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From: Shakeel Butt To: Tejun Heo Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Vlastimil Babka , Alexei Starovoitov , Sebastian Andrzej Siewior , Michal =?utf-8?Q?Koutn=C3=BD?= , 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: <35ppn2muk4bsyosca4nxnbv5l6qv4ov2cxg5ksypst5ldf5zc4@vwrpziws4wjy> References: <20250609225611.3967338-1-shakeel.butt@linux.dev> <20250609225611.3967338-3-shakeel.butt@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: A8F4640010 X-Rspamd-Server: rspam03 X-Rspam-User: X-Stat-Signature: rb876hkjs7xff748jp6m4xp55kyf3jjy X-HE-Tag: 1749594676-367283 X-HE-Meta: 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 uXa/yo1y tpWTPtITAuSdqM64y8iNQjWrn1pceCYYpKgL4gakN9kwuUA+PPGhPZ3lhk54FnZPU9JB3WFeDvinvRRqqLy76tYXrTXrqxfOFp/bQ7L15ZhzjlXMw3pziubTWHTyqvRu+uyyyiRX055HDkFVREC2f0f4ZwtOjyIhyU62x4oqOoTF+zZJa5ouwaDWcNs/jU1cnIJ4Dk1Dz3s2esNsLN4218t9IRUGUoKOsb+6kg4L3GHE87aSd+sNMvyYTSoLwdja6XMNKUyuvRlRSuCs= 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: 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