linux-kernel - Re: [PATCH v9 01/12] asm-generic: barrier: Add smp_cond_load_relaxed

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Message-ID: <87h5rn9b7r.fsf@oracle.com>
Date: Wed, 11 Feb 2026 14:17:28 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, bpf@...r.kernel.org, arnd@...db.de,
        will@...nel.org, peterz@...radead.org, akpm@...ux-foundation.org,
        mark.rutland@....com, harisokn@...zon.com, cl@...two.org,
        ast@...nel.org, rafael@...nel.org, daniel.lezcano@...aro.org,
        memxor@...il.com, zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v9 01/12] asm-generic: barrier: Add
 smp_cond_load_relaxed_timeout()


Catalin Marinas <catalin.marinas@....com> writes:

> On Sun, Feb 08, 2026 at 06:31:42PM -0800, Ankur Arora wrote:
>> Add smp_cond_load_relaxed_timeout(), which extends
>> smp_cond_load_relaxed() to allow waiting for a duration.
>>
>> We loop around waiting for the condition variable to change while
>> peridically doing a time-check. The loop uses cpu_poll_relax() to slow
>> down the busy-waiting, which, unless overridden by the architecture
>> code, amounts to a cpu_relax().
>>
>> Note that there are two ways for the time-check to fail: the usual
>> timeout case or, @time_expr_ns returning an invalid value (negative
>> or zero). The second failure mode allows for clocks attached to the
>> clock-domain of @cond_expr, which might cease to operate meaningfully
>> once some state internal to @cond_expr has changed.
>>
>> Evaluation of @time_expr_ns: in the fastpath we want to keep the
>> performance close to smp_cond_load_relaxed(). To do that we defer
>> evaluation of the potentially costly @time_expr_ns to when we hit
>> the slowpath.
>>
>> This also means that there will always be some hardware dependent
>> duration that has passed in cpu_poll_relax() iterations at the time of
>> first evaluation. Additionally cpu_poll_relax() is not guaranteed to
>> return at timeout boundary. In sum, expect timeout overshoot when we
>> exit due to expiration of the timeout.
>>
>> The number of spin iterations before time-check, SMP_TIMEOUT_POLL_COUNT
>> is chosen to be 200 by default. With a cpu_poll_relax() iteration
>> taking ~20-30 cycles (measured on a variety of x86 platforms), we expect
>> a tim-check every ~4000-6000 cycles.
>>
>> The outer limit of the overshoot is double that when working with the
>> parameters above. This might be higher or lower depending on the
>> implementation of cpu_poll_relax() across architectures.
>>
>> Lastly, config option ARCH_HAS_CPU_RELAX indicates availability of a
>> cpu_poll_relax() that is cheaper than polling. This might be relevant
>> for cases with a prolonged timeout.
>>
>> Cc: Arnd Bergmann <arnd@...db.de>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: linux-arch@...r.kernel.org
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>
> This series evolved a bit since last time I looked, so going through it
> again:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Great. Thanks for the (re-)review!

--
ankur