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

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Date:   Tue, 11 Apr 2023 21:34:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        Mark Rutland <mark.rutland@....com>
CC:     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>,
        "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>,
        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: 11 April 2023 14:44
> 
> On 4/11/23 04:35, Mark Rutland wrote:
> > I agree it'd be nice to have performance figures, but I think those would only
> > need to demonstrate a lack of a regression rather than a performance
> > improvement, and I think it's fairly clear from eyeballing the generated
> > instructions that a regression isn't likely.
> 
> Thanks for the additional context.
> 
> I totally agree that there's zero burden here to show a performance
> increase.  If anyone can think of a quick way to do _some_ kind of
> benchmark on the code being changed and just show that it's free of
> brown paper bags, it would be appreciated.  Nothing crazy, just think of
> one workload (synthetic or not) that will stress the paths being changed
> and run it with and without these changes.  Make sure there are not
> surprises.
> 
> I also agree that it's unlikely to be brown paper bag material.

The only thing I can think of is that, on x86, the locked
variant may actually be faster!
Both require exclusive access to the cache line (the unlocked
variant always does the write! [1]).
So if the cache line is contended between cpu the unlocked
variant might ping-pong the cache line twice!
Of course, if the line is shared like that then performance
is horrid.

[1] I checked on an uncached PCIe address on which I can monitor
the TLP. The write always happens so you can use cmpxchg18b
with a 'known bad value' to do a 16 byte read as a single TLP
(without using an SSE register).

	David

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