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

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Message-ID: <73c1f2160911250753q6c64791exabd79e994b315076@mail.gmail.com>
Date:	Wed, 25 Nov 2009 10:53:56 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Tim Blechmann <tim@...ngt.org>
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
Subject: Re: [tip:sched/core] sched, x86: Optimize branch hint in 
	__switch_to()

On Wed, Nov 25, 2009 at 4:03 AM, Tim Blechmann <tim@...ngt.org> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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

Some early kernel threads inherited KERNEL_DS from the boot process.
Patch coming soon.

--
Brian Gerst
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