linux-kernel - Re: [patch] speed up / fix the new generic semaphore code (fix AIM7 40% regression with 2.6.26-rc1)

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Date:	Thu, 8 May 2008 14:28:02 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch] speed up / fix the new generic semaphore code (fix
	AIM7 40% regression with 2.6.26-rc1)


* Ingo Molnar <mingo@...e.hu> wrote:

> +	if (unlikely(sem->count <= 0))
>  		__down(sem);
> +	sem->count--;

Peter pointed it out that because sem->count is u32, the <= 0 is in fact 
a "== 0" condition - the patch below does that. As expected gcc figured 
out the same thing too so the resulting code output did not change. (so 
this is just a cleanup)

i've got this lined up in sched.git and it's undergoing testing right 
now. If that testing goes fine and if there are no objections i'll send 
a pull request for it later today.

	Ingo

---------------->
Subject: semaphores: improve code
From: Ingo Molnar <mingo@...e.hu>
Date: Thu May 08 14:19:23 CEST 2008

No code changed:

kernel/semaphore.o:

   text	   data	    bss	    dec	    hex	filename
   1207	      0	      0	   1207	    4b7	semaphore.o.before
   1207	      0	      0	   1207	    4b7	semaphore.o.after

md5:
   c10198c2952bd345a1edaac6db891548  semaphore.o.before.asm
   c10198c2952bd345a1edaac6db891548  semaphore.o.after.asm

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/semaphore.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: linux/kernel/semaphore.c
===================================================================
--- linux.orig/kernel/semaphore.c
+++ linux/kernel/semaphore.c
@@ -54,7 +54,7 @@ void down(struct semaphore *sem)
 	unsigned long flags;
 
 	spin_lock_irqsave(&sem->lock, flags);
-	if (unlikely(sem->count <= 0))
+	if (unlikely(!sem->count))
 		__down(sem);
 	sem->count--;
 	spin_unlock_irqrestore(&sem->lock, flags);
@@ -76,7 +76,7 @@ int down_interruptible(struct semaphore 
 	int result = 0;
 
 	spin_lock_irqsave(&sem->lock, flags);
-	if (unlikely(sem->count <= 0))
+	if (unlikely(!sem->count))
 		result = __down_interruptible(sem);
 	if (!result)
 		sem->count--;
@@ -102,7 +102,7 @@ int down_killable(struct semaphore *sem)
 	int result = 0;
 
 	spin_lock_irqsave(&sem->lock, flags);
-	if (unlikely(sem->count <= 0))
+	if (unlikely(!sem->count))
 		result = __down_killable(sem);
 	if (!result)
 		sem->count--;
@@ -156,7 +156,7 @@ int down_timeout(struct semaphore *sem, 
 	int result = 0;
 
 	spin_lock_irqsave(&sem->lock, flags);
-	if (unlikely(sem->count <= 0))
+	if (unlikely(!sem->count))
 		result = __down_timeout(sem, jiffies);
 	if (!result)
 		sem->count--;
--
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