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SubjectRe: AIM7 40% regression with 2.6.26-rc1

* 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;
}

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