Date:	Tue, 17 Jun 2008 11:31:19 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: FIO: kjournald blocked for more than 120 seconds

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@...cle.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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