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    Subject[tip: timers/core] lib/vdso: Add unlikely() hint into vdso_read_begin()
    The following commit has been merged into the timers/core branch of tip:

    Commit-ID: 79472606d6fba66c20299f4121a657ce73efc302
    Gitweb: https://git.kernel.org/tip/79472606d6fba66c20299f4121a657ce73efc302
    Author: Andrei Vagin <avagin@gmail.com>
    AuthorDate: Tue, 12 Nov 2019 01:26:50
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitterDate: Mon, 13 Jan 2020 08:10:46 +01:00

    lib/vdso: Add unlikely() hint into vdso_read_begin()

    Place the branch with no concurrent write before the contended case.

    Performance numbers for Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
    (more clock_gettime() cycles - the better):
    | before | after
    -----------------------------------
    | 150252214 | 153242367
    | 150301112 | 153324800
    | 150392773 | 153125401
    | 150373957 | 153399355
    | 150303157 | 153489417
    | 150365237 | 153494270
    -----------------------------------
    avg | 150331408 | 153345935
    diff % | 2 | 0
    -----------------------------------
    stdev % | 0.3 | 0.1

    Co-developed-by: Dmitry Safonov <dima@arista.com>
    Signed-off-by: Andrei Vagin <avagin@gmail.com>
    Signed-off-by: Dmitry Safonov <dima@arista.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
    Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
    Link: https://lore.kernel.org/r/20191112012724.250792-2-dima@arista.com

    ---
    include/vdso/helpers.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/include/vdso/helpers.h b/include/vdso/helpers.h
    index 01641db..9a2af9f 100644
    --- a/include/vdso/helpers.h
    +++ b/include/vdso/helpers.h
    @@ -10,7 +10,7 @@ static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
    {
    u32 seq;

    - while ((seq = READ_ONCE(vd->seq)) & 1)
    + while (unlikely((seq = READ_ONCE(vd->seq)) & 1))
    cpu_relax();

    smp_rmb();
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