linux-kernel - Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num

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Message-ID: <20250319191638.GD1876369@cmpxchg.org>
Date: Wed, 19 Mar 2025 15:16:38 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Greg Thelen <gthelen@...gle.com>, Tejun Heo <tj@...nel.org>,
	Michal Koutný <mkoutny@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu)

On Wed, Mar 19, 2025 at 06:35:02PM +0000, Yosry Ahmed wrote:
> On Wed, Mar 19, 2025 at 02:06:43PM -0400, Johannes Weiner wrote:
> > (btw, why do we not have any locking around the root stats in
> > cgroup_base_stat_cputime_show()? There isn't anything preventing a
> > reader from seeing all zeroes if another reader runs the memset() on
> > cgrp->bstat, is there? Or double times...)
> 
> (I think root_cgroup_cputime() operates on a stack allocated bstat, not
> cgrp->bstat)

That was the case until:

commit b824766504e49f3fdcbb8c722e70996a78c3636e
Author: Chen Ridong <chenridong@...wei.com>
Date:   Thu Jul 4 14:01:19 2024 +0000

    cgroup/rstat: add force idle show helper

Now it's doing this:

void cgroup_base_stat_cputime_show(struct seq_file *seq)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	if (cgroup_parent(cgrp)) {
		...
	} else {
		/* cgrp->bstat of root is not actually used, reuse it */
		root_cgroup_cputime(&cgrp->bstat);
		usage = cgrp->bstat.cputime.sum_exec_runtime;
		utime = cgrp->bstat.cputime.utime;
		stime = cgrp->bstat.cputime.stime;
		ntime = cgrp->bstat.ntime;
	}
}

and then:

static void root_cgroup_cputime(struct cgroup_base_stat *bstat)
{
	struct task_cputime *cputime = &bstat->cputime;
	int i;

	memset(bstat, 0, sizeof(*bstat));

	/* various += on bstat and cputime members */
}

So all readers are mucking with the root cgroup's bstat.