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

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On Tue, Sep 18, 2018 at 04:25:33PM +0800, jianchao.wang wrote:
> 
> 
> 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@xxxxxxxxxx> 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 ?

The adminq request can be quisced in the scsi_device level, just as normal IO
request, because the scsi_device instance is same with normal IO.

But in blk-mq level, this admin queue won't be quiesced or frozen at all.

> 
> > 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.

As I mentioned, there isn't any change in scsi level about handling the
admin request because the scsi_device is shared, and scsi_host too.

Thanks,
Ming



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