Re: Hung tasks with multiple partitions

Salman Qazi
2020-01-30T21:03:10Z

On Thu, Jan 30, 2020 at 12:49 PM Bart Van Assche <bvanassche@acm.org> wrote:
>
> On 1/30/20 11:34 AM, Salman Qazi wrote:
> > I am writing on behalf of the Chromium OS team at Google. We found
> > the root cause for some hung tasks we were experiencing and we would
> > like to get your opinion on potential solutions. The bugs were
> > encountered on 4.19 kernel.
> > However my reading of the code suggests that the relevant portions of the
> > code have not changed since then.
> >
> > We have an eMMC flash drive that has been carved into partitions on an
> > 8 CPU system. The repro case that we came up with, is to use 8
> > threaded fio write-mostly workload against one partition, let the
> > system use the other partition as the read-write filesystem (i.e. just
> > background activity) and then run the following loop:
> >
> > while true; do sync; sleep 1 ; done
> >
> > The hung task stack traces look like the following:
> >
> > [ 128.994891] jbd2/dm-1-8 D 0 367 2 0x00000028
> > last_sleep: 96340206998. last_runnable: 96340140151
> > [ 128.994898] Call trace:
> > [ 128.994903] __switch_to+0x120/0x13c
> > [ 128.994909] __schedule+0x60c/0x7dc
> > [ 128.994914] schedule+0x74/0x94
> > [ 128.994919] io_schedule+0x1c/0x40
> > [ 128.994925] bit_wait_io+0x18/0x58
> > [ 128.994930] __wait_on_bit+0x78/0xdc
> > [ 128.994935] out_of_line_wait_on_bit+0xa0/0xcc
> > [ 128.994943] __wait_on_buffer+0x48/0x54
> > [ 128.994948] jbd2_journal_commit_transaction+0x1198/0x1a4c
> > [ 128.994956] kjournald2+0x19c/0x268
> > [ 128.994961] kthread+0x120/0x130
> > [ 128.994967] ret_from_fork+0x10/0x18
> >
> > I added some more information to trace points to understand what was
> > going on. It turns out that blk_mq_sched_dispatch_requests had
> > checked hctx->dispatch, found it empty, and then began consuming
> > requests from the io scheduler (in blk_mq_do_dispatch_sched).
> > Unfortunately, the deluge from the I/O scheduler (BFQ in our case)
> > doesn't stop for 30 seconds and there is no mechanism present in
> > blk_mq_do_dispatch_sched to terminate early or reconsider
> > hctx->dispatch contents. In the meantime, a flush command arrives in
> > hctx->dispatch (via insertion in blk_mq_sched_bypass_insert) and
> > languishes there. Eventually the thread waiting on the flush triggers
> > the hung task watchdog.
> >
> > The solution that comes to mind is to periodically check
> > hctx->dispatch in blk_mq_do_dispatch_sched and exit early if it is
> > non-empty. However, not being an expert in this subsystem, I am not
> > sure if there would be other consequences.
>
> The call stack shown in your e-mail usually means that an I/O request
> got stuck. How about determining first whether this is caused by the BFQ
> scheduler or by the eMMC driver? I think the developers of these
> software components need that information anyway before they can step in.

As I mentioned in my previous email, I did use trace points to arrive
at my conclusion. I added trace points in
blk_mq_sched_dispatch_requests to
detect both the start and the end of that function, as well as where
the dispatched commands were picked from. I also traced
blk_mq_sched_bypass_insert and saw a flush enter hctx->dispatch after
blk_mq_sched_dispatch_requests had started but before it
finished. After reaching my conclusion, I also tried a simple fix by
introducing an exit path in blk_mq_do_dispatch_sched, if
we detect that hctx->dispatch has become non-empty. This made the
problem go away.

>
> The attached script may help to identify which requests got stuck.
>
> Bart.