Re: AIM7 40% regression with 2.6.26-rc1

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > But my preferred option would indeed be just turning it back into a 
> > spinlock - and screw latency and BKL preemption - and having the RT 
> > people who care deeply just work on removing the BKL in the long 
> > run.
> 
> Here's a trial balloon patch to do that.

here's a simpler trial baloon test-patch (well, hack) that is also 
reasonably well tested. It turns the BKL into a "spin-semaphore". If 
this resolves the performance problem then it's all due to the BKL's 
scheduling/preemption properties.

this approach is ugly (it's just a more expensive spinlock), but has an 
advantage: the code logic is obviously correct, and it would also make 
it much easier later on to turn the BKL back into a sleeping lock again 
- once the TTY code's BKL use is fixed. (i think Alan said it might 
happen in the next few months) The BKL is more expensive than a simple 
spinlock anyway.

	Ingo

------------->
Subject: BKL: spin on acquire
From: Ingo Molnar <mingo@elte.hu>
Date: Wed May 07 19:05:40 CEST 2008

NOT-Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 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;
 }