From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753283AbcHNOSV (ORCPT <rfc822;w@1wt.eu>);
	Sun, 14 Aug 2016 10:18:21 -0400
Received: from mail-oi0-f67.google.com ([209.85.218.67]:36545 "EHLO
	mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752595AbcHNOST (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sun, 14 Aug 2016 10:18:19 -0400
MIME-Version: 1.0
In-Reply-To: <20160813184534.GA15037@gmail.com>
References: <1471106302-10159-1-git-send-email-brgerst@gmail.com>
 <CA+55aFwb=Lk6xB-gd0eWutu5NM53RX34uAToKaokm5hRRSTsfw@mail.gmail.com>
 <CAMzpN2jLBZHwkhoi_aoV4ZBEJPhFXjB3US4GqmQFAaiGeX+kYQ@mail.gmail.com> <20160813184534.GA15037@gmail.com>
From: Brian Gerst <brgerst@gmail.com>
Date: Sun, 14 Aug 2016 10:18:18 -0400
Message-ID: <CAMzpN2hSoebz26z+ithTCc-PRGpDEcLX2Q=63rgW3h9d9y+3vw@mail.gmail.com>
Subject: Re: [PATCH v3 0/7] x86: Rewrite switch_to()
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>, Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sat, Aug 13, 2016 at 2:45 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
>> On Sat, Aug 13, 2016 at 1:16 PM, Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>> > On Sat, Aug 13, 2016 at 9:38 AM, Brian Gerst <brgerst@gmail.com> wrote:
>> >> This patch set simplifies the switch_to() code, by moving the stack switch
>> >> code out of line into an asm stub before calling __switch_to().  This ends
>> >> up being more readable, and using the C calling convention instead of
>> >> clobbering all registers improves code generation.  It also allows newly
>> >> forked processes to construct a special stack frame to seamlessly flow
>> >> to ret_from_fork, instead of using a test and branch, or an unbalanced
>> >> call/ret.
>> >
>> > Do you have performance numbers? Is it noticeable/measurable?
>>
>> How do I measure it?  The perf documentation isn't easy to understand.
>
> Something like this:
>
>   taskset 1 perf stat -a -e '{instructions,cycles}' --repeat 10 perf bench sched pipe
>
> ... will give a very good idea about the general impact of these changes on
> context switch overhead.

Before:
 Performance counter stats for 'system wide' (10 runs):

    12,010,932,128      instructions              #    1.03  insn per
cycle                                              ( +-  0.31% )
    11,691,797,513      cycles
               ( +-  0.76% )

       3.487329979 seconds time elapsed
          ( +-  0.78% )

After:
 Performance counter stats for 'system wide' (10 runs):

    12,097,706,506      instructions              #    1.04  insn per
cycle                                              ( +-  0.14% )
    11,612,167,742      cycles
               ( +-  0.81% )

       3.451278789 seconds time elapsed
          ( +-  0.82% )

The numbers with or without this patch series are roughly the same.
There is noticeable variation in the numbers each time I run it, so
I'm not sure how good of a benchmark this is.

--
Brian Gerst