linux-kernel - Re: [PATCH v2 32/36] x86/entry: Micro-optimize compat fast syscall arg fetch

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Date:	Sat, 10 Oct 2015 11:05:46 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH v2 32/36] x86/entry: Micro-optimize compat fast syscall
 arg fetch


* Andy Lutomirski <luto@...capital.net> wrote:

> On Fri, Oct 9, 2015 at 12:32 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Andy Lutomirski <luto@...nel.org> wrote:
> >
> >> we're following a 32-bit pointer, and the uaccess code isn't smart
> >> enough to figure out that the access_ok check isn't needed.
> >>
> >> This saves about three cycles on a cache-hot fast syscall.
> >
> > Another request: could you please stick the benchmarking code of the various x86
> > system call variants into 'perf bench' - under tools/perf/bench/, so that
> > measurements can be done on more hardware and can be reproduced easily?
> >
> > I'd suggest we dedicate an entirely new benchmark family to it: 'perf bench x86'
> > and then have:
> >
> >    perf bench x86 syscall vdso
> >    perf bench x86 syscall int80
> >    perf bench x86 syscall vdso-compat
> 
> I'll play with this.  I'm not too familiar with the perf bench stuff.

So the perf bench stuff is meant to be a familiar home to kernel developers we'd 
like to slap a micro (or macro) benchmark into an easy to modify place.

Over the years it has gathered a number of benchmarks - but more are always 
welcome.

Just copy one of the existing benchmark modules (the tools/perf/bench/numa.c one 
is the most advanced one, tools/perf/bench/sched-pipe.c is the simplest one) and 
off you go.

Here's a commit that adds a new benchmark suite:

  a043971141f1 ("perf bench: Add futex-hash microbenchmark")

There are no big restrictions on the benchmarks: just put your existing code in 
that produces stdout output and it will be likely very close to upstream 
acceptable.

Can help should you get stuck anywhere.

Thanks,

	Ingo
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