linux-kernel - RE: [PATCH v2 0/5] locking: Introduce local{,64}_try

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Message-ID: <5c10520ac747430cb421badcb293c706@AcuMS.aculab.com>
Date:   Thu, 6 Apr 2023 08:25:57 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        Uros Bizjak <ubizjak@...il.com>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Richard Henderson <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Nicholas Piggin" <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        "Namhyung Kim" <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Jun Yi <yijun@...ngson.cn>
Subject: RE: [PATCH v2 0/5] locking: Introduce local{,64}_try_cmpxchg

From: Dave Hansen
> Sent: 05 April 2023 17:37
> 
> 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()?

I'm assuming the local_xxx operations only have to be save wrt interrupts?
On x86 it is possible that an alternate instruction sequence
that doesn't use a locked instruction may actually be faster!

Although, maybe, any kind of locked cmpxchg just needs to ensure
the cache line isn't 'stolen', so apart from possible slight
delays on another cpu that gets a cache miss for the line in
all makes little difference.
The cache line miss costs a lot anyway, line bouncing more
and is best avoided.
So is there actually much of a benefit at all?

Clearly the try_cmpxchg help - but that is a different issue.

	David

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