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SubjectRE: FIO: kjournald blocked for more than 120 seconds
Date
From
>>-----Original Message-----
>>From: Jens Axboe [mailto:jens.axboe@oracle.com]
>>Sent: Tuesday, June 17, 2008 3:30 AM
>>To: Lin, Ming M
>>Cc: Zhang, Yanmin; Linux Kernel Mailing List
>>Subject: Re: FIO: kjournald blocked for more than 120 seconds
>>
>>On Mon, Jun 16 2008, Lin Ming wrote:
>>> Hi, Jens
>>>
>>> When runnig FIO benchmark, kjournald blocked for more than 120
seconds.
>>> Detailed root cause analysis and proposed solutions as below.
>>>
>>> Any comment is appreciated.
>>>
>>> Hardware Environment
>>> ---------------------
>>> 13 SEAGATE ST373307FC disks in a JBOD, connected by a Qlogic ISP2312
>>> Fibe Channel HBA.
>>>
>>> Bug description
>>> ----------------
>>> fio vsync random read 4K in 13 disks, 4 processes per disk, fio
global
>>> paramter as below,
>>> Tested 4 IO schedulers, issue is only seen in CFQ.
>>>
>>> INFO: task kjournald:20558 blocked for more than 120 seconds.
>>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
>>> message.
>>> kjournald D ffff810010820978 6712 20558 2
>>> ffff81022ddb1d10 0000000000000046 ffff81022e7baa10 ffffffff803ba6f2
>>> ffff81022ecd0000 ffff8101e6dc9160 ffff81022ecd0348 000000008048b6cb
>>> 0000000000000086 ffff81022c4e8d30 0000000000000000 ffffffff80247537
>>> Call Trace:
>>> [<ffffffff803ba6f2>] kobject_get+0x12/0x17
>>> The disks of my testing machine are tagged devices, so the CFQ idle
>>> window is disabled. In other words, the active queue of tagged
>>> devices(cfqd->hw_tag=1) never idle for a new request.
>>>
>>> This causes active queue be expired immediately if it's empty,
although
>>> it has not run out of time. CFQ will select next queue as active
queue.
>>> In this testcase, there are thousands of FIO read requests in sync
>>> queues, only a few write requests by journal_write_commit_record in
>>> async queues.
>>>
>>> In the other hand, all processes use the default io class and
priority.
>>> They share the async queue for the same device, but have their own
sync
>>> queue, so the sync queue number is 4 while asyn queue number is just
1
>>> for the same device.
>>>
>>> So sync queue has much more chances be selected as new active queue
than
>>> async queue.
>>>
>>> Sync queues do not idle and they are dispatched all the time. This
leads
>>> to many unfinished requests in external queue,
>>> namely, cfqd->sync_flight > 0.
>>>
>>> static int cfq_dispatch_requests (...) {
>>> ....
>>> while ((cfqq = cfq_select_queue(cfqd)) != NULL) {
>>> ....
>>> if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
>>> break;
>>> ....
>>> __cfq_dispatch_requests(cfqq)
>>> }
>>> ....
>>> }
>>>
>>> When cfq_select_queue selects the async queue which includes
kjournald's
>>> write request, this selected async queue will never be dispatched
since
>>> cfqd->sync_flight > 0, so kjournald is blocked.
>>>
>>> Proposed 3 solutions
>>> ------------------
>>> 1. Do not check cfqd->sync_flight
>>>
>>> - if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
>>> - break;
>>>
>>> 2. If we do need to check cfqd->sync_flight, then for tagged
devices, we
>>> should give a little more chances to async queue to be dispatched.
>>>
>>> @@ -1102,7 +1102,7 @@ static int cfq_dispatch_requests(struct
>>> request_queue *q, int force)
>>> break;
>>> }
>>>
>>> - if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
>>> + if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq) && !
>>> cfqd->hw_tag)
>>> break;
>>>
>>> 3. Force write request issued by journal_write_commit_record as sync
>>> request. As a matter of fact, it looks like most write requests
>>> submitted by kjournald is async request. We need convert them to
sync
>>> requests.
>>
>>Thanks for the very detailed analysis of the problem, complete with
>>suggestions. While I think that any code that does:
>>
>> submit async io
>> wait for it
>>
>>should be issuing sync IO (or, better, automatically upgrade the
request
>>from async -> sync), we cannot rely on that.
[YM] We can talk case by case. We could convert some important async io
codes
to sync io codes at least. For example, kjournald calls
sync_dirty_buffer what
we captured in this case.

Another case is writeback. If processes do mmapped I/O and they might
stop in
page fault to wait writeback finishing. Or a buffer write might trigger
a dirty
page balance. As the latest kernel is more aggressive to start
writeback, it might
be an issue now.

>>
>>This problem is similar in nature to device starvation, and a classic
>>solution to that problem is to issue occasional ordered tags to
prevent
>>indefinite starvation. Perhaps we can apply some similar logic here.
>>
>>For 2.6.26, the simple approach of just removing the sync_flight check
>>is probably the safest.



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