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

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Message-ID: <20250319194610.GF1876369@cmpxchg.org>
Date: Wed, 19 Mar 2025 15:46:10 -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 03:16:42PM -0400, Johannes Weiner wrote:
> 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

Nevermind, Tejun pointed me to the follow-up fix he's got already
queued up:

https://web.git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git/commit/?id=c4af66a95aa3bc1d4f607ebd4eea524fb58946e3

That brings it all back on the stack.