linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

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Date:	Thu, 8 May 2008 08:43:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>
Subject: Re: AIM7 40% regression with 2.6.26-rc1


* Zhang, Yanmin <yanmin_zhang@...ux.intel.com> wrote:

> > Here's a trial balloon patch to do that.
> > 
> > Yanmin - this is not well tested, but the code is fairly obvious, 
> > and it would be interesting to hear if this fixes the performance 
> > regression. Because if it doesn't, then it's not the BKL, or 
> > something totally different is going on.
>
> Congratulations! The patch really fixes the regression completely! 
> vmstat showed cpu idle is 0%, just like 2.6.25's.

great! Yanmin, could you please also check the other patch i sent (also 
attached below), does it solve the regression similarly?

	Ingo

---
 lib/kernel_lock.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: linux/lib/kernel_lock.c
===================================================================
--- linux.orig/lib/kernel_lock.c
+++ linux/lib/kernel_lock.c
@@ -46,7 +46,8 @@ int __lockfunc __reacquire_kernel_lock(v
 	task->lock_depth = -1;
 	preempt_enable_no_resched();
 
-	down(&kernel_sem);
+	while (down_trylock(&kernel_sem))
+		cpu_relax();
 
 	preempt_disable();
 	task->lock_depth = saved_lock_depth;
@@ -67,11 +68,13 @@ void __lockfunc lock_kernel(void)
 	struct task_struct *task = current;
 	int depth = task->lock_depth + 1;
 
-	if (likely(!depth))
+	if (likely(!depth)) {
 		/*
 		 * No recursion worries - we set up lock_depth _after_
 		 */
-		down(&kernel_sem);
+		while (down_trylock(&kernel_sem))
+			cpu_relax();
+	}
 
 	task->lock_depth = depth;
 }
--
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