linux-kernel - [PATCHv3 25/27] x86/vdso: Align VDSO functions by CPU L1 cache line

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Message-Id: <20190425161416.26600-26-dima@arista.com>
Date:   Thu, 25 Apr 2019 17:14:14 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     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>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.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: [PATCHv3 25/27] x86/vdso: Align VDSO functions by CPU L1 cache line

From: Andrei Vagin <avagin@...il.com>

After performance testing VDSO patches a noticeable 20% regression was
found on gettime_perf selftest with a cold cache.
As it turns to be, before time namespaces introduction, VDSO functions
were quite aligned to cache lines, but adding a new code to adjust
timens offset inside namespace created a small shift and vdso functions
become unaligned on cache lines.

Add align to vdso functions with gcc option to fix performance drop.

Coping the resulting numbers from cover letter:

Hot CPU cache (more gettime_perf.c cycles - the better):
        | before     | CONFIG_TIME_NS=n | host        | inside timens
--------|------------|------------------|-------------|-------------
cycles  | 139887013  | 139453003        | 139899785   | 128792458
diff (%)| 100        | 99.7             | 100         | 92

Cold cache (lesser tsc per gettime_perf_cold.c cycle - the better):
        | before     | CONFIG_TIME_NS=n | host        | inside timens
--------|------------|------------------|-------------|-------------
tsc     | 6748       | 6718             | 6862        | 12682
diff (%)| 100        | 99.6             | 101.7       | 188

Measured on Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz

Co-developed-by: Dmitry Safonov <dima@...sta.com>
Signed-off-by: Andrei Vagin <avagin@...il.com>
Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 arch/x86/entry/vdso/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index bc0bdbf49397..9b45aafdfcfe 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -4,6 +4,7 @@
 #
 
 KBUILD_CFLAGS += $(DISABLE_LTO)
+KBUILD_CFLAGS += -falign-functions=$(CONFIG_X86_L1_CACHE_SHIFT)
 KASAN_SANITIZE			:= n
 UBSAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
-- 
2.21.0