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

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Message-ID: <CAP01T74XjRJGzT7BV3PYQOQOwZVud3nL7HfcW3kzU_fPFekNHg@mail.gmail.com>
Date: Thu, 11 Sep 2025 20:54:59 +0200
From: Kumar Kartikeya Dwivedi <memxor@...il.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, 
	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()

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.

> > [...]