linux-kernel - Re: [tip:sched/core] sched, x86: Optimize branch hint in __switch

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Message-ID: <4B0CF2D0.90600@klingt.org>
Date:	Wed, 25 Nov 2009 10:03:12 +0100
From:	Tim Blechmann <tim@...ngt.org>
To:	Brian Gerst <brgerst@...il.com>
CC:	mingo@...hat.com, hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	a.p.zijlstra@...llo.nl, efault@....de, tglx@...utronix.de, tim@kl
Subject: Re: [tip:sched/core] sched, x86: Optimize branch hint in    __switch_to()

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On 11/24/2009 06:28 PM, Brian Gerst wrote:
> On Tue, Nov 24, 2009 at 11:57 AM, tip-bot for Tim Blechmann
> <tim@...ngt.org> wrote:
>> Commit-ID:  a3a1de0c34de6f5f8332cd6151c46af7813c0fcb
>> Gitweb:     http://git.kernel.org/tip/a3a1de0c34de6f5f8332cd6151c46af7813c0fcb
>> Author:     Tim Blechmann <tim@...ngt.org>
>> AuthorDate: Tue, 24 Nov 2009 11:55:15 +0100
>> Committer:  Ingo Molnar <mingo@...e.hu>
>> CommitDate: Tue, 24 Nov 2009 12:20:04 +0100
>>
>> sched, x86: Optimize branch hint in __switch_to()
>>
>> Branch hint profiling on my nehalem machine showed 96%
>> incorrect branch hints:
>>
>>  6548732 174664120  96 __switch_to                    process_64.c
>>    406
>>  6548745 174565593  96 __switch_to                    process_64.c
>>    410
>>
>> Signed-off-by: Tim Blechmann <tim@...ngt.org>
>> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>> Cc: Mike Galbraith <efault@....de>
>> Cc: Paul Mackerras <paulus@...ba.org>
>> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
>> Cc: Frederic Weisbecker <fweisbec@...il.com>
>> LKML-Reference: <4B0BBB93.3080307@...ngt.org>
>> Signed-off-by: Ingo Molnar <mingo@...e.hu>
>> ---
>>  arch/x86/kernel/process_64.c |    5 ++---
>>  1 files changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>> index ad535b6..d9db104 100644
>> --- a/arch/x86/kernel/process_64.c
>> +++ b/arch/x86/kernel/process_64.c
>> @@ -406,11 +406,10 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>>         * This won't pick up thread selector changes, but I guess that is ok.
>>         */
>>        savesegment(es, prev->es);
>> -       if (unlikely(next->es | prev->es))
>> +       if (next->es | prev->es)
>>                loadsegment(es, next->es);
>> -
>>        savesegment(ds, prev->ds);
>> -       if (unlikely(next->ds | prev->ds))
>> +       if (next->ds | prev->ds)
>>                loadsegment(ds, next->ds);
>>
>>
> 
> 64-bit tasks should have %ds and %es set to null selectors.  The only
> time they should be different is for 32-bit tasks.
	
this doesn't seem to be the case on my machine for 96% of the calls ...
i am just running 64bit binaries on this machine, no 32bit programs
(that i know of (ubuntu karmic amd64))

tim

- -- 
tim@...ngt.org
http://tim.klingt.org

Relying on the government to protect your privacy is like asking a
peeping tom to install your window blinds.
  John Perry Barlow
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