linux-kernel - Re: [PATCH 2/3] cgroup: make css_rstat

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Message-ID: <aEi0FplA6eZUHF01@slm.duckdns.org>
Date: Tue, 10 Jun 2025 12:39:18 -1000
From: Tejun Heo <tj@...nel.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Vlastimil Babka <vbabka@...e.cz>,
	Alexei Starovoitov <ast@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Michal Koutný <mkoutny@...e.com>,
	Harry Yoo <harry.yoo@...cle.com>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>, bpf@...r.kernel.org,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 2/3] cgroup: make css_rstat_updated nmi safe

Hello,

On Tue, Jun 10, 2025 at 03:31:03PM -0700, Shakeel Butt wrote:
...
> Couple of lines above I have llist_on_list(&rstatc->lnode) check which
> should be as cheap as data_race(css_rstat_cpu(css, cpu)->updated_next). 

Ah, I missed that.

> So, I can add lnode for nmi and non-nmi contexts (with irqs disabled)
> but I think that is not needed. Actually I ran the netperf benchmark (36
> parallel instances) and I see no significant differences with and
> without the patch.

Yeah, as long as the hot path doesn't hit the extra cmpxchg, I think it
should be fine. Can you fortify the comments a bit that the synchronization
is against the stacking contexts on the same CPU. The use of cmpxchg for
something like this is a bit unusual and it'd be nice to have explanation on
why it's done this way and why the overhead doesn't matter.

Thanks.

-- 
tejun