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

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On 2022/7/26 12:16, Ming Lei wrote:
On Tue, Jul 26, 2022 at 11:31:34AM +0800, Yu Kuai wrote:
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

In the case, blk_mq_try_issue_list_directly() returns to
blk_mq_sched_insert_requests() immediately, then blk_mq_insert_requests()
and blk_mq_run_hw_queue() will be run from blk_mq_sched_insert_requests()
because the list isn't empty.

Right?


hi Ming,

Here rq2 fail from blk_mq_plug_issue_direct() in blk_mq_flush_plug_list(),
not blk_mq_sched_insert_requests

blk_mq_flush_plug_list

    if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
        struct request_queue *q;

        rq = rq_list_peek(&plug->mq_list);
        q = rq->q;

        /*
         * Peek first request and see if we have a ->queue_rqs() hook.
         * If we do, we can dispatch the whole plug list in one go. We
         * already know at this point that all requests belong to the
         * same queue, caller must ensure that's the case.
         *
         * Since we pass off the full list to the driver at this point,
         * we do not increment the active request count for the queue.
         * Bypass shared tags for now because of that.
         */
        if (q->mq_ops->queue_rqs &&
            !(rq->mq_hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED)) {
            blk_mq_run_dispatch_ops(q,
                __blk_mq_flush_plug_list(q, plug));
            if (rq_list_empty(plug->mq_list))
                return;
        }

        blk_mq_run_dispatch_ops(q,
                blk_mq_plug_issue_direct(plug, false));  //rq2 insert into dispatch list
        if (rq_list_empty(plug->mq_list))
            return;
    }

    do {
        blk_mq_dispatch_plug_list(plug, from_schedule);  //continue issue rq3 and success
    } while (!rq_list_empty(plug->mq_list));


Thanks,
Ming

.



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