From: Uros Bizjak <ubizjak@gmail.com>
To: Dave Hansen <dave.hansen@intel.com>,
Steven Rostedt <rostedt@goodmis.org>
Cc: linux-alpha@vger.kernel.org, loongarch@lists.linux.dev,
linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
x86@kernel.org, linux-arch@vger.kernel.org,
linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
Richard Henderson <richard.henderson@linaro.org>,
Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
Matt Turner <mattst88@gmail.com>,
Huacai Chen <chenhuacai@kernel.org>,
WANG Xuerui <kernel@xen0n.name>,
Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
Michael Ellerman <mpe@ellerman.id.au>,
Nicholas Piggin <npiggin@gmail.com>,
Christophe Leroy <christophe.leroy@csgroup.eu>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.c>
Subject: Re: [PATCH v2 0/5] locking: Introduce local{,64}_try_cmpxchg
Date: Wed, 5 Apr 2023 20:53:45 +0200 [thread overview]
Message-ID: <CAFULd4a6u=LB0ivfHtHt=jRxeJeLWuBot=Pync6pbrvKi=CdjA@mail.gmail.com> (raw)
In-Reply-To: <7360ffd2-a5aa-1373-8309-93e71ff36cbb@intel.com>
On Wed, Apr 5, 2023 at 6:37 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 4/5/23 07:17, Uros Bizjak wrote:
> > Add generic and target specific support for local{,64}_try_cmpxchg
> > and wire up support for all targets that use local_t infrastructure.
>
> I feel like I'm missing some context.
>
> What are the actual end user visible effects of this series? Is there a
> measurable decrease in perf overhead? Why go to all this trouble for
> perf? Who else will use local_try_cmpxchg()?
This functionality was requested by perf people [1], so perhaps Steven
can give us some concrete examples. In general, apart from the removal
of unneeded compare instruction on x86, usage of try_cmpxchg also
results in slightly better code on non-x86 targets [2], since the code
now correctly identifies fast-path through the cmpxchg loop.
Also important is that try_cmpxchg code reuses the result of cmpxchg
instruction in the loop, so a read from the memory in the loop is
eliminated. When reviewing the cmpxchg usage sites, I found numerous
places where unnecessary read from memory was present in the loop, two
examples can be seen in the last patch of this series.
Also, using try_cmpxchg prevents inconsistencies of the cmpxchg loop,
where the result of the cmpxchg is compared with the wrong "old" value
- one such bug is still lurking in x86 APIC code, please see [3].
Please note that apart from perf subsystem, event subsystem can also
be improved by using local_try_cmpxchg. This is the reason that the
last patch includes a change in events/core.c.
> I'm all for improving things, and perf is an important user. But, if
> the goal here is improving performance, it would be nice to see at least
> a stab at quantifying the performance delta.
[1] https://lore.kernel.org/lkml/20230301131831.6c8d4ff5@gandalf.local.home/
[2] https://lore.kernel.org/lkml/Yo91omfDZtTgXhyn@FVFF77S0Q05N.cambridge.arm.com/
[3] https://lore.kernel.org/lkml/20230227160917.107820-1-ubizjak@gmail.com/
Uros.
next prev parent reply other threads:[~2023-04-05 18:53 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-05 14:17 [PATCH v2 0/5] locking: Introduce local{,64}_try_cmpxchg Uros Bizjak
2023-04-05 14:17 ` [PATCH v2 1/5] locking/atomic: Add generic try_cmpxchg{,64}_local support Uros Bizjak
2023-04-11 11:10 ` Mark Rutland
2023-04-05 14:17 ` [PATCH v2 2/5] locking/generic: Wire up local{,64}_try_cmpxchg Uros Bizjak
2023-04-11 11:13 ` Mark Rutland
2023-04-05 14:17 ` [PATCH v2 3/5] locking/arch: Wire up local_try_cmpxchg Uros Bizjak
2023-04-12 11:32 ` Peter Zijlstra
2023-04-12 13:37 ` Uros Bizjak
2023-04-12 13:40 ` Peter Zijlstra
2023-05-17 7:41 ` Charlemagne Lasse
2023-04-05 14:17 ` [PATCH v2 4/5] locking/x86: Define arch_try_cmpxchg_local Uros Bizjak
2023-04-05 14:17 ` [PATCH v2 5/5] events: Illustrate the transition to local{,64}_try_cmpxchg Uros Bizjak
2023-04-05 16:37 ` [PATCH v2 0/5] locking: Introduce local{,64}_try_cmpxchg Dave Hansen
2023-04-05 18:53 ` Uros Bizjak [this message]
2023-04-06 8:25 ` David Laight
2023-04-06 8:38 ` Uros Bizjak
2023-04-06 9:01 ` David Laight
2023-04-11 11:35 ` Mark Rutland
2023-04-11 13:43 ` Dave Hansen
2023-04-11 21:34 ` David Laight
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