linux-kernel - [tip: timers/core] lib/vdso: Add unlikely() hint into vdso_read

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Date:   Tue, 14 Jan 2020 13:02:21 -0000
From:   "tip-bot2 for Andrei Vagin" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Andrei Vagin <avagin@...il.com>, Dmitry Safonov <dima@...sta.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
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@...il.com>
AuthorDate:    Tue, 12 Nov 2019 01:26:50 
Committer:     Thomas Gleixner <tglx@...utronix.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@...sta.com>
Signed-off-by: Andrei Vagin <avagin@...il.com>
Signed-off-by: Dmitry Safonov <dima@...sta.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Vincenzo Frascino <vincenzo.frascino@....com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....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();