Re: Hung tasks with multiple partitions

Bart Van Assche
2020-01-30T20:49:46Z

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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.

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#!/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