Re: [PATCH] blk-mq: run queue after issuing the last request of the plug list

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在 2022/07/26 9:46, Ming Lei 写道:
On Tue, Jul 26, 2022 at 09:08:19AM +0800, Yu Kuai wrote:
Hi, Ming!

在 2022/07/25 23:43, Ming Lei 写道:
On Sat, Jul 23, 2022 at 10:50:03AM +0800, Yu Kuai wrote:
Hi, Ming!

在 2022/07/19 17:26, Ming Lei 写道:
On Mon, Jul 18, 2022 at 08:35:28PM +0800, Yufen Yu wrote:
We do test on a virtio scsi device (/dev/sda) and the default mq
scheduler is 'none'. We found a IO hung as following:

blk_finish_plug
     blk_mq_plug_issue_direct
         scsi_mq_get_budget
         //get budget_token fail and sdev->restarts=1

			     	 scsi_end_request
				   scsi_run_queue_async
                                      //sdev->restart=0 and run queue

        blk_mq_request_bypass_insert
           //add request to hctx->dispatch list

Here the issue shouldn't be related with scsi's get budget or
scsi_run_queue_async.

If blk-mq adds request into ->dispatch_list, it is blk-mq core's
responsibility to re-run queue for moving on. Can you investigate a
bit more why blk-mq doesn't run queue after adding request to
hctx dispatch list?

I think Yufen is probably thinking about the following Concurrent
scenario:

blk_mq_flush_plug_list
# assume there are three rq
   blk_mq_plug_issue_direct
    blk_mq_request_issue_directly
    # dispatch rq1, succeed
    blk_mq_request_issue_directly
    # dispatch rq2
     __blk_mq_try_issue_directly
      blk_mq_get_dispatch_budget
       scsi_mq_get_budget
        atomic_inc(&sdev->restarts);
        # rq2 failed to get budget
        # restarts is 1 now
                                          scsi_end_request
                                          # rq1 is completed
                                          ┊scsi_run_queue_async
                                          ┊ atomic_cmpxchg(&sdev->restarts,
old, 0) == old
                                          ┊ # set restarts to 0
                                          ┊ blk_mq_run_hw_queues
                                          ┊ # hctx->dispatch list is empty
    blk_mq_request_bypass_insert
    # insert rq2 to hctx->dispatch list

After rq2 is added to ->dispatch_list in blk_mq_try_issue_list_directly(),
no matter if list_empty(list) is empty or not, queue will be run either from
blk_mq_request_bypass_insert() or blk_mq_sched_insert_requests().

1) while inserting rq2 to dispatch list, blk_mq_request_bypass_insert()
is called from blk_mq_try_issue_list_directly(), list_empty() won't
pass, thus thus blk_mq_request_bypass_insert() won't run queue.

Yeah, but in blk_mq_try_issue_list_directly() after rq2 is inserted to dispatch
list, the loop is broken and blk_mq_try_issue_list_directly() returns to
blk_mq_sched_insert_requests() in which list_empty() is false, so
blk_mq_insert_requests() and blk_mq_run_hw_queue() are called, queue
is still run.

Also not sure why you make rq3 involved, since the list is local list on
stack, and it can be operated concurrently.

I make rq3 involved because there are some conditions that
blk_mq_insert_requests() and blk_mq_run_hw_queue() won't be called from
blk_mq_sched_insert_requests():

This is the case I'm thinking, if blk_mq_try_issue_list_directly()
succeed from blk_mq_sched_insert_requests(), and list become empty.

Thanks,
Kuai

Thanks,
Ming

.




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