linux-kernel - Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG

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Message-ID: <Yo91omfDZtTgXhyn@FVFF77S0Q05N.cambridge.arm.com>
Date:   Thu, 26 May 2022 13:42:35 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Waiman.Long@...com,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP
 macro

On Thu, May 26, 2022 at 10:14:59PM +1000, Michael Ellerman wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> > On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak@...il.com> wrote:
> >>
> >> Use try_cmpxchg64 instead of cmpxchg64 in CMPXCHG_LOOP macro.
> >> x86 CMPXCHG instruction returns success in ZF flag, so this
> >> change saves a compare after cmpxchg (and related move instruction
> >> in front of cmpxchg). The main loop of lockref_get improves from:
> >
> > Ack on this one regardless of the 32-bit x86 question.
> >
> > HOWEVER.
> >
> > I'd like other architectures to pipe up too, because I think right now
> > x86 is the only one that implements that "arch_try_cmpxchg()" family
> > of operations natively, and I think the generic fallback for when it
> > is missing might be kind of nasty.
> >
> > Maybe it ends up generating ok code, but it's also possible that it
> > just didn't matter when it was only used in one place in the
> > scheduler.
> 
> This patch seems to generate slightly *better* code on powerpc.
> 
> I see one register-to-register move that gets shifted slightly later, so
> that it's skipped on the path that returns directly via the SUCCESS
> case.

FWIW, I see the same on arm64; a register-to-register move gets moved out of
the success path. That changes the register allocation, and resulting in one
fewer move, but otherwise the code generation is the same.

Thanks,
Mark.