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

[Yu Kuai <yukuai3@huawei.com>]

在 2022/07/26 11:21, Ming Lei 写道:
> On Tue, Jul 26, 2022 at 11:14:23AM +0800, Yu Kuai wrote:
>> Hi, Ming
>>
>> 在 2022/07/26 11:02, Ming Lei 写道:
>>> On Tue, Jul 26, 2022 at 10:52:56AM +0800, Yu Kuai wrote:
>>>> Hi, Ming
>>>> 在 2022/07/26 10:32, Ming Lei 写道:
>>>>> On Tue, Jul 26, 2022 at 10:08:13AM +0800, Yu Kuai wrote:
>>>>>> 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():
>>>>>
>>>>> The two won't be called if list_empty() is true, and will be called if
>>>>> !list_empty().
>>>>>
>>>>> That is why I mentioned run queue has been done after rq2 is added to
>>>>> ->dispatch_list.
>>>>
>>>> I don't follow here, it's right after rq2 is inserted to dispatch list,
>>>> list is not empty, and blk_mq_sched_insert_requests() will be called.
>>>> However, do you think that it's impossible that
>>>> blk_mq_sched_insert_requests() can dispatch rq in the list and list
>>>> will become empty?
>>>
>>> Please take a look at blk_mq_sched_insert_requests().
>>>
>>> When codes runs into blk_mq_sched_insert_requests(), the following
>>> blk_mq_run_hw_queue() will be run always, how does list empty or not
>>> make a difference there?
>>
>> This is strange, always blk_mq_run_hw_queue() is exactly what Yufen
>> tries to do in this patch, are we look at different code?
> 
> No.
> 
>>
>> I'm copying blk_mq_sched_insert_requests() here, the code is from
>> latest linux-next:
>>
>> 461 void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
>> 462                                 ┊ struct blk_mq_ctx *ctx,
>> 463                                 ┊ struct list_head *list, bool
>> run_queue_async)
>> 464 {
>> 465         struct elevator_queue *e;
>> 466         struct request_queue *q = hctx->queue;
>> 467
>> 468         /*
>> 469         ┊* blk_mq_sched_insert_requests() is called from flush plug
>> 470         ┊* context only, and hold one usage counter to prevent queue
>> 471         ┊* from being released.
>> 472         ┊*/
>> 473         percpu_ref_get(&q->q_usage_counter);
>> 474
>> 475         e = hctx->queue->elevator;
>> 476         if (e) {
>> 477                 e->type->ops.insert_requests(hctx, list, false);
>> 478         } else {
>> 479                 /*
>> 480                 ┊* try to issue requests directly if the hw queue isn't
>> 481                 ┊* busy in case of 'none' scheduler, and this way may
>> save
>> 482                 ┊* us one extra enqueue & dequeue to sw queue.
>> 483                 ┊*/
>> 484                 if (!hctx->dispatch_busy && !run_queue_async) {
>> 485                         blk_mq_run_dispatch_ops(hctx->queue,
>> 486                                 blk_mq_try_issue_list_directly(hctx,
>> list));
>> 487                         if (list_empty(list))
>> 488                                 goto out;
>> 489                 }
>> 490                 blk_mq_insert_requests(hctx, ctx, list);
>> 491         }
>> 492
>> 493         blk_mq_run_hw_queue(hctx, run_queue_async);
>> 494  out:
>> 495         percpu_ref_put(&q->q_usage_counter);
>> 496 }
>>
>> Here in line 487, if list_empty() is true, out label will skip
>> run_queue().
> 
> If list_empty() is true, run queue is guaranteed to run
> in blk_mq_try_issue_list_directly() in case that BLK_STS_*RESOURCE
> is returned from blk_mq_request_issue_directly().
> 
> 		ret = blk_mq_request_issue_directly(rq, list_empty(list));
> 		if (ret != BLK_STS_OK) {
> 			if (ret == BLK_STS_RESOURCE ||
> 					ret == BLK_STS_DEV_RESOURCE) {
> 				blk_mq_request_bypass_insert(rq, false,
> 							list_empty(list));	//run queue
> 				break;
> 			}
> 			blk_mq_end_request(rq, ret);
> 			errors++;
> 		} else
> 			queued++;
> 
> So why do you try to add one extra run queue?

Hi, Ming

Perhaps I didn't explain the scenario clearly, please notice that list
contain three rq is required.

1) rq1 is dispatched successfuly
2) rq2 failed to dispatch due to no budget, in this case
    - rq2 will insert to dispatch list
    - list is not emply yet, run queue won't called
3) finally, blk_mq_sched_insert_requests() dispatch rq3 successfuly,
and list will become empty, thus run queue still won't be called.

Thanks,
Kuai