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

[Shakeel Butt <shakeel.butt@linux.dev>]

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