[tip: timers/core] lib/vdso: Add unlikely() hint into vdso_read_begin()

[tip-bot2 for Andrei Vagin]

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@xxxxxxxxx>
AuthorDate:    Tue, 12 Nov 2019 01:26:50 
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Andrei Vagin <avagin@xxxxxxxxx>
Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Link: https://lore.kernel.org/r/20191112012724.250792-2-dima@xxxxxxxxxx

---
 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();