Re: [PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[dongli.zhang@oracle.com]

On 2/19/20 2:10 PM, Ming Lei wrote:
> On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote:
>> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote:
>>> For some reason, device may be in one situation which can't handle
>>> FS request, so STS_RESOURCE is always returned and the FS request
>>> will be added to hctx->dispatch. However passthrough request may
>>> be required at that time for fixing the problem. If passthrough
>>> request is added to scheduler queue, there isn't any chance for
>>> blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
>>> Then the FS IO request may never be completed, and IO hang is caused.
>>>
>>> So passthrough request has to be added to hctx->dispatch directly.
>>>
>>> Fix this issue by inserting passthrough request into hctx->dispatch
>>> directly. Then it becomes consistent with original legacy IO request
>>> path, in which passthrough request is always added to q->queue_head.
>>
>> Do you have a description of an actual problem this fixes?  Maybe even
>> a reproducer for blktests?
>>
> 
> It is reported by one RH customer in the following test case:
> 
> 	1) Start IO on Emulex FC host
> 	2) Fail one controller, wait 5 minutes
> 	3) Bring controller back online
> 
> When we trace the problem, it is found that FS request started in device_add_disk()
> from scsi disk probe context stuck because scsi_queue_rq() always return
> STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn().
> 
> The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see
> BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn().
> 
> However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work
> when the FS request can't be dispatched to LLD. And SCSI stack should
> have been allowed to handle this passthrough rquest. But it can't reach SCSI stack
> via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is
> empty.
> 
> The legacy IO request code always added passthrough request into head of q->queue_head
> directly instead of scheduler queue or sw queue, so no such issue.
> 
> So far not figured out one blktests test case, but the problem is real.
> 
> BTW, I just found we need the extra following change:
> 
> @@ -1301,7 +1301,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>                         q->mq_ops->commit_rqs(hctx);
> 
>                 spin_lock(&hctx->lock);
> -               list_splice_init(list, &hctx->dispatch);
> +               list_splice_tail_init(list, &hctx->dispatch);
>                 spin_unlock(&hctx->lock);
> 

Is it fine to add to tail as the requests on dispatch would be reordered?

A, B, C and D are on the list. Suppose A is failed and the new list would become
B, C D, A?

Dongli Zhang