From: Marc Zyngier <maz@kernel.org>
To: zhangqiao22@huawei.com, Zhan Xusheng <zhanxusheng1024@gmail.com>
Cc: tglx@linutronix.de, jstultz@google.com, sboyd@kernel.org,
peterz@infradead.org, mark.rutland@arm.com,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, zhanxusheng@xiaomi.com
Subject: Re: [QUESTION] sched/clock: Potential timestamp jump due to inter-CPU counter skew?
Date: Mon, 26 Oct 2026 09:05:00 +0000 [thread overview]
Message-ID: <86zew0ygqr.wl-maz@kernel.org> (raw)
In-Reply-To: <20261026082730.2117173-1-zhanxusheng1024@gmail.com>
On Mon, 26 Oct 2026 08:27:30 +0000,
Zhan Xusheng <zhanxusheng1024@gmail.com> wrote:
>
> On arm64 the system counter is meant to give all PEs a common view of
> time, and keeping the copies on separate dies in step is the
> implementation's job. The kernel has no tolerance for skew. The counter
> errata handled in arm_arch_timer.c (Freescale A-008585, HiSilicon
> 161010101, Cortex-A73 858921, Allwinner A64) are all reads on a single
> CPU returning a wrong value, worked around by reading twice; none of them
> is skew between CPUs.
Indeed. If the implementation cannot give any two arbitrary CPUs
anywhere in the system the exact same view of the counter, then this
implementation is broken, and unsupported. No discussion.
M.
--
Without deviation from the norm, progress is not possible.
next prev parent reply other threads:[~2026-10-26 9:05 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-26 6:53 [QUESTION] sched/clock: Potential timestamp jump due to inter-CPU counter skew? Zhang Qiao
2026-10-26 8:27 ` Zhan Xusheng
2026-10-26 9:05 ` Marc Zyngier [this message]
2026-10-26 10:23 ` Peter Zijlstra
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