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

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Message-ID: <35ppn2muk4bsyosca4nxnbv5l6qv4ov2cxg5ksypst5ldf5zc4@vwrpziws4wjy>
Date: Tue, 10 Jun 2025 15:31:03 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Tejun Heo <tj@...nel.org>
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

On Tue, Jun 10, 2025 at 11:26:35AM -1000, Tejun Heo wrote:
> 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?

Yes.

> 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.
> 

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). 
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.

Thanks for taking a look.
Shakeel