From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by ug-out-1314.google.com with SMTP id s2so244953uge for ; Wed, 11 Apr 2007 23:17:42 -0700 (PDT) Message-ID: Date: Thu, 12 Apr 2007 14:17:37 +0800 From: "Zhao Forrest" Subject: Re: Why kmem_cache_free occupy CPU for more than 10 seconds? In-Reply-To: <1176287911.6893.47.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <84144f020704102353r7dcc3538u2e34237d3496630e@mail.gmail.com> <1176287911.6893.47.camel@twins> Sender: owner-linux-mm@kvack.org Return-Path: To: Peter Zijlstra Cc: Pekka Enberg , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On 4/11/07, Peter Zijlstra wrote: > On Wed, 2007-04-11 at 17:53 +0800, Zhao Forrest wrote: > > I got some new information: > > Before soft lockup message is out, we have: > > [root@nsgsh-dhcp-149 home]# cat /proc/slabinfo |grep buffer_head > > buffer_head 10927942 10942560 120 32 1 : tunables 32 > > 16 8 : slabdata 341955 341955 6 : globalstat 37602996 11589379 > > 1174373 6 0 1 6918 12166031 1013708 > > : cpustat 35254590 2350698 13610965 907286 > > > > Then after buffer_head is freed, we have: > > [root@nsgsh-dhcp-149 home]# cat /proc/slabinfo |grep buffer_head > > buffer_head 9542 36384 120 32 1 : tunables 32 16 > > 8 : slabdata 1137 1137 245 : globalstat 37602996 11589379 > > 1174373 6 0 1 6983 20507478 > > 1708818 : cpustat 35254625 2350704 16027174 1068367 > > > > Does this huge number of buffer_head cause the soft lockup? > > > __blkdev_put() takes the BKL and bd_mutex > invalidate_mapping_pages() tries to take the PageLock > > But no other looks seem held while free_buffer_head() is called > > All these locks are preemptible (CONFIG_PREEMPT_BKL?=y) and should not > hog the cpu like that, what preemption mode have you got selected? > (CONFIG_PREEMPT_VOLUNTARY?=y) These 2 kernel options are turned on by default in my kernel. Here's snip from .config # CONFIG_PREEMPT_NONE is not set CONFIG_PREEMPT_VOLUNTARY=y # CONFIG_PREEMPT is not set CONFIG_PREEMPT_BKL=y CONFIG_NUMA=y CONFIG_K8_NUMA=y > > Does this fix it? > > --- fs/buffer.c~ 2007-02-01 12:00:34.000000000 +0100 > +++ fs/buffer.c 2007-04-11 12:35:48.000000000 +0200 > @@ -3029,6 +3029,8 @@ out: > struct buffer_head *next = bh->b_this_page; > free_buffer_head(bh); > bh = next; > + > + cond_resched(); > } while (bh != buffers_to_free); > } > return ret; > So far I have run the test with patched kernel for 6 rounds, and didn't see the soft lockup. I think this patch should fix the problem. But what still confused me is that why do we need to invoke cond_resched() voluntarily since CONFIG_PREEMPT_VOLUNTARY and CONFIG_PREEMPT_BKL are both turned on? From my understanding these 2 options should make schedule happen even if CPU is under heavy load...... Thanks, Forrest -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org