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b=anIpDvCbLz1al5lEwizeQ9TRqV0GfEwS3ElIBSUJbK9mwo6IQiwKGEfx9O3lKpIe5 6rLdNm+ZejMW8uukRRb/U3GnkcSRU5N4Qgrcrbhej0DmvEgRPRShipVaDF+ifEO29O un+yx37gRX/Ir3fQ5Occp8CBb7sXLKEvdaBZCGGb0EBsAiEBMJtcJ2rPre27JqQaow j5dDnMSADfw7+CXmEYORzsAS3nPPS4nYt4Cs/K35WAbVCEZAfOOtMTVDYKH/DNuyCz zcBZ3GzUGKJAdAqEAhdP1CCEk3DSYInSPWkW6cpK9Ae6V+Fn5ShxZOAaJZfpi5l3cq q6FXjyJt6QDtQ== Message-ID: Date: Tue, 4 Aug 2026 23:47:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 PATCH 0/16] Optimize this_cpu_*() ops for non-x86 (ARM64 for this series) To: "Christoph Lameter (Ampere)" Cc: "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, hca@linux.ibm.com, 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 References: <20260715180455.515692-1-yang@os.amperecomputing.com> <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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspam-User: X-Stat-Signature: 8xfyu8dffa1syrfdmnus7miiwfydg3gj X-Rspamd-Queue-Id: 7096012000E X-HE-Tag: 1785880032-618611 X-HE-Meta: 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 VVOwxaxV 0Q4+TozTcx1lU+3q741JJScBU4XQBLAGOIZLvje/BAovV+cK5CxqjCP6T4NGj5GVvfCFnRZwWRskvWCeDUNT3gtZ5skrBImLvb03ESXj5s0qkshZ2j5ItI+ZlwekyDSMZipARhxmJFDjUnp7+iGYMunr1yyVJTxtIjiiiLOFSF0/wR3iP3l92on55jjkxPh4hk8ykWgh+c6clSXvqm9TCeFvEZcMyg5lWnjsHCVkeOD1xNgttew1aUNLLQ4G27oR3/MYzSGa96snlFEq2AUUgOHth+5HCl5YEEgxSc62jS569dNRSqJ5R6NXBbA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/4/26 23:25, Christoph Lameter (Ampere) wrote: > On Tue, 4 Aug 2026, David Hildenbrand (Arm) wrote: > >>> rather hacky restart logic. It relies on a long preemable and postscript >>> to each per cpu operations. > >> But I am sure you did you homework and have data to back up your claims. Please >> share that data, because I am very curious. > > The comparison here is a single instruction on one side and abnd the per > cpu operation prefixed with a preeamble and another instruction / set of > instructions on the other side. > > Even if its only one before and one after then we are still looking at 3 > instructions instead of one. This is a frequently used operation in the > VM. > > You need data to show that one instruction is faster than 3??? I want you to show that the difference *matters* in *practice* enough that we should *care* about accepting the additional *complexity*. No hand-waving about "3 > 1". IIRC people were asking that previously but received no results back. So to be clear: claiming that the additional instructions are unacceptable I'd expect something minimal like: "removing these 3 instructions speeds up micro-benchmark (a, b, c) by X% and macro-benchmarks (d, e, f) by Y% compared to Mark's approach". Maybe there really is a big difference. > >> Mark's solution is the obvious improvement to the problem, doing it just like >> s390 already does. > > Well there is interest by the S390 folks to move to what we proposed from > what I can tell. Heiko said that as reply to v1, bit he's been replying a lot on Mark's patches. Only Heiko can tell. (on CC) > > >> If the proposed solution has a significant benefit over Mark's solution it could >> be discussed as a next step; and it must be backed with sufficient data to >> warrant all this complexity here. > > You cannot accept that one instruction is faster than the same > instruction prefixed with another and something else at its tail? I am pretty sure that you got exactly what I meant, you likely just don't have the data to back it up. Proof me wrong! -- Cheers, David