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From:   Uros Bizjak <ubizjak@gmail.com>
Date:   Wed, 5 Apr 2023 20:53:45 +0200
Message-ID: <CAFULd4a6u=LB0ivfHtHt=jRxeJeLWuBot=Pync6pbrvKi=CdjA@mail.gmail.com>
Subject: Re: [PATCH v2 0/5] locking: Introduce local{,64}_try_cmpxchg
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.com>, Arnd Bergmann <arnd@arndb.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Jun Yi <yijun@loongson.cn>
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On Wed, Apr 5, 2023 at 6:37=E2=80=AFPM 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.ar=
m.com/
[3] https://lore.kernel.org/lkml/20230227160917.107820-1-ubizjak@gmail.com/

Uros.