linux-kernel - Re: Why kmem_cache_free occupy CPU for more than 10 seconds?

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Date:	Wed, 11 Apr 2007 12:38:31 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Zhao Forrest <forrest.zhao@...il.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: Why kmem_cache_free occupy CPU for more than 10 seconds?

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@...sh-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@...sh-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)

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;




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