From: Tejun Heo <tj@kernel.org>
To: Shakeel Butt <shakeel.butt@linux.dev>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
"Johannes Weiner" <hannes@cmpxchg.org>,
"Michal Hocko" <mhocko@kernel.org>,
"Roman Gushchin" <roman.gushchin@linux.dev>,
"Muchun Song" <muchun.song@linux.dev>,
"Vlastimil Babka" <vbabka@suse.cz>,
"Alexei Starovoitov" <ast@kernel.org>,
"Sebastian Andrzej Siewior" <bigeasy@linutronix.de>,
"Michal Koutný" <mkoutny@suse.com>,
"Harry Yoo" <harry.yoo@oracle.com>,
"Yosry Ahmed" <yosry.ahmed@linux.dev>,
bpf@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org,
linux-kernel@vger.kernel.org,
"Meta kernel team" <kernel-team@meta.com>
Subject: Re: [PATCH 2/3] cgroup: make css_rstat_updated nmi safe
Date: Tue, 10 Jun 2025 12:39:18 -1000 [thread overview]
Message-ID: <aEi0FplA6eZUHF01@slm.duckdns.org> (raw)
In-Reply-To: <35ppn2muk4bsyosca4nxnbv5l6qv4ov2cxg5ksypst5ldf5zc4@vwrpziws4wjy>
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
next prev parent reply other threads:[~2025-06-10 22:39 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-09 22:56 [PATCH 0/3] cgroup: nmi safe css_rstat_updated Shakeel Butt
2025-06-09 22:56 ` [PATCH 1/3] cgroup: support to enable nmi-safe css_rstat_updated Shakeel Butt
2025-06-09 22:56 ` [PATCH 2/3] cgroup: make css_rstat_updated nmi safe Shakeel Butt
2025-06-10 21:26 ` Tejun Heo
2025-06-10 22:31 ` Shakeel Butt
2025-06-10 22:39 ` Tejun Heo [this message]
2025-06-10 23:28 ` Shakeel Butt
2025-06-10 23:33 ` Tejun Heo
2025-06-11 5:23 ` JP Kobryn
2025-06-11 13:56 ` Shakeel Butt
2025-06-09 22:56 ` [PATCH 3/3] memcg: cgroup: call memcg_rstat_updated irrespective of in_nmi() Shakeel Butt
2025-06-09 23:44 ` [PATCH 0/3] cgroup: nmi safe css_rstat_updated Andrew Morton
2025-06-09 23:51 ` Shakeel Butt
2025-06-10 10:53 ` Michal Koutný
2025-06-10 16:24 ` Shakeel Butt
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