Commit 0898a16a authored Nov 12, 2019 by Andrei Vagin Committed by Thomas Gleixner Jan 14, 2020

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

parent cdb7c5a9

include/vdso/helpers.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -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();

Admin message