Messages in this thread | | | Date | Tue, 17 Jun 2008 11:31:19 +0200 | From | Jens Axboe <> | Subject | Re: FIO: kjournald blocked for more than 120 seconds |
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On Tue, Jun 17 2008, Lin Ming wrote: > > On Tue, 2008-06-17 at 10:36 +0200, Jens Axboe wrote: > > On Tue, Jun 17 2008, Zhang, Yanmin wrote: > > > >>-----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. > > > > I agree, we should fix the obvious cases. My point was merely that there > > will probably always be missed cases, so we should attempt to handle it > > in the scheduler as well. Does the below buffer patch make it any > > better? > > Yes, kjournald blocked issue is gone with below patch applied.
I think it's obviously the right thing to do, but I'm also a bit worried about applying it so close to 2.6.26 release. OTOH, we need to do SOMETHING for 2.6.26 release, so...
> > Lin Ming > > > > > > 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. > > > > Sync process getting stuck in async writeout is another problem of the > > same variety. > > > > diff --git a/fs/buffer.c b/fs/buffer.c > > index a073f3f..1957a8f 100644 > > --- a/fs/buffer.c > > +++ b/fs/buffer.c > > @@ -2978,7 +2978,7 @@ int sync_dirty_buffer(struct buffer_head *bh) > > if (test_clear_buffer_dirty(bh)) { > > get_bh(bh); > > bh->b_end_io = end_buffer_write_sync; > > - ret = submit_bh(WRITE, bh); > > + ret = submit_bh(WRITE_SYNC, bh); > > wait_on_buffer(bh); > > if (buffer_eopnotsupp(bh)) { > > clear_buffer_eopnotsupp(bh); > > >
-- Jens Axboe
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