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bh=7bX+ckDiWzV0UXiUsEpBvHAAjmkCz9ocHzwV5JJ1Gn4=; b=Op8M+Ud/ecXwHOTy7YoUTH4slRc/9pzpxC5sgAbCKhScsfynbjtvzjn7cI2VkPtpaHNByX 6lDCZXjvLs1v5TxMEN0E5LIQFZmHVb8+FsmBvsCGeD9e55FZ0mtwfU0KHpqR/3mwHJNyj5 RxL8beQJgpw/sg3Cx9smitqJm9CAimomZxHQycly883Y7B0DFZiDDnX6887epqDFHeXqyP 6XY5FHyUejTAz1HsZYNaI/BcL6pnFZyIYyC4c1KuHBemn88f9eBxsFnYyuPydGqo9nhV9Q VbvIEW4dOOiGVjm7zccRVtSfsK5R8tJDEYbkkVE3k6CIHccCilknBxwF42Q3Fg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1726573969; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=7bX+ckDiWzV0UXiUsEpBvHAAjmkCz9ocHzwV5JJ1Gn4=; b=Nv/BsxsS8N1MMLS0Mu4yS1B+iTF5E4Cmt408LhLBA/k+P/2xsFm9tPkhB/8+UpN3tp9U+F oG+6WW/C6AQoWtCw== To: Christoph Lameter via B4 Relay , Catalin Marinas , Will Deacon , Peter Zijlstra , Ingo Molnar , Waiman Long , Boqun Feng Cc: Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, "Christoph Lameter (Ampere)" Subject: Re: [PATCH v3] Avoid memory barrier in read_seqcount() through load acquire In-Reply-To: <20240912-seq_optimize-v3-1-8ee25e04dffa@gentwo.org> References: <20240912-seq_optimize-v3-1-8ee25e04dffa@gentwo.org> Date: Tue, 17 Sep 2024 13:52:48 +0200 Message-ID: <87o74m1oq7.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain X-Stat-Signature: gmxexsa8pjwjs956pt8ty91d5m9ypdmy X-Rspamd-Queue-Id: A134C180003 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1726573974-452392 X-HE-Meta: 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 7yJeue3r bqMNlPf4/H8bl97gC8XxuIf/MqgnefjwbTfpSFpRISrhFiJcZfVd+MvWzs307advnV+Y+RaCMrTO6oO/9UnwgNmgcZStjqootUW78i+i+xS2jo6PTend97wz5RGESiXi6x2BqgTDPU82SwlxMGkD+gkO1QVoVanON9oFBNrEcBdS0+2Y3rYaDYcQexh+U/I2o7Eh/LGkXCiSPMwoXtmLYgmhlQZKt06oVjLOEZ2xl/htM4CDNmlme+0Yejzk+7Na5v/gSzm5luRms82sbLHZONbe7iBMVuZSNVHmzuKrjhUC5hT97N4kYw2jWFLoHXU43d/nxYMUGkNaQqHrsLqIVmZrLgPgXE48qiv7cWr/NJgyaYbBqhlJZkb+g6AQTUj2XcbGaSye7vddlnMuhVGHXI86o4yTpYfGyJzR5WUDlqLdoJ2g= 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 Thu, Sep 12 2024 at 15:44, Christoph Lameter via wrote: $Subject: ..... You still fail to provide a proper subsystem prefix. It's not rocket science. https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-subject > From: "Christoph Lameter (Ampere)" > > Some architectures support load acquire which can save us a memory > barrier and save some cycles. > > A typical sequence > > do { > seq = read_seqcount_begin(&s); > > } while (read_seqcount_retry(&s, seq); > > requires 13 cycles on ARM64 for an empty loop. Two read memory > barriers are needed. One for each of the seqcount_* functions. > > We can replace the first read barrier with a load acquire of > the seqcount which saves us one barrier. We .... Please write changelogs using passive voice. 'We' means nothing here. Thanks, tglx