linux-kernel - Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

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Message-ID: <20130701122954.GD23515@pd.tnic>
Date:	Mon, 1 Jul 2013 14:29:54 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Wedson Almeida Filho <wedsonaf@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

On Mon, Jul 01, 2013 at 01:11:22PM +0200, Ingo Molnar wrote:
> Hm, a 6 seconds win looks _way_ too much - we don't execute that much 
> mutex code, let alone a portion of it.
> 
> This could perhaps be a bootup-to-bootup cache layout systematic jitter 
> artifact, which isn't captured by stddev observations?
> 
> Doing something like this with a relatively fresh version of perf:
> 
>   perf stat --repeat 10 -a --sync \
>    --pre 'make -s O=defconfig-build/ clean; echo 1 > /proc/sys/vm/drop_caches' \
>    make -s -j64 O=defconfig-build/ bzImage
> 
> ... might do the trick (untested!). (Also note the use of -a: this should 
> run on an otherwise quiescent system.)

Yep, I didn't run -a since I wanted to trace only the build process.
Btw, the build-kernel.sh script looks like this:

#!/bin/bash

NUM_CPUS=$(cat /proc/cpuinfo | grep processor | wc -l)
MAKE_OPTS=-j$(($NUM_CPUS+1))

echo 3 > /proc/sys/vm/drop_caches
make $MAKE_OPTS mrproper
make $MAKE_OPTS oldconfig
make $MAKE_OPTS
<EOF>

Let me try your perf tracing variant.

> As a sidenote, we could add this as a convenience feature, triggered via:
> 
>    perf stat --flush-vm-caches
> 
> ... or so, in addition to the already existing --sync option.

Is this something which we want to use a lot? Also, there's 1, 2 and 3
as arg to drop_caches:

drop_caches

Writing to this will cause the kernel to drop clean caches, dentries and
inodes from memory, causing that memory to become free.

To free pagecache:
	echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
	echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
	echo 3 > /proc/sys/vm/drop_caches

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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