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?
Thanks,
Ming
