Messages in this thread Patch in this message | | | Date | Wed, 7 May 2008 19:14:43 +0200 | From | Ingo Molnar <> | Subject | Re: AIM7 40% regression with 2.6.26-rc1 |
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* 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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