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

[Yu Kuai <yukuai3@huawei.com>]

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.

2) after blk_mq_sched_insert_requests() dispatchs rq3, list_empty() will
pass, thus blk_mq_sched_insert_requests() won't run queue. (That's what
yufen tries to fix.)
> 
> And rq2 should be visible to the run queue, just wondering why rq2 isn't
> issued finally?
> 
> 
> Thanks,
> Ming
> 
> .
>