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

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Message-ID: <aMPyiKuzGh4L4gD8@arm.com>
Date: Fri, 12 Sep 2025 11:14:32 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.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, Sep 11, 2025 at 02:58:22PM -0700, Ankur Arora wrote:
> 
> 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.

Does this spin count work for poll_idle()? I don't remember where the
200 value came from.

-- 
Catalin