linux-kernel - [PATCH] x86/processor.h: Force inlining of cpu

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Date:	Thu, 24 Sep 2015 14:02:29 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] x86/processor.h: Force inlining of cpu_relax()

On x86, cpu_relax() simply calls rep_nop(), which generates one
instruction, PAUSE (aka REP NOP).

With this config:
http://busybox.net/~vda/kernel_config_OPTIMIZE_INLINING_and_Os
gcc-4.7.2 does not always inline rep_nop(): it generates
several copies of this:

<rep_nop> (16 copies, 194 calls):
       55                      push   %rbp
       48 89 e5                mov    %rsp,%rbp
       f3 90                   pause
       5d                      pop    %rbp
       c3                      retq

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122

This patch fixes this via s/inline/__always_inline/
on rep_nop() and cpu_relax().
(Forcing inlining only on rep_nop() causes gcc to
deinline cpu_relax(), with almost no change in generated code).

    text     data      bss       dec     hex filename
88118971 19905208 36421632 144445811 89c1173 vmlinux.before
88118139 19905208 36421632 144444979 89c0e33 vmlinux

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Ingo Molnar <mingo@...nel.org>
CC: Andy Lutomirski <luto@...capital.net>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Borislav Petkov <bp@...en8.de>
CC: Brian Gerst <brgerst@...il.com>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
---
 arch/x86/include/asm/processor.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 19577dd..b55f309 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -556,12 +556,12 @@ static inline unsigned int cpuid_edx(unsigned int op)
 }
 
 /* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
-static inline void rep_nop(void)
+static __always_inline void rep_nop(void)
 {
 	asm volatile("rep; nop" ::: "memory");
 }
 
-static inline void cpu_relax(void)
+static __always_inline void cpu_relax(void)
 {
 	rep_nop();
 }
-- 
1.8.1.4

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