linux-kernel - Re: [PATCH] kernel/cgroup/pids: add "pids.forks" counter

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Message-ID: <bbublxzpnqrzjq5rmmbp772w2darjoahlewqky7caanaknglbx@6wuy5nidsnu3>
Date: Fri, 2 May 2025 14:55:51 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Max Kellermann <max.kellermann@...os.com>
Cc: tj@...nel.org, hannes@...xchg.org, cgroups@...r.kernel.org.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/cgroup/pids: add "pids.forks" counter

Hello Max.

On Fri, May 02, 2025 at 02:19:30PM +0200, Max Kellermann <max.kellermann@...os.com> wrote:
> Counts the number of fork()/clone() calls, similar to the "processes"
> row in /proc/stat, but per cgroup.

> This helps with analyzing who was responsible for peaks in the global
> "processes" counter.

bpftrace <<EOD
kprobe:copy_process
{
        @forks[cgroup] = count();
}
END {
        for ($kv : @forks) {
                printf("%s\t%d\n", cgroup_path($kv.0), (int64)$kv.1);
        }
        clear(@forks);
}
EOD

> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
...
> +  pids.forks
> +	A read-only single value file which exists on non-root cgroups.
> +
> +	The number of fork()/clone() calls (whether successful or not).
> +

It would conceptually fit better as a pids.stat:forks (but there's no
pids.stat unlike other controllers')

But I don't know this new field justifies introductions of a cgroup
attribute and (yet) another atomic operation on the forking path.

Does the value have long-term or broad use?


Thanks,
Michal

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