Re: [PATCH 2/3] cgroup: make css_rstat_updated nmi safe

[Tejun Heo]

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