[PATCHv8,01/34] lib/vdso: Add unlikely() hint into vdso_read_begin()

Submitted by Jann Horn via Containers on Nov. 12, 2019, 1:26 a.m.

Details

Message ID 20191112012724.250792-2-dima@arista.com
State New
Series "kernel: Introduce Time Namespace"
Headers show

Commit Message

Jann Horn via Containers Nov. 12, 2019, 1:26 a.m.
From: Andrei Vagin <avagin@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

Patch hide | download patch | download mbox

diff %  | 2	    | 0
-----------------------------------
stdev % | 0.3	    | 0.1

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Co-developed-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Dmitry Safonov <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 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();