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.

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

Bart.