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

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Message-ID: <aEijC1iHehAxdsfi@slm.duckdns.org>
Date: Tue, 10 Jun 2025 11:26:35 -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 Mon, Jun 09, 2025 at 03:56:10PM -0700, Shakeel Butt wrote:
...
> +	self = &rstatc->lnode;
> +	if (!try_cmpxchg(&(rstatc->lnode.next), &self, NULL))
>  		return;
>  
> +	llist_add(&rstatc->lnode, lhead);

I may be missing something but when you say multiple inserters, you mean the
function being re-entered from stacked contexts - ie. process context, BH,
irq, nmi? If so, would it make sense to make the nmi and non-nmi paths use
separate lnode? In non-nmi path, we can just disable irq and test whether
lnode is empty and add it. nmi path can just test whether its lnode is empty
and add it. I suppose nmi's don't nest, right? If they do, we can do
try_cmpxchg() there I suppose.

While the actual addition to the list would be relatively low frequency,
css_rstat_updated() itself can be called pretty frequently. Before, the hot
path was early exit after data_race(css_rstat_cpu(css, cpu)->updated_next).
After, the hot path is now !try_cmpxchg() which doesn't seem great.

Thanks.

-- 
tejun