Re: [PATCH V3 11/17] SCSI: track pending admin commands

["jianchao.wang" <jianchao.w.wang@oracle.com>]

On 09/18/2018 03:55 PM, Ming Lei wrote:
> On Tue, Sep 18, 2018 at 03:51:10PM +0800, jianchao.wang wrote:
>>
>>
>> On 09/18/2018 03:39 PM, Ming Lei wrote:
>>> On Tue, Sep 18, 2018 at 09:22:32AM +0800, jianchao.wang wrote:
>>>> Hi Ming
>>>>
>>>> On 09/17/2018 07:35 PM, Ming Lei wrote:
>>>>> On Mon, Sep 17, 2018 at 10:46:34AM +0800, jianchao.wang wrote:
>>>>>> Hi Ming
>>>>>>
>>>>>> On 09/14/2018 07:33 PM, Ming Lei wrote:
>>>>>>> On Fri, Sep 14, 2018 at 11:40 AM jianchao.wang
>>>>>>> <jianchao.w.wang@oracle.com> wrote:
>>>>>>>>
>>>>>>>> Hi Ming
>>>>>>>>
>>>>>>>> On 09/13/2018 08:15 PM, Ming Lei wrote:
>>>>>>>>>  EXPORT_SYMBOL(__scsi_execute);
>>>>>>>>> @@ -3246,6 +3251,7 @@ static int scsi_internal_device_block(struct scsi_device *sdev)
>>>>>>>>>               else
>>>>>>>>>                       scsi_wait_for_queuecommand(sdev);
>>>>>>>>>       }
>>>>>>>>> +     wait_event(sdev->admin_wq, !atomic_read(&sdev->nr_admin_pending));
>>>>>>>>>       mutex_unlock(&sdev->state_mutex);
>>>>>>>>>
>>>>>>>>>       return err;
>>>>>>>> ...
>>>>>>>>> diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
>>>>>>>>> index 3aee9464a7bf..8bcb7ecc0c06 100644
>>>>>>>>> --- a/drivers/scsi/scsi_sysfs.c
>>>>>>>>> +++ b/drivers/scsi/scsi_sysfs.c
>>>>>>>>> @@ -1393,6 +1393,7 @@ void __scsi_remove_device(struct scsi_device *sdev)
>>>>>>>>>
>>>>>>>>>       blk_cleanup_queue(sdev->request_queue);
>>>>>>>>>       cancel_work_sync(&sdev->requeue_work);
>>>>>>>>> +     wait_event(sdev->admin_wq, !atomic_read(&sdev->nr_admin_pending))
>>>>>>>>
>>>>>>>> This nr_admin_pending could drain the ongoing scsi_request_fn/scsi_queue_rq,
>>>>>>>> but I'm afraid it cannot stop new ones coming in, such as the ones that have passed
>>>>>>>> the sdev state checking and have not crossed the atomic_inc(&sdev->nr_admin_pending).
>>>>>>>>
>>>>>>>
>>>>>>> The counter of .nr_admin_pending is introduced for draining queued
>>>>>>> admin requests to this scsi device.
>>>>>>>
>>>>>>> Actually new requests have been prevented from entering scsi_queue_rq(),
>>>>>>> please see the two callers of wait_event(sdev->admin_wq,
>>>>>>> !atomic_read(&sdev->nr_admin_pending)).
>>>>>>>
>>>>>> For example
>>>>>>
>>>>>> _scsi_execute
>>>>>> ...
>>>>>>                                           scsi_internal_device_block
>>>>>>                                             scsi_internal_device_block_nowait
>>>>>>                                             blk_mq_quiesce_queue
>>>>>>                                             wait_event(sdev->admin_wq, !atomic_read(&sdev->nr_admin_pending))
>>>>>>   &sdev->nr_admin_pending;
>>>>>>
>>>>>>   blk_execute_rq（...）
>>>>>>
>>>>>>   atomic_dec(&sdev->nr_admin_pending);
>>>>>>   wake_up_all(&sdev->admin_wq);
>>>>>>
>>>>>> Or do you mean the scsi_queue_rq -> scsi_prep_state_check could gate out of ?
>>>>>
>>>>> I got it, then this issue can be fixed simply by moving atomic_inc/dec(&sdev->nr_admin_pending)
>>>>> and related wake_up_all(&sdev->admin_wq) into scsi_admin_queue_rq().
>>>>>
>>>>
>>>> I don't think so. It is a similar scenario.
>>>>
>>>> I guess a state checking is needed after atomic_inc(&sdev->nr_admin_pending), like:
>>>>
>>>>  _scsi_execute
>>>>  ...
>>>>                                            scsi_internal_device_block
>>>>                                              scsi_internal_device_block_nowait
>>>>                                              blk_mq_quiesce_queue
>>>>                                              wait_event(sdev->admin_wq, !atomic_read(&sdev->nr_admin_pending))
>>>>    atomic_inc(&sdev->nr_admin_pending);
>>>>    if state checking fails
>>>>      goto done
>>>
>>> The check will be done in scsi_admin_queue_rq().
>>>
>>>>
>>>>    blk_execute_rq（...）
>>>>
>>>>    atomic_dec(&sdev->nr_admin_pending);
>>>>    wake_up_all(&sdev->admin_wq);
>>>
>>> I guess you may misunderstand the purpose of .nr_admin_pending, which is
>>> for draining requests to .queue_rq(). So it is enough to just move the
>>> inc/dec of .nr_admin_pending into scsi_admin_queue_rq(), right?
>> Yes.
> 
> Thanks for your confirmation.
> 
>>
>> But I just think of how to assign with queue quiesce.
>> The existence of per-host adminq seems to break it.
> 
> The per-host adminq won't be frozen or quiesced at all,

This is the place which makes me confused.

Before this patchset, when scsi_internal_device_block quiesces the request_queue,
Both normal IO or admin requests cannot be processed any more.

Given the per-adminq, we could say every scsi device has two request_queues, or
there could be two request_queues sending requests to the a scsi_device:
 - normal IO request_queue
 - shared per-host adminq

We could quiesce the normal IO request_queue, but what's about the per-host adminq ?

> could you explain
> your concern in a bit detail about 'assign with queue quiesce'? 
>

The scsi_queue_rq->scsi_prep_state_check could stop requests entering further, but
the scsi_queue_rq is still invoked.


Thanks
Jianchao