Re: [PATCH 0/4] blk-mq: support to use hw tag for scheduling

On 05/03/2017 11:08 AM, Bart Van Assche wrote:

On Thu, 2017-05-04 at 00:52 +0800, Ming Lei wrote:

quoted

Looks v4.11 plus your for-linus often triggers the following hang during
boot, and it seems caused by the change in (blk-mq: unify hctx delayed_run_work
and run_work)


BUG: scheduling while atomic: kworker/0:1H/704/0x00000002
Modules linked in:
Preemption disabled at:
[<ffffffffaf5607bb>] virtio_queue_rq+0xdb/0x350
CPU: 0 PID: 704 Comm: kworker/0:1H Not tainted 4.11.0-04508-ga1f35f46164b #132
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.9.3-1.fc25 04/01/2014
Workqueue: kblockd blk_mq_run_work_fn
Call Trace:
 dump_stack+0x65/0x8f
 ? virtio_queue_rq+0xdb/0x350
 __schedule_bug+0x76/0xc0
 __schedule+0x610/0x820
 ? new_slab+0x2c9/0x590
 schedule+0x40/0x90
 schedule_timeout+0x273/0x320
 ? ___slab_alloc+0x3cb/0x4f0
 wait_for_completion+0x97/0x100
 ? wait_for_completion+0x97/0x100
 ? wake_up_q+0x80/0x80
 flush_work+0x104/0x1a0
 ? flush_workqueue_prep_pwqs+0x130/0x130
 __cancel_work_timer+0xeb/0x160
 ? vp_notify+0x16/0x20
 ? virtqueue_add_sgs+0x23c/0x4a0
 cancel_delayed_work_sync+0x13/0x20
 blk_mq_stop_hw_queue+0x16/0x20
 virtio_queue_rq+0x316/0x350
 blk_mq_dispatch_rq_list+0x194/0x350
 blk_mq_sched_dispatch_requests+0x118/0x170
 ? finish_task_switch+0x80/0x1e0
 __blk_mq_run_hw_queue+0xa3/0xc0
 blk_mq_run_work_fn+0x2c/0x30
 process_one_work+0x1e0/0x400
 worker_thread+0x48/0x3f0
 kthread+0x109/0x140
 ? process_one_work+0x400/0x400
 ? kthread_create_on_node+0x40/0x40
 ret_from_fork+0x2c/0x40

Callers of blk_mq_quiesce_queue() really need blk_mq_stop_hw_queue() to
cancel delayed work synchronously.

Right.

The above call stack shows that we have to do something about the
blk_mq_stop_hw_queue() calls from inside .queue_rq() functions for
queues for which BLK_MQ_F_BLOCKING has not been set. I'm not sure what
the best approach would be: setting BLK_MQ_F_BLOCKING for queues that
call blk_mq_stop_hw_queue() from inside .queue_rq() or creating two
versions of blk_mq_stop_hw_queue().

Regardless of whether BLOCKING is set or not, we don't have to hard
guarantee the flush from the drivers. If they do happen to get a 2nd
invocation before being stopped, that doesn't matter. So I think we're
fine with the patch I sent out 5 minutes ago, would be great if Ming
could test it though.

-- 
Jens Axboe