linux-kernel - Re: [RFC] cgroup: Fix /proc/cgroups count for v2

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Message-ID: <h5xdtfh7dc4rjh74b4cwkpjszro73hfbxzdobwtivyx4hl4hyn@p5lp5h5gzjuj>
Date: Fri, 24 May 2024 16:23:09 +0200
From: Michal Koutný <mkoutny@...e.com>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>, 
	Johannes Weiner <hannes@...xchg.org>, shakeel.butt@...ux.dev, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] cgroup: Fix /proc/cgroups count for v2

Hello.

On Sun, May 19, 2024 at 05:46:48PM GMT, "T.J. Mercier" <tjmercier@...gle.com> wrote:
> The number of cgroups using a controller is an important metric since
> kernel memory is used for each cgroup, and some kernel operations scale
> with the number of cgroups for some controllers (memory, io). So users
> have an interest in minimizing/tracking the number of them.

I agree this is good for debugging or quick checks of unified hierarchy
enablement status.

> To deal with num_cgroups being reported as 1 for those utility
> controllers regardless of the number of cgroups that exist and support
> their use,

But '1' is correct number no? Those utility controllers are v1-only and
their single group only exists on (default) root.

> @@ -675,11 +699,19 @@ int proc_cgroupstats_show(struct seq_file *m, void *v)
>  	 * cgroup_mutex contention.
>  	 */
>  
> -	for_each_subsys(ss, i)
> +	for_each_subsys(ss, i) {
> +		int count;
> +
> +		if (!cgroup_on_dfl(&ss->root->cgrp) || is_v2_utility_controller(i))
> +			count = atomic_read(&ss->root->nr_cgrps);

I think is_v2_utility_controller(ssid) implies
!cgroup_on_dfl(&ss->root->cgrp). I'd only decide based on the
cgroup_on_dfl() predicate.

> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -2047,6 +2047,8 @@ void init_cgroup_root(struct cgroup_fs_context *ctx)
>  
>  	INIT_LIST_HEAD_RCU(&root->root_list);
>  	atomic_set(&root->nr_cgrps, 1);
> +	for (int i = 0; i < CGROUP_SUBSYS_COUNT; ++i)
> +		atomic_set(&root->nr_css[i], 0);

Strictly not needed, non-dfl roots are kzalloc'd and dfl root is global
variable (zeroed).

HTH,
Michal

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