From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: AIM7 40% regression with 2.6.26-rc1 Date: Wed, 7 May 2008 20:17:14 +0200 Message-ID: <20080507181714.GA14980@elte.hu> References: <20080506120934.GH19219@parisc-linux.org> <20080506162332.GI19219@parisc-linux.org> <20080506102153.5484c6ac.akpm@linux-foundation.org> <20080507163811.GY19219@parisc-linux.org> <20080507172246.GA13262@elte.hu> <20080507174900.GB13591@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Matthew Wilcox , Andrew Morton , "J. Bruce Fields" , "Zhang, Yanmin" , LKML , Alexander Viro , linux-fsdevel@vger.kernel.org To: Linus Torvalds Return-path: Received: from mx3.mail.elte.hu ([157.181.1.138]:59785 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753495AbYEGSRb (ORCPT ); Wed, 7 May 2008 14:17:31 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: * Linus Torvalds wrote: > On Wed, 7 May 2008, Ingo Molnar wrote: > > > > another idea: my trial-baloon patch should test your theory too, > > because the generic down_trylock() is still the 'fat' version, it > > does: > > I agree that your trial-balloon should likely get rid of the big > regression, since it avoids the scheduler. > > So with your patch, lock_kernel() ends up being just a rather > expensive spinlock. And yes, I'd expect that it should get rid of the > 40% cost, because while it makes lock_kernel() more expensive than a > spinlock and you might end up having a few more cacheline bounces on > the lock due to that, that's still the "small" expense compared to > going through the whole scheduler on conflicts. > > So I'd expect that realistically the performance difference between > your version and just plain spinlocks shouldn't be *that* big. I'd > expect it to be visible, but in the (low) single-digit percentage > range rather than in any 40% range. That's just a guess. third attempt - the patch below ontop of v2.6.25 should be quite similar fastpath atomic overhead to what generic semaphores do? So if Yanmin tests this patch ontop of v2.6.25, we should see the direct fastpath overhead - without any changes to the semaphore wakeup/scheduling logic otherwise. [ this patch should in fact be a bit worse, because there's two more atomics in the fastpath - the fastpath atomics of the old semaphore code. ] Ingo ------------------> Subject: v2.6.25 BKL: add atomic overhead From: Ingo Molnar Date: Wed May 07 20:09:13 CEST 2008 --- lib/kernel_lock.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) Index: linux-2.6.25/lib/kernel_lock.c =================================================================== --- linux-2.6.25.orig/lib/kernel_lock.c +++ linux-2.6.25/lib/kernel_lock.c @@ -24,6 +24,7 @@ * Don't use in new code. */ static DECLARE_MUTEX(kernel_sem); +static DEFINE_SPINLOCK(global_lock); /* * Re-acquire the kernel semaphore. @@ -47,6 +48,9 @@ int __lockfunc __reacquire_kernel_lock(v down(&kernel_sem); + spin_lock(&global_lock); + spin_unlock(&global_lock); + preempt_disable(); task->lock_depth = saved_lock_depth; @@ -55,6 +59,9 @@ int __lockfunc __reacquire_kernel_lock(v void __lockfunc __release_kernel_lock(void) { + spin_lock(&global_lock); + spin_unlock(&global_lock); + up(&kernel_sem); } @@ -66,12 +73,16 @@ 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); + spin_lock(&global_lock); + spin_unlock(&global_lock); + } + task->lock_depth = depth; }