From: Heiko Carstens <hca@linux.ibm.com>
To: "Christoph Lameter (Ampere)" <cl@gentwo.org>
Cc: Dev Jain <dev.jain@arm.com>, Will Deacon <will@kernel.org>,
Jisheng Zhang <jszhang@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
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
Date: Mon, 23 Feb 2026 10:19:13 +0100 [thread overview]
Message-ID: <20260223091913.8139A0b-hca@linux.ibm.com> (raw)
In-Reply-To: <6d30176d-9673-287e-4658-a5e9e6e5d649@gentwo.org>
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.
next prev parent reply other threads:[~2026-02-23 9:19 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-15 3:39 Jisheng Zhang
2026-02-16 10:59 ` Dev Jain
2026-02-16 11:00 ` Will Deacon
2026-02-16 15:29 ` Dev Jain
2026-02-17 13:53 ` Catalin Marinas
2026-02-17 15:00 ` Will Deacon
2026-02-17 16:48 ` Catalin Marinas
2026-02-18 4:01 ` K Prateek Nayak
2026-02-18 9:29 ` Catalin Marinas
2026-02-17 17:19 ` Christoph Lameter (Ampere)
2026-02-23 9:19 ` Heiko Carstens [this message]
2026-02-20 6:14 ` Jisheng Zhang
2026-02-18 22:07 ` Shakeel Butt
2026-02-20 6:20 ` Jisheng Zhang
2026-02-20 23:27 ` Shakeel Butt
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