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Mon, 23 Feb 2026 09:19:15 +0000 (GMT) Date: Mon, 23 Feb 2026 10:19:13 +0100 From: Heiko Carstens To: "Christoph Lameter (Ampere)" Cc: Dev Jain , Will Deacon , Jisheng Zhang , Catalin Marinas , Dennis Zhou , Tejun Heo , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, maz@kernel.org Subject: Re: [PATCH] arm64: remove HAVE_CMPXCHG_LOCAL Message-ID: <20260223091913.8139A0b-hca@linux.ibm.com> References: <20260215033944.16374-1-jszhang@kernel.org> <89606308-3c03-4dcf-a89d-479258b710e4@arm.com> <6d30176d-9673-287e-4658-a5e9e6e5d649@gentwo.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6d30176d-9673-287e-4658-a5e9e6e5d649@gentwo.org> X-TM-AS-GCONF: 00 X-Authority-Analysis: v=2.4 cv=TNRIilla c=1 sm=1 tr=0 ts=699c1b96 cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=kj9zAlcOel0A:10 a=HzLeVaNsDn8A:10 a=VkNPw1HP01LnGYTKEx00:22 a=Mpw57Om8IfrbqaoTuvik:22 a=GgsMoib0sEa3-_RKJdDe:22 a=FhHsSIHnO-3NYp-ZkXEA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-GUID: FfOG356PllBGcL4cFZoTPHDdr2AldGay X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMjIzMDA4MSBTYWx0ZWRfX0UoJva7jCva3 z9d41zV4OBne2pTiVEEhjv5cb5YHPlpt7K+P7rSz15b1qEQbW6Jpo1ge+Lm0BQnyW9fRhYP6CKH HZ4QpIfQeGXfj0tMd8dV418IKUdcq3Po6Pupx9dfAuHhwBNGdeClkWUlMyDWoOiTiDR6rcY7wuc vXv9v9qaDOiTso5w+px82r4J+peiUdh0seZCfNv5dpd7V6Uj7Ga1Q1b8LWI1Lz2ruVLoDE596f0 syF+WAvv8JDYUhxBiRTPwpUb+jqhJ9sszEReTuGuaR1EgSDm2G/DYqc1r4JZjinn//lh0rs9+4v bMiNTEFjFxLhP0AP40LRn/Y5G/hoR8wiOfFhxXQqOArg/fiGa2e+aY0aq2qlg5ApL594CKGpokb 1+5lJ7fV0ZJ0QTk5Xp+HoK2sJC08hinckBU0MK6gvELhay5aXl5h+EE9v8Lc/2kXUYR/lTHa6LH wqV6Ki6fO0bOxge9ebA== X-Proofpoint-ORIG-GUID: FfOG356PllBGcL4cFZoTPHDdr2AldGay 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 malwarescore=0 suspectscore=0 adultscore=0 priorityscore=1501 impostorscore=0 spamscore=0 clxscore=1011 bulkscore=0 lowpriorityscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2601150000 definitions=main-2602230081 On Tue, Feb 17, 2026 at 09:19:22AM -0800, Christoph Lameter (Ampere) wrote: > On Mon, 16 Feb 2026, Dev Jain wrote: > > > By coincidence, Yang Shi has been discussing the this_cpu_* overhead > > at [2]. > > > Yang Shi is on vacation but we have a patchset that removes > preempt_enable/disable from this_cpu operations on ARM64. > > The performance of cmpxchg varies by platform in use and with the kernel > config. The measurements that I did 2 years ago indicated that the cmpxchg > use with Ampere processors did not cause a regression. > > Note that distro kernels often do not enable PREEMPT_FULL and therefore > preempt_disable/enable overhead is not incurred in production systems. > > PREEMPT_VOLUNTARY does not use preemption for this_cpu ops. Note that with commit 7dadeaa6e851 ("sched: Further restrict the preemption modes") at least PREEMPT_LAZY is enforced, which comes together with PREEMPT_BUILD and full preempt_disable()/preempt_enable() overhead for this_cpu_* ops for every "up-to-date" architecture (except x86). PREEMPT_NONE and PREEMPT_VOLUNTARY are gone for those architectures.