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

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Message-ID: <lmjsy6fp25bhno62mg3hz7z2ysggg4z66yhhpd6mxpzksthsbz@55hjcvz2jymh>
Date: Tue, 10 Jun 2025 16:28:23 -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 12:39:18PM -1000, Tejun Heo wrote:
> 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.

I was actually thinking of using this_cpu_cmpxchg but then I need to
also check for CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS. However if you
prefer that, I can try this_cpu_cmpxchg in the next version.

I will also fix the comment with additional information about stacking
context.