On 10/11/19 2:23 AM, Dmitry Safonov wrote:
> From: Andrei Vagin <ava...@gmail.com>
>
> Place the branch with no concurrent write before 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
>
> Signed-off-by: Andrei Vagin <ava...@gmail.com>
> Co-developed-by: Dmitry Safonov <d...@arista.com>
> Signed-off-by: Dmitry Safonov <d...@arista.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frasc...@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frasc...@arm.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 01641dbb68ef..9a2af9fca45e 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();
>
--
Regards,
Vincenzo
