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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: 0898a16a362d436464b34fa644d0d46efc81df92
Gitweb: https://git.kernel.org/tip/0898a16a362d436464b34fa644d0d46efc81df92
Author: Andrei Vagin <avagin@gmail.com>
AuthorDate: Tue, 12 Nov 2019 01:26:50
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 14 Jan 2020 12:20:47 +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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