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
.
