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Mon, 23 Feb 2026 09:50:21 +0000 (GMT) Date: Mon, 23 Feb 2026 10:50:20 +0100 From: Heiko Carstens To: Yang Shi Cc: lsf-pc@lists.linux-foundation.org, Linux MM , "Christoph Lameter (Ampere)" , dennis@kernel.org, Tejun Heo , urezki@gmail.com, Catalin Marinas , Will Deacon , Ryan Roberts , Yang Shi , Vasily Gorbik , Alexander Gordeev Subject: Re: [LSF/MM/BPF TOPIC] Improve this_cpu_ops performance for ARM64 (and potentially other architectures) Message-ID: <20260223095020.8139Bd5-hca@linux.ibm.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMjIzMDA4NCBTYWx0ZWRfX66bRpwjgWXRn Q4+a8tzmO/mA9tq2qYXC1ODUwJ5bOK24ET+vHjWz2MAkFiaCm5p4JMRQy33XxStkdpQqmQpb8ZX LeQbgLUtlwKpgqYIn7/dB0Ih6BE7h1zyms772M7Q8goM7tm6kKoLfyd9sDofN9z0fu7ug8n88a7 AOaCvWyZmkgxlRaDd1pfzD1GbwI3YOIuz7Y5cmmL4iPs+T6OR154Q+eg450Y91ND8pbIjbibl3m Y57YZLOiBG6PdXYN5iudg/QuInXu5UK1TWHf500hBRfkU0gB5z7/TxFQ5aXyHs4i0pqb1FeK0z0 UCmWj5uc3ONJDQFM4eQbJYGbIMskTuZgXgs5skzQfxpBOP3+30pby221U59w4DfD84V+k6Azjqv Hjk8k0da3Qm36uboES6OW8u16HAGjsEmrpcUXD8Za8VYAh2K0A2kTHccT6dsTetelVbFsHiI0wX TRQcT5VmMAKEfmvfBpQ== X-Proofpoint-ORIG-GUID: ochqu-XZkKCnlVXvsC_Xl1gV38A75zmw X-Authority-Analysis: v=2.4 cv=F9lat6hN c=1 sm=1 tr=0 ts=699c22e3 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=kj9zAlcOel0A:10 a=HzLeVaNsDn8A:10 a=VkNPw1HP01LnGYTKEx00:22 a=Mpw57Om8IfrbqaoTuvik:22 a=GgsMoib0sEa3-_RKJdDe:22 a=Xhgu7lUpGomXScTDuk8A:9 a=CjuIK1q_8ugA:10 X-Proofpoint-GUID: cqO9iRPjS1shf7Ud8vRYWF-llPJZgMQ4 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-02-23_01,2026-02-20_04,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 phishscore=0 bulkscore=0 adultscore=0 spamscore=0 clxscore=1011 suspectscore=0 malwarescore=0 lowpriorityscore=0 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2601150000 definitions=main-2602230084 X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 898ED100009 X-Stat-Signature: hdsdaymz1qbhbc7zkh56pb4rng9fq3k1 X-HE-Tag: 1771840233-69438 X-HE-Meta: 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 q/f8gjOX Odlo8BCXq8nIdZUJG2l8Wvd7FRMXJk77hEktxVqP762E6POVnTTpZG8qpbVqhTlXFCg58dLUOV2f5nFmKiQQ1jPRzZ2nvqaMF81nzHqAjmmTJLETjHnOghaqbEdtEwndHQpjQivnknv/HQInLhDCH+1zEK+2K/Uba1NOJuJ2zhkU6fJVRPIK2EEpGwUiI8RukOrxh0KPQI2A1bKr7pPRjzt4Yf+K88LZtEGKVHSX+TAEWXwJhXjTVdToKgJtmjxOApEAbmflKhZhX6Uj//F5i9GO5DXJAfhHgIhCi1jUae6lD+UYziNgOpQyEypfg2mDWoGWmrp5PzsR7dSZJP1lLuPFWMd7FsQFGOVjKZiqK+5BVDgE= 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, Feb 11, 2026 at 03:14:57PM -0800, Yang Shi wrote: > Design > ====== > To improve the performance for this_cpu_ops on ARM64 and potentially > some other non-x86 architectures, I and Christopher Lameter proposed > the below solution. ... > I have done some PoC on ARM64. Hopefully I can post them to the > mailing list to ease the discussion before the conference. FWIW, from an s390 perspective we have been relying on that preempt_disable() and preempt_enable() pairs for this_cpu*() ops are optimized away at compile time for PREEMPT_NONE and PREEMPT_VOLUNTARY. Which is what all distributions did, until now. With 7dadeaa6e851 ("sched: Further restrict the preemption modes") this is not the case anymore. So the overhead will be there for every "up-to-date" architecture except x86. I've been looking at some ways how to get rid of this extra overhead for s390. E.g. we have an "access register mode", which can be sort of compared to the x86 segment register approach, but switching the CPU to this mode and back is more expensive than the preempt_disable() and preempt_enable() pairs. To make the long story short: I'm very interested in your approch from an s390 perspective (aka "other non-x86 architectures").