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Wed, 5 Aug 2026 08:45:04 +0000 (GMT) Date: Wed, 5 Aug 2026 10:45:03 +0200 From: Heiko Carstens To: "David Hildenbrand (Arm)" Cc: "Christoph Lameter (Ampere)" , "Lorenzo Stoakes (ARM)" , Mark Rutland , Yang Shi , Ryan Roberts , dennis@kernel.org, tj@kernel.org, urezki@gmail.com, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, gor@linux.ibm.com, agordeev@linux.ibm.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Linus Torvalds , Jason Gunthorpe Subject: Re: [RFC v2 PATCH 0/16] Optimize this_cpu_*() ops for non-x86 (ARM64 for this series) Message-ID: <20260805084503.37016Aa7-hca@linux.ibm.com> References: <0344c559-1959-4531-9265-d5a5180eb7cd@arm.com> <25d1e09b-53e4-7cd5-87db-b58437e4e690@gentwo.org> <4887267b-dc26-4c33-96ca-8dff054a0d1f@kernel.org> <63ea8156-a109-b2a2-6d35-1c17ad7f1a9d@gentwo.org> <84b31836-1d10-4bec-a469-468b91fd4645@kernel.org> <9f146ce0-4adc-0c00-a2d8-c01b47edc9d1@gentwo.org> MIME-Version: 1.0 Content-Type: text/plain; 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(on CC) I said that before Peter Zijlstra proposed the restartable sequence approach for the kernel. I took that as input, and provided the current solution for s390, which does not restart anything, but only fix up register contents if required. Now Mark provided an improved implementation for arm64. That said, there are some performance numbers [1] for s390 available as posted by Mete. A performance improvement is indeed there, but looking at the current s390 implementation I'm not sure there will be much of a benefit if we now go to percpu page tables. Currently we have this code sequence for e.g. this_cpu_add(...) (%r2 contains the to be added value). larl %r4,1b33300 <-- load address of percpu var mviy 960,4 <-- mark start of percpu op section ag %r4,952 <-- add percpu offset laag %r5,%r2,0(%r4) <-- atomic add mviy 960,0 <-- mark end of percpu op section With the proposed percpu page tables I would guess / hope we would end up with something like this: larl %r4,1b33300 <-- load address of percpu var (same on all cpus) laag %r5,%r2,0(%r4) <-- atomic add That's certainly better to what we have compared to now (no performance numbers available), but I'm not sure if only this would justify all the added complexity to common code currently discussed. Read: I'm more or less fine with the current s390 solution, and not pushing for percpu page tables. [1] https://lore.kernel.org/all/18ca706f-ef45-4207-a55d-025f90133978@linux.ibm.com/