Messages in this thread | | | Subject | Re: Hung tasks with multiple partitions | From | Bart Van Assche <> | Date | Thu, 30 Jan 2020 12:49:38 -0800 |
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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. #!/bin/bash
show_state() { local a dev=$1
for a in device/state queue/scheduler; do [ -e "$dev/$a" ] && grep -aH . "$dev/$a" done }
if [ -e /sys/kernel/debug/block ]; then devs=($(cd /sys/kernel/debug/block && echo ./*)) else devs=($(cd /sys/class/block && echo ./*)) fi
cd /sys/class/block || exit $? for dev in "${devs[@]}"; do dev="${dev#./}" echo "$dev" pending=0 if [ -e "$dev/mq" ]; then for f in "$dev"/mq/*/{pending,*/rq_list}; do [ -e "$f" ] || continue if { read -r line1 && read -r line2; } <"$f"; then echo "$f" echo "$line1 $line2" >/dev/null head -n 9 "$f" ((pending++)) fi done fi ( busy=0 cd /sys/kernel/debug/block >&/dev/null && { grep -aH . $dev/requeue_list; true; } && for d in "$dev"/mq/hctx* "$dev"/hctx*; do [ ! -d "$d" ] && continue { [ ! -e "$d/tags" ] || grep -q '^busy=0$' "$d/tags"; } && { [ ! -e "$d/sched_tags" ] || [ "$(<"$d/sched_tags")" = "" ] || grep -q '^busy=0$' "$d/sched_tags"; } && continue ((busy++)) for f in "$d"/{active,busy,dispatch,flags,requeue_list,sched_tags,state,tags*,cpu*/rq_list,sched/*rqs}; do [ -e "$f" ] && grep -aH . "$f" done done exit $busy ) pending=$((pending+$?)) if [ "$pending" -gt 0 ]; then ( cd /sys/kernel/debug/block >&/dev/null && if [ -e "$dev/mq/state" ]; then grep -aH . "$dev/mq/state" else grep -aH . "$dev/state" fi ) show_state "$dev" fi done
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