linux-kernel - [PATCHv7 18/33] lib/vdso: Add unlikely() hint into vdso_read

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Message-Id: <20191011012341.846266-19-dima@arista.com>
Date:   Fri, 11 Oct 2019 02:23:26 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: [PATCHv7 18/33] lib/vdso: Add unlikely() hint into vdso_read_begin()

From: Andrei Vagin <avagin@...il.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
diff %  | 2	    | 0
-----------------------------------
stdev % | 0.3	    | 0.1

Signed-off-by: Andrei Vagin <avagin@...il.com>
Co-developed-by: Dmitry Safonov <dima@...sta.com>
Signed-off-by: Dmitry Safonov <dima@...sta.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();
-- 
2.23.0