RE: [PATCH] scsi: ufs: Fix proper API to send HPB pre-request

[Daejun Park]

> On Fri, Oct 29, 2021 at 11:50:12AM +0900, Daejun Park wrote:
> > > On Fri, Oct 29, 2021 at 10:50:15AM +0900, Daejun Park wrote:
> > > > > On Thu, Oct 28, 2021 at 07:36:19AM +0900, Daejun Park wrote:
> > > > > > This patch addresses the issue of using the wrong API to create a
> > > > > > pre_request for HPB READ.
> > > > > > HPB READ candidate that require a pre-request will try to allocate a
> > > > > > pre-request only during request_timeout_ms (default: 0). Otherwise, it is
> > > > >  
> > > > > Can you explain about 'only during request_timeout_ms'?
> > > > >  
> > > > > From the following code in ufshpb_prep(), the pre-request is allocated
> > > > > for each READ IO in case of (!ufshpb_is_legacy(hba) && ufshpb_is_required_wb(hpb,
> > > > > transfer_len)).
> > > > >  
> > > > >    if (!ufshpb_is_legacy(hba) &&
> > > > >             ufshpb_is_required_wb(hpb, transfer_len)) {
> > > > >                 err = ufshpb_issue_pre_req(hpb, cmd, &read_id);
> > > > >  
> > > > > > passed as normal READ, so deadlock problem can be resolved.
> > > > > > 
> > > > > > Signed-off-by: Daejun Park <daejun7.park@xxxxxxxxxxx>
> > > > > > ---
> > > > > >  drivers/scsi/ufs/ufshpb.c | 11 +++++------
> > > > > >  drivers/scsi/ufs/ufshpb.h |  1 +
> > > > > >  2 files changed, 6 insertions(+), 6 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/scsi/ufs/ufshpb.c b/drivers/scsi/ufs/ufshpb.c
> > > > > > index 02fb51ae8b25..3117bd47d762 100644
> > > > > > --- a/drivers/scsi/ufs/ufshpb.c
> > > > > > +++ b/drivers/scsi/ufs/ufshpb.c
> > > > > > @@ -548,8 +548,7 @@ static int ufshpb_execute_pre_req(struct ufshpb_lu *hpb, struct scsi_cmnd *cmd,
> > > > > >                                   read_id);
> > > > > >          rq->cmd_len = scsi_command_size(rq->cmd);
> > > > > >  
> > > > > > -        if (blk_insert_cloned_request(q, req) != BLK_STS_OK)
> > > > > > -                return -EAGAIN;
> > > > > > +        blk_execute_rq_nowait(NULL, req, true, ufshpb_pre_req_compl_fn);
> > > > >  
> > > > > Be care with above change, blk_insert_cloned_request() allocates
> > > > > driver tag and issues the request to LLD directly, then returns the
> > > > > result. If anything fails in the code path, -EAGAIN is returned.
> > > > >  
> > > > > But blk_execute_rq_nowait() simply queued the request in block layer,
> > > > > and run hw queue. It doesn't allocate driver tag, and doesn't issue it
> > > > > to LLD.
> > > > >  
> > > > > So ufshpb_execute_pre_req() may think the pre-request is issued to LLD
> > > > > successfully, but actually not, maybe never. What will happen after the
> > > > > READ IO is issued to device, but the pre-request(write buffer) isn't
> > > > > sent to device?
> > > > 
> > > > In that case, the HPB READ cannot get benefit from pre-request. But it is not
> > > > common case.
> > >  
> > > OK, so the device will ignore the pre-request if it isn't received in
> > > time, not sure it is common or not, since blk_execute_rq_nowait()
> > > doesn't provide any feedback. Here looks blk_insert_cloned_request()
> > > is better.
> > 
> > Yor're right.
> > 
> > > > 
> > > > > Can you explain how this change solves the deadlock?
> > > > 
> > > > The deadlock is happen when the READ waiting allocation of pre-request. But
> > > > the timeout code makes to stop waiting after given time later.
> > >  
> > > If you mean blk-mq timeout code will be triggered, I think it won't.
> > > Meantime, LLD may see nothing to timeout too.
> > 
> > I mean timeout of the HPB code. Please refer following code:
> > 
> > if (!ufshpb_is_legacy(hba) &&
> >         ufshpb_is_required_wb(hpb, transfer_len)) {
> >         err = ufshpb_issue_pre_req(hpb, cmd, &read_id);
> >         if (err) {
> >                 unsigned long timeout;
> > 
> >                 timeout = cmd->jiffies_at_alloc + msecs_to_jiffies(
> >                           hpb->params.requeue_timeout_ms);
> > 
> >                 if (time_before(jiffies, timeout))
> >                         return -EAGAIN;
> > 
> >                 hpb->stats.miss_cnt++;
> >                 return 0;
> >         }
> > }
> > 
> > Although the return value of ufshpb_issue_pre_req() is -EAGAIN, the code
> > ignores the return value and issues READ not HPB READ.
>  
> OK, got it, this way should avoid the deadlock. But just be curious why
> you change hpb->throttle_pre_req to 4, seems it isn't necessary for
> avoiding the deadlock?

Because blk_execute_rq_nowait calls blk_mq_run_hw_queue, not dispatchs WRITE_BUFFER directly.
So, if the next request requires pre-request, it makes the latency of first read longer.
Therefore, it prevents this extreme case by limiting number of pre-request.

Thanks,
Daejun