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From: Heiko Carstens <hca@linux.ibm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Uros Bizjak <ubizjak@gmail.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Waiman.Long@hp.com, Paul McKenney <paulmck@linux.vnet.ibm.com>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro
Date: Fri, 27 May 2022 11:36:08 +0200	[thread overview]
Message-ID: <YpCbiBCx1o38uhhX@osiris> (raw)
In-Reply-To: <Yo91omfDZtTgXhyn@FVFF77S0Q05N.cambridge.arm.com>

On Thu, May 26, 2022 at 01:42:35PM +0100, Mark Rutland wrote:
> On Thu, May 26, 2022 at 10:14:59PM +1000, Michael Ellerman wrote:
> > Linus Torvalds <torvalds@linux-foundation.org> writes:
> > > On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak@gmail.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.

Just for the records: s390 code generation changes the same like on
powerpc; so looks good.

WARNING: multiple messages have this Message-ID (diff)
From: Heiko Carstens <hca@linux.ibm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Waiman.Long@hp.com,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Uros Bizjak <ubizjak@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linuxppc-dev@lists.ozlabs.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro
Date: Fri, 27 May 2022 11:36:08 +0200	[thread overview]
Message-ID: <YpCbiBCx1o38uhhX@osiris> (raw)
In-Reply-To: <Yo91omfDZtTgXhyn@FVFF77S0Q05N.cambridge.arm.com>

On Thu, May 26, 2022 at 01:42:35PM +0100, Mark Rutland wrote:
> On Thu, May 26, 2022 at 10:14:59PM +1000, Michael Ellerman wrote:
> > Linus Torvalds <torvalds@linux-foundation.org> writes:
> > > On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak@gmail.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.

Just for the records: s390 code generation changes the same like on
powerpc; so looks good.

  reply	other threads:[~2022-05-27  9:37 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-25 14:40 [PATCH 0/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro Uros Bizjak
2022-05-25 14:40 ` [PATCH 1/2] " Uros Bizjak
2022-05-25 16:47   ` Linus Torvalds
2022-05-26  8:54     ` Uros Bizjak
2022-05-26 12:14     ` Michael Ellerman
2022-05-26 12:14       ` Michael Ellerman
2022-05-26 12:42       ` Mark Rutland
2022-05-26 12:42         ` Mark Rutland
2022-05-27  9:36         ` Heiko Carstens [this message]
2022-05-27  9:36           ` Heiko Carstens
2022-05-26 16:52       ` Linus Torvalds
2022-05-26 16:52         ` Linus Torvalds
2022-05-26 16:56     ` Linus Torvalds
2022-05-25 14:40 ` [PATCH 2/2] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86_32 && X86_CMPXCHG64 Uros Bizjak
2022-05-25 16:29   ` Linus Torvalds
2022-05-26  8:30     ` David Laight
2022-05-26  9:12       ` Uros Bizjak
2022-05-26 17:23       ` Linus Torvalds

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