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

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Message-ID: <20250319180643.GC1876369@cmpxchg.org>
Date: Wed, 19 Mar 2025 14:06:43 -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>,
	Eric Dumazet <edumzaet@...gle.com>, 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 05:16:02PM +0000, Yosry Ahmed wrote:
> @@ -365,9 +352,8 @@ __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
>  void cgroup_rstat_flush_hold(struct cgroup *cgrp)
>  	__acquires(&cgroup_rstat_lock)
>  {
> -	might_sleep();
> +	cgroup_rstat_flush(cgrp);
>  	__cgroup_rstat_lock(cgrp, -1);
> -	cgroup_rstat_flush_locked(cgrp);
>  }

Might as well remove cgroup_rstat_flush_hold/release entirely? There
are no external users, and the concept seems moot when the lock is
dropped per default. cgroup_base_stat_cputime_show() can open-code the
lock/unlock to stabilize the counts while reading.

(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...)