On 02/17/2010 09:39 PM, Jan Kara wrote:
>> On 2/13/2010 11:51 AM, Michael Breuer wrote:
>>      
>>> Scenario:
>>>
>>> 1. raid6 (software - 6 1Tb sata drives) doing a resync (multi core
>>> enabled)
>>> 2. rebuilding kernel (rc8)
>>> 3. system became sluggish - top&  vmstat showed all 12Gb ram used -
>>> albeit 10g of fs cache. It seemed as though relcaim of fs cache became
>>> really slow once there were no more "free" pages.
>>> vmstat<after hung task reported - don't have from before>
>>> procs -----------memory---------- ---swap-- -----io---- --system--
>>> -----cpu-----
>>>     r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us
>>> sy id wa st
>>>     0  1    808 112476 347592 9556952    0    0    39   388  158  189
>>> 1 18 77  4  0
>>> 4. Worrying a bit about the looming instability, I typed, "sync."
>>> 5. sync took a long time, and was reported by the kernel as a hung
>>> task (repeatedly) - see below.
>>> 6. entering additional sync commands also hang (unsuprising, but
>>> figured I'd try as non-root).
>>> 7. The running sync (pid 11975) cannot be killed.
>>> 8. echo 1>  drop_caches does clear the fs cache. System behaves better
>>> after this (but sync is still hung).
>>>
>>> config attached.
>>>
>>> Running with sky2 dma patches (in rc8) and increased the audit name
>>> space to avoid the flood of name space maxed warnings.
>>>
>>> My current plan is to let the raid rebuild complete and then reboot
>>> (to rc8 if the bits made it to disk)... maybe with a backup of
>>> recently changed files to an external system.
>>>
>>> Feb 13 10:54:13 mail kernel: INFO: task sync:11975 blocked for more
>>> than 120 seconds.
>>> Feb 13 10:54:13 mail kernel: "echo 0>
>>> /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>> Feb 13 10:54:13 mail kernel: sync          D 0000000000000002     0
>>> 11975   6433 0x00000000
>>> Feb 13 10:54:13 mail kernel: ffff8801c45f3da8 0000000000000082
>>> ffff8800282f5948 ffff8800282f5920
>>> Feb 13 10:54:13 mail kernel: ffff88032f785d78 ffff88032f785d40
>>> 000000030c37a771 0000000000000282
>>> Feb 13 10:54:13 mail kernel: ffff8801c45f3fd8 000000000000f888
>>> ffff88032ca00000 ffff8801c61c9750
>>> Feb 13 10:54:13 mail kernel: Call Trace:
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81154730>] ?
>>> bdi_sched_wait+0x0/0x20
>>> Feb 13 10:54:13 mail kernel: [<ffffffff8115473e>] bdi_sched_wait+0xe/0x20
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81537b4f>] __wait_on_bit+0x5f/0x90
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81154730>] ?
>>> bdi_sched_wait+0x0/0x20
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81537bf8>]
>>> out_of_line_wait_on_bit+0x78/0x90
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81078650>] ?
>>> wake_bit_function+0x0/0x50
>>> Feb 13 10:54:13 mail kernel: [<ffffffff8104ac55>] ?
>>> wake_up_process+0x15/0x20
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81155daf>]
>>> bdi_sync_writeback+0x6f/0x80
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81155de2>]
>>> sync_inodes_sb+0x22/0x100
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81159902>]
>>> __sync_filesystem+0x82/0x90
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81159a04>]
>>> sync_filesystems+0xf4/0x120
>>> Feb 13 10:54:13 mail kernel: [<ffffffff81159a91>] sys_sync+0x21/0x40
>>> Feb 13 10:54:13 mail kernel: [<ffffffff8100b0f2>]
>>> system_call_fastpath+0x16/0x1b
>>>
>>> <this repeats every 120 seconds - all the same traceback>
>>>
>>>
>>>
>>>
>>>        
>> Note: this cleared after about 90 minutes - sync eventually completed.
>> I'm thinking that with multicore enabled the resync is able to starve
>> out normal system activities that weren't starved w/o multicore.
>>      
>    Hmm, it is a bug in writeback code. But as Linus pointed out, it's not really
> clear why it's *so* slow. So when it happens again, could you please sample for
> a while (like every second for 30 seconds) stacks of blocked tasks via
> Alt-Sysrq-W? I'd like to see where flusher threads are hanging... Thanks.
>
> 								Honza
>    
Ok - got it. Sync is still spinning, btw... attaching log extract as 
well as dmesg output.