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Wed, 23 Oct 2024 16:42:36 -0700 (PDT) Date: Wed, 23 Oct 2024 16:42:36 -0700 (PDT) From: "Christoph Lameter (Ampere)" To: Peter Zijlstra cc: Linus Torvalds , Will Deacon , Thomas Gleixner , Catalin Marinas , Ingo Molnar , Waiman Long , Boqun Feng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire In-Reply-To: <20241023194543.GD11151@noisy.programming.kicks-ass.net> Message-ID: References: <20240912-seq_optimize-v3-1-8ee25e04dffa@gentwo.org> <20240917071246.GA27290@willie-the-truck> <4b546151-d5e1-22a3-a6d5-167a82c5724d@gentwo.org> <20241023194543.GD11151@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Queue-Id: E9C404001B X-Rspamd-Server: rspam01 X-Stat-Signature: e14cm3nby1jwr8kbn4ppj4qfsh3f7hdn X-HE-Tag: 1729727465-466432 X-HE-Meta: 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 CZGnqGGS /i2auiLGkEHobDQ/G1jo6BrIE/kkF4BxyvjFTgCjhpXoAvWHmqZICbdGfOGsEdaXrHnkKTAwwVRnwIsLjeMEHpNJn7OuIFXyn9iddkHJCVWGXOreH+EkUlDdKWzbYhu89RYC8ix01cDN4hIp4ZFqzTVMNrBjuI5h2UcCOqCYIHHioXa8wWb5O8J8cdHCmvCYj6RzqaxodYs7QRYxyT2CECRxTVZ6662bs/lnfZQfWhHSmRvMyHvVtb5L6hABoDE7BRRKrPAOnAfXJYXOd0nJPMrHTzrXH9WeXAlkIa4DTEnu5Hk5SHyR+g4fnFpH27J91Ik4S 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 Wed, 23 Oct 2024, Peter Zijlstra wrote: > > I doubt anybody will notice, and smp_load_acquire() is the future. Any > > architecture that does badly on it just doesn't matter (and, as > > mentioned, I don't think they even exist - "smp_rmb()" is generally at > > least as expensive). > > Do we want to do the complementing patch and make write_seqcount_end() > use smp_store_release() ? > > I think at least ARM (the 32bit thing) has wmb but uses mb for > store_release. But I also think I don't really care about that. The proper instruction would be something like atomic_inc_release(&seqcount) The current atomics do not provide such a macro. The closest in the current tree is atomic_inc_return_release(). We would prefer atomic_inc_release(&seqcount) because such an atomic may be executed as a far atomic in the ARM mesh. This could be cheaper than a local atomic and could f.e. be executed on the memory controller of a remote NUMA node in order to avoid a costly transfer of cacheline ownership. The code generated is a atomic that also does a release. So there would be no extra barrier etc needed.