[PATCH v2 0/3] Fix some starvation problems in block layer

Muchun Song
2024-09-03T08:17:08Z

We encounter a problem on our servers where hundreds of UNINTERRUPTED
processes are all waiting in the WBT wait queue. And the IO hung detector
logged so many messages about "blocked for more than 122 seconds". The
call trace is as follows:

Call Trace:
__schedule+0x959/0xee0
schedule+0x40/0xb0
io_schedule+0x12/0x40
rq_qos_wait+0xaf/0x140
wbt_wait+0x92/0xc0
__rq_qos_throttle+0x20/0x30
blk_mq_make_request+0x12a/0x5c0
generic_make_request_nocheck+0x172/0x3f0
submit_bio+0x42/0x1c0
...

The WBT module is used to throttle buffered writeback, which will block
any buffered writeback IO request until the previous inflight IOs have
been completed. So I checked the inflight IO counter. That was one meaning
one IO request was submitted to the downstream interface like block core
layer or device driver (virtio_blk driver in our case). We need to figure
out why the inflight IO is not completed in time. I confirmed that all
the virtio ring buffers of virtio_blk are empty and the hardware dispatch
list had one IO request, so the root cause is not related to the block
device or the virtio_blk driver since the driver has never received that
IO request.

We know that block core layer could submit IO requests to the driver through
kworker (the callback function is blk_mq_run_work_fn). I thought maybe the
kworker was blocked by some other resources causing the callback to not be
evoked in time. So I checked all the kworkers and workqueues and confirmed
there was no pending work on any kworker or workqueue.

Integrate all the investigation information, the problem should be in the
block core layer missing a chance to submit that IO request. After
some investigation of code, I found some scenarios which could cause the
problem.

Changes in v2:
- Collect RB tag from Ming Lei.
- Use barrier-less approach to fix QUEUE_FLAG_QUIESCED ordering problem
suggested by Ming Lei.
- Apply new approach to fix BLK_MQ_S_STOPPED ordering for easier maintenance.
- Add Fixes tag to each patch.

Muchun Song (3):
block: fix missing dispatching request when queue is started or
unquiesced
block: fix ordering between checking QUEUE_FLAG_QUIESCED and adding
requests
block: fix ordering between checking BLK_MQ_S_STOPPED and adding
requests

block/blk-mq.c | 55 ++++++++++++++++++++++++++++++++++++++------------
block/blk-mq.h | 13 ++++++++++++
2 files changed, 55 insertions(+), 13 deletions(-)

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2.20.1