void cgroup_do_freeze(struct cgroup *cgrp, bool freeze)
>  	lockdep_assert_held(&cgroup_mutex);
>  
>  	spin_lock_irq(&css_set_lock);
> -	if (freeze)
> +	write_seqcount_begin(&cgrp->freezer.freeze_seq);
> +	if (freeze) {
>  		set_bit(CGRP_FREEZE, &cgrp->flags);
> -	else
> +		cgrp->freezer.freeze_time_start_ns = ktime_get_ns();

I wonder whether it wouldn't achieve more stable results if the
reference timestamp was taken only once in cgroup_freeze().
Measuring the rate of cgroup traversal is rather noise in this case.

Thanks,
Michal

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