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



Vivek Goyal wrote:
> On Wed, Sep 09, 2009 at 03:38:25PM +0800, Gui Jianfeng wrote:
>> 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).
> 
> Thanks Gui. As Jens said, it does look like a case of missing queue
> restart somewhere and now we are stuck, no requests are being dispatched
> to the disk and queue is already unplugged.
> 
> Can you please also try capturing the trace of events at io scheduler
> (blktrace) to see how did we get into that situation.

  Ok, I'll try.

> 
> Are you using ide drivers and not libata? As jens said, I will try to make
> use of ide drivers and see if I can reproduce it.

  Hi Vivek, Jens,

  yes, i used the old ide driver. So I switch to libata instead(deactivate the 
  whole “ATA/ATAPI/MFM/RLL support”), the bug still exists and I can reproduce
  it by that script.

> 
> Thanks
> Vivek
> 
> 
> 

-- 
Regards
Gui Jianfeng



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