Re: [PATCH v19 2/3] scsi: ufs: L2P map management for HPB read

[Can Guo]

On 2021-02-09 09:27, Daejun Park wrote:
> > > > > > @@ -342,13 +1208,14 @@ void ufshpb_suspend(struct ufs_hba *hba)
> > > > > > >          struct scsi_device *sdev;
> > > > > > >
> > > > > > >          shost_for_each_device(sdev, hba->host) {
> > > > > > > -                hpb = sdev->hostdata;
> > > > > > > +                hpb = ufshpb_get_hpb_data(sdev);
> > > > > > >                  if (!hpb)
> > > > > > >                          continue;
> > > > > > >
> > > > > > >                  if (ufshpb_get_state(hpb) != HPB_PRESENT)
> > > > > > >                          continue;
> > > > > > >                  ufshpb_set_state(hpb, HPB_SUSPEND);
> > > > > > > +                ufshpb_cancel_jobs(hpb);
> > > > > >
> > > > > > Here may have a dead lock problem - in the case of runtime 
> > > > > > suspend,
> > > > > > when ufshpb_suspend() is invoked, all of hba's children scsi 
> > > > > > devices
> > > > > > are in RPM_SUSPENDED state. When this line tries to cancel a 
> > > > > > running
> > > > > > map work, i.e. when ufshpb_get_map_req() calls below lines, it 
> > > > > > will
> > > > > > be stuck at blk_queue_enter().
> > > > > >
> > > > > > req = blk_get_request(hpb->sdev_ufs_lu->request_queue,
> > > > > >                       REQ_OP_SCSI_IN, 0);
> > > > > >
> > > > > > Please check block layer power management, and see also commit
> > > > > > d55d15a33
> > > > > > ("scsi: block: Do not accept any requests while suspended").
> > > > >
> > > > > I am agree with your comment.
> > > > > How about add BLK_MQ_REQ_NOWAIT flag on blk_get_request() to avoid
> > > > > hang?
> > > >
> > > > That won't work - BLK_MQ_REQ_NOWAIT allows one to fast fail from
> > > > blk_mq_get_tag(),
> > > > but blk_queue_enter() comes before __blk_mq_alloc_request();
> > > In blk_queue_enter(), BLK_MQ_REQ_NOWAIT flag can make error than wait
> > > rpm
> > > resume. Please refer following code.
> >
> > Oops, sorry, my memory needs to be refreshed on that part.
> >
> > But will BLK_MQ_REQ_NOWAIT flag breaks your original purpose? When
> > runtime suspend is out of the picture, if traffic is heavy on the
> > request queue, map_work() will be stopped frequently once it is
> > not able to get a request from the queue - that shall pull down the
> > efficiency of one map_work(), that may hurt random performance...
>
> I think deadlock prevention is the most important. So I want to add
> BLK_MQ_REQ_NOWAIT flag.
> Starvation of map request can be distinguish by return value of
> blk_get_request(). -EWOULDBLOCK means there is no available tags for 
> this
> request. -EBUSY means failed on blk_queue_enter(). To overcome 
> starvation
> of map request, we can try N times in heavy traffic situation (maybe 
> N=3?).

LGTM. You make the call.

Regards,
Can Guo.

> Thanks,
> Daejun