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    SubjectRe: [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@klingt.org> wrote:
    >> Commit-ID: a3a1de0c34de6f5f8332cd6151c46af7813c0fcb
    >> Gitweb: http://git.kernel.org/tip/a3a1de0c34de6f5f8332cd6151c46af7813c0fcb
    >> Author: Tim Blechmann <tim@klingt.org>
    >> AuthorDate: Tue, 24 Nov 2009 11:55:15 +0100
    >> Committer: Ingo Molnar <mingo@elte.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@klingt.org>
    >> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    >> Cc: Mike Galbraith <efault@gmx.de>
    >> Cc: Paul Mackerras <paulus@samba.org>
    >> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    >> Cc: Frederic Weisbecker <fweisbec@gmail.com>
    >> LKML-Reference: <4B0BBB93.3080307@klingt.org>
    >> Signed-off-by: Ingo Molnar <mingo@elte.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@klingt.org
    http://tim.klingt.org

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    peeping tom to install your window blinds.
    John Perry Barlow
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