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Date:	Tue, 25 Jun 2013 09:52:33 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Jan Kara <jack@...e.cz>
CC:	<linux-rt-users@...r.kernel.org>, <linux-ext4@...r.kernel.org>
Subject: Re: ext4/jbd2 hangs in __jbd2_log_wait_for_space on 3.4-RT/3.6-RT

On 13-06-25 09:18 AM, Jan Kara wrote:
> On Fri 31-05-13 14:34:12, Paul Gortmaker wrote:
>> This problem is seen on vanilla 3.4-RT and 3.6-RT kernels. It is
>> not clear to me whether this is an RT issue, or whether (as usual)
>> RT has managed to shake out an issue in mainline code.  So I've
>> looped in the ext4 list as well as the RT list, since at the
>> moment it appears this can impact anyone using RT and ext4...
>>
>> What happens is that under reasonable load, the jbd2/sda1-8 thread
>> goes D state, and then lots of regular processes follow suit, after
>> calling __jbd2_log_wait_for_space.  As can be seen at the bottom
>> of the sysrq-t output, j_checkpoint_mutex is implicated.  All
>> future processes trying to do I/O to/from that filesystem go D.
>>
>> More testing details:
>> Even though debug_rt_mutex_print_deadlock shows up in each stalled
>> process backtrace, no output is seen from debug_rt_mutex_print_deadlock.
>> There are no messages in dmesg at all, until I trigger a SysRQ-t.
>>
>> I've reproduced this on v3.4.42-rt57, v3.4.47-rt62, and v3.6.11.3-rt35.
>>
>> The two separate versions of v3.4.x are because I noticed the 3.4.47
>> pulled in some jbd2 commits via stable, like 794446c6 "jbd2: fix race
>> between jbd2_journal_remove_checkpoint and ->j_commit_callback". It
>> looked promising, but having that present didn't change things.
>>
>> I'm using a yocto build, configured for six parallel package builds,
>> each pkg in turn with "make -j6" to create I/O.  I've found that also
>> running an "rm -rf" of an old build (several gigs of data) at the
>> same time increases the probability of it.  Typically it will fail
>> within about 15m or so.  The test box is a dell optiplex 990 with
>> a single disk as ext4.  The box stays alive for basic sysrq operations
>> and anything else that doesn't touch the locked filesystem.  The build
>> halts with a static load average equal to the number of blocked D procs.
>>
>> I've deleted the sysrq-t output from the irrelevant sleeping processes
>> in order to reduce the noise.  I'll keep looking at this but I'm hoping
>> more experienced eyes on the problem will help, since it seems common
>> to all RT users and hence of interest to everyone (I've not yet tried
>> 3.8.x-RT, mind you.)
>   Hum, this sounds familiar... I was already debugging this with RT kernel
> and I also remember it was RT specific issue. Let me try to remember the
> whole story... yes, while wandering over the traces I think I remember what
> was the problem: In standard kernel, whenever we scheduler process out from
> CPU, we unplug its IO queue in sched_submit_work(). However in RT kernel
> that was not the case. So it could happen that a process has IOs queued
> and was sent to sleep waiting for jbd2 thread to free some journal space
> and jbd2 thread was waiting for some IO to complete - however that never
> happened because the IO was sitting in the sleeping process' queue.

Do you have a link to that older discussion?  I did search around before
posting, but came up empty.  I'll try and fold your description into my
thoughts as I return to looking at it (got dragged into other things
as of late, and haven't been spending time on this as of late...)

> 
> From a quick look into the traces you've provided this seems to be your
> case as well. I think newer RT kernels should have the bug fixed but I
> wasn't really watching closely after I handed over the problem to RT folks.

I was able to reproduce it on 3.4.x and 3.6.x -- but not on 3.8.x-rt.
However, it seemed harder to trigger on 3.6 than 3.4, and hence I'm
never 100% confident that the problem isn't there vs. just hard to
trigger.  The 3.8.x-rt is (as I understand it) largely technically
equivalent to the 3.6.x-rt kernel -- so I can't explain why it can't
happen there in principle.

Thanks,
Paul.
--

> 
> 								Honza
> 
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