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[dm-devel] Re: [RFC] IO scheduler based IO controller V9



Vivek Goyal wrote:
> On Mon, Sep 07, 2009 at 03:40:53PM +0800, Gui Jianfeng wrote:
>> Hi Vivek,
>>
>> I happened to encount a bug when i test IO Controller V9.
>> When there are three tasks to run concurrently in three group,
>> that is, one is parent group, and other two tasks are running 
>> in two different child groups respectively to read or write 
>> files in some disk, say disk "hdb", The task may hang up, and 
>> other tasks which access into "hdb" will also hang up.
>>
>> The bug only happens when using AS io scheduler.
>> The following scirpt can reproduce this bug in my box.
>>
> 
> Hi Gui,
> 
> I tried reproducing this on my system and can't reproduce it. All the
> three processes get killed and system does not hang.
> 
> Can you please dig deeper a bit into it. 
> 
> - If whole system hangs or it is just IO to disk seems to be hung.

    Only when the task is trying do IO to disk it will hang up.

> - Does io scheduler switch on the device work

    yes, io scheduler can be switched, and the hung task will be resumed.

> - If the system is not hung, can you capture the blktrace on the device.
>   Trace might give some idea, what's happening.

I run a "find" task to do some io on that disk, it seems that task hangs 
when it is issuing getdents() syscall.
kernel generates the following message:

INFO: task find:3260 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
find          D a1e95787  1912  3260   2897 0x00000004
 f6af2db8 00000096 f660075c a1e95787 00000032 f6600270 f6600508 c2037820
 00000000 c09e0820 f655f0c0 f6af2d8c fffebbf1 00000000 c0447323 f7152a1c
 0006a144 f7152a1c 0006a144 f6af2e04 f6af2db0 c04438df c2037820 c2037820
Call Trace:
 [<c0447323>] ? getnstimeofday+0x57/0xe0
 [<c04438df>] ? ktime_get_ts+0x4a/0x4e
 [<c068ab68>] io_schedule+0x47/0x79
 [<c04c12ee>] sync_buffer+0x36/0x3a
 [<c068ae14>] __wait_on_bit+0x36/0x5d
 [<c04c12b8>] ? sync_buffer+0x0/0x3a
 [<c068ae93>] out_of_line_wait_on_bit+0x58/0x60
 [<c04c12b8>] ? sync_buffer+0x0/0x3a
 [<c0440fa4>] ? wake_bit_function+0x0/0x43
 [<c04c1249>] __wait_on_buffer+0x19/0x1c
 [<f81e4186>] ext3_bread+0x5e/0x79 [ext3]
 [<f81e77a8>] htree_dirblock_to_tree+0x1f/0x120 [ext3]
 [<f81e7923>] ext3_htree_fill_tree+0x7a/0x1bb [ext3]
 [<c04a01f9>] ? kmem_cache_alloc+0x86/0xf3
 [<c044c428>] ? trace_hardirqs_on_caller+0x107/0x12f
 [<c044c45b>] ? trace_hardirqs_on+0xb/0xd
 [<f81e09e4>] ? ext3_readdir+0x9e/0x692 [ext3]
 [<f81e0b34>] ext3_readdir+0x1ee/0x692 [ext3]
 [<c04b1100>] ? filldir64+0x0/0xcd
 [<c068b86a>] ? mutex_lock_killable_nested+0x2b1/0x2c5
 [<c068b874>] ? mutex_lock_killable_nested+0x2bb/0x2c5
 [<c04b12db>] ? vfs_readdir+0x46/0x94
 [<c04b12fd>] vfs_readdir+0x68/0x94
 [<c04b1100>] ? filldir64+0x0/0xcd
 [<c04b1387>] sys_getdents64+0x5e/0x9f
 [<c04028b4>] sysenter_do_call+0x12/0x32
1 lock held by find/3260:
 #0:  (&sb->s_type->i_mutex_key#7){+.+.+.}, at: [<c04b12db>] vfs_readdir+0x46/0x94

ext3 calls wait_on_buffer() to wait buffer, and schedule the task out in TASK_UNINTERRUPTIBLE
state, and I found this task will be resumed after a quite long period(more than 10 mins).

-- 
Regards
Gui Jianfeng



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