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

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Date:	Tue, 24 Nov 2009 12:28:18 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	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@...ngt.org, mingo@...e.hu
Subject: Re: [tip:sched/core] sched, x86: Optimize branch hint in 
	__switch_to()

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.  It lookx like some
versions of GCC just aren't using the hint.  I've tested it with 4.4.2
and the generated assembly is the same with or without the hint.

--
Brian Gerst
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