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b=iA45rXjfts3BL4hUeIUxxe+byrtKWMod75y24CPGNN9SvmqJ6rqAd+CqWKZW3yG6w kaDyCJ4wjLA+FqXK7EkN76If5n4Hj0k4ILNtarEO1jq/st6cUjbPwTKFtKsoajE9H2 HecPOJr4de83Yml+8tI9KJuPPpw+eCcGSKt615dRrpaiE77SGPxk608iLyZdDCfRGT q+VmnC2mbqeyj6GZGUQT/iG1PVQrFDvNNQsteEeadKnFyDy9EEVYYoOI94hbkRiys5 +PgqlvtSk9XDM/zk8Qd/9GQu1+WeLghEI4ZH3sUSslOZTwEhaM87BBg6QfjElWcET7 QZm5qvYbr0JtA== Date: Tue, 10 Jun 2025 12:39:18 -1000 From: Tejun Heo To: Shakeel Butt Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Vlastimil Babka , Alexei Starovoitov , Sebastian Andrzej Siewior , Michal =?iso-8859-1?Q?Koutn=FD?= , Harry Yoo , Yosry Ahmed , bpf@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team Subject: Re: [PATCH 2/3] cgroup: make css_rstat_updated nmi safe Message-ID: References: <20250609225611.3967338-1-shakeel.butt@linux.dev> <20250609225611.3967338-3-shakeel.butt@linux.dev> <35ppn2muk4bsyosca4nxnbv5l6qv4ov2cxg5ksypst5ldf5zc4@vwrpziws4wjy> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <35ppn2muk4bsyosca4nxnbv5l6qv4ov2cxg5ksypst5ldf5zc4@vwrpziws4wjy> X-Stat-Signature: au56hb1nsxs78bsjto17jex83urj41mb X-Rspamd-Queue-Id: 09D5E18000D X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1749595160-504336 X-HE-Meta: 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 LqdWtzBP jFD7paQG6WveYIpfjZvxZjvAJGl3MAiT7sw/vKvkG94dmCKPyTl2HLFakEYJ4q0UHvJvBI8SnQbA2tHdmyvXT4g36+vZxYgDcOCkS4xksZD9cijAsFH/F41uyoePAwxajejHsEiiGA4NOfUdA6SlRIk0mU2Srrco9biWxqAoaFCI6s05zz/49gu0qRC81xrXrAk6ThHbqT9dG6U2kY+tRgr9n9mZKTq9ixdxL+0eljq4Fuf8P1ePybkDc86UVgnw+btrOcUWHQ6N2V8g3vhGSozzSSHiouHIZWwpB X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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