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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49035c22d97sm12933315e9.1.2026.05.21.03.23.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 May 2026 03:23:26 -0700 (PDT) Date: Thu, 21 May 2026 11:23:25 +0100 From: David Laight To: Yang Shi Cc: Heiko Carstens , Alexander Gordeev , Sven Schnelle , Vasily Gorbik , Christian Borntraeger , Juergen Christ , "Christoph Lameter (Ampere)" , Peter Zijlstra , Shrikanth Hegde , linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org Subject: Re: [PATCH v3 0/9] s390: Improve this_cpu operations Message-ID: <20260521112325.230959c9@pumpkin> In-Reply-To: References: <20260520092243.264847-1-hca@linux.ibm.com> <9d503c6f-5641-4b28-998e-01e38b3622a9@os.amperecomputing.com> <20260520233409.0683f595@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 20 May 2026 17:23:37 -0700 Yang Shi wrote: > On 5/20/26 3:34 PM, David Laight wrote: .. > >> So it seems have more overhead than the percpu page table approach IIUC. > >> We don't need all the steps with percpu page table. And there is no > >> penalty for migration. > > This code looks like it relies on 'page zero' already being percpu. > > So it probably isn't really that different. > > Some values like the 'preemption disable count' and 'current' could be > > (maybe are?) written into page zero to give fast access. > > I don't quite get what you mean about 'page zero'. 'page zero' is (at least for some cpu) the memory that can be accessed using a small offset embedded in the instruction. This is equivalent to using offsets from an 'always zero' %r0. The code relies on the accesses to 960(%r0) being per-cpu. -- David