linux-kernel - Re: [PATCH v5 5/5] rqspinlock: Use smp_cond_load_acquire

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Message-ID: <87o6rgk5xd.fsf@oracle.com>
Date: Thu, 11 Sep 2025 14:58:22 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
        Ankur Arora
 <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.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,
        zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v5 5/5] rqspinlock: Use smp_cond_load_acquire_timeout()


Kumar Kartikeya Dwivedi <memxor@...il.com> writes:

> On Thu, 11 Sept 2025 at 16:32, Catalin Marinas <catalin.marinas@....com> wrote:
>>
>> On Wed, Sep 10, 2025 at 08:46:55PM -0700, Ankur Arora wrote:
>> > Switch out the conditional load inerfaces used by rqspinlock
>> > to smp_cond_read_acquire_timeout().
>> > This interface handles the timeout check explicitly and does any
>> > necessary amortization, so use check_timeout() directly.
>>
>> It's worth mentioning that the default smp_cond_load_acquire_timeout()
>> implementation (without hardware support) only spins 200 times instead
>> of 16K times in the rqspinlock code. That's probably fine but it would
>> be good to have confirmation from Kumar or Alexei.
>>
>
> This looks good, but I would still redefine the spin count from 200 to
> 16k for rqspinlock.c, especially because we need to keep
> RES_CHECK_TIMEOUT around which still uses 16k spins to amortize
> check_timeout.

By my count that amounts to ~100us per check_timeout() on x86
systems I've tested with cpu_relax(). Which seems quite reasonable.

16k also seems safer on CPUs where cpu_relax() is basically a NOP.

--
ankur