Re: [PATCH] scsi: ufs: mark HPB support as BROKEN

Keith Busch
2021-10-27T16:16:38Z

On Wed, Oct 27, 2021 at 11:58:23PM +0800, Ming Lei wrote:
> On Wed, Oct 27, 2021 at 11:44:04AM -0400, Martin K. Petersen wrote:
> >
> > Ming,
> >
> > > request with scsi_cmnd may be allocated by the ufshpb driver, even it
> > > should be fine to call ufshcd_queuecommand() directly for this driver
> > > private IO, if the tag can be reused. One example is scsi_ioctl_reset().
> >
> > scsi_ioctl_reset() allocates a new request, though, so that doesn't
> > solve the forward progress guarantee. Whereas eh puts the saved request
> > on the stack.
>
> What I meant is to use one totally ufshpb private command allocated from
> private slab to replace the spawned request, which is sent to ufshcd_queuecommand()
> directly, so forward progress is guaranteed if the blk-mq request's tag can be
> reused for issuing this private command. This approach takes a bit effort,
> but avoids tags reservation.
>
> Yeah, it is cleaner to use reserved tag for the spawned request, but we
> need to know:
>
> 1) how many queue depth for the hba? If it is small, even 1 reservation
> can affect performance.
>
> 2) how many inflight write buffer commands are to be supported? Or how many
> is enough for obtaining expected performance? If the number is big, reserved
> tags can't work.

The original and clone are not dispatched to hardware concurrently, so I
don't think the reserved_tags need to subtract from the generic ones.
The original request already accounts for the hardware resource, so the
clone doesn't need to consume another one.