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?
Thanks,
Ming
