On Thu, Jan 30, 2020 at 12:49 PM Bart Van Assche <bvanassche@xxxxxxx> 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.
