Re: [PATCH v4 01/21] ibmvfc: add vhost fields and defaults for MQ enablement

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jan 13, 2021 at 11:13:07AM -0600, Brian King wrote:
> On 1/12/21 6:33 PM, Tyrel Datwyler wrote:
> > On 1/12/21 2:54 PM, Brian King wrote:
> >> On 1/11/21 5:12 PM, Tyrel Datwyler wrote:
> >>> Introduce several new vhost fields for managing MQ state of the adapter
> >>> as well as initial defaults for MQ enablement.
> >>>
> >>> Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
> >>> ---
> >>>  drivers/scsi/ibmvscsi/ibmvfc.c | 8 ++++++++
> >>>  drivers/scsi/ibmvscsi/ibmvfc.h | 9 +++++++++
> >>>  2 files changed, 17 insertions(+)
> >>>
> >>> diff --git a/drivers/scsi/ibmvscsi/ibmvfc.c b/drivers/scsi/ibmvscsi/ibmvfc.c
> >>> index ba95438a8912..9200fe49c57e 100644
> >>> --- a/drivers/scsi/ibmvscsi/ibmvfc.c
> >>> +++ b/drivers/scsi/ibmvscsi/ibmvfc.c
> >>> @@ -3302,6 +3302,7 @@ static struct scsi_host_template driver_template = {
> >>>  	.max_sectors = IBMVFC_MAX_SECTORS,
> >>>  	.shost_attrs = ibmvfc_attrs,
> >>>  	.track_queue_depth = 1,
> >>> +	.host_tagset = 1,
> >>
> >> This doesn't seem right. You are setting host_tagset, which means you want a
> >> shared, host wide, tag set for commands. It also means that the total
> >> queue depth for the host is can_queue. However, it looks like you are allocating
> >> max_requests events for each sub crq, which means you are over allocating memory.
> > 
> > With the shared tagset yes the queue depth for the host is can_queue, but this
> > also implies that the max queue depth for each hw queue is also can_queue. So,
> > in the worst case that all commands are queued down the same hw queue we need an
> > event pool with can_queue commands.
> > 
> >>
> >> Looking at this closer, we might have bigger problems. There is a host wide
> >> max number of commands that the VFC host supports, which gets returned on
> >> NPIV Login. This value can change across a live migration event.
> > 
> > From what I understand the max commands can only become less.
> > 
> >>
> >> The ibmvfc driver, which does the same thing the lpfc driver does, modifies
> >> can_queue on the scsi_host *after* the tag set has been allocated. This looks
> >> to be a concern with ibmvfc, not sure about lpfc, as it doesn't look like
> >> we look at can_queue once the tag set is setup, and I'm not seeing a good way
> >> to dynamically change the host queue depth once the tag set is setup. 
> >>
> >> Unless I'm missing something, our best options appear to either be to implement
> >> our own host wide busy reference counting, which doesn't sound very good, or
> >> we need to add some API to block / scsi that allows us to dynamically change
> >> can_queue.
> > 
> > Changing can_queue won't do use any good with the shared tagset becasue each
> > queue still needs to be able to queue can_queue number of commands in the worst
> > case.
> 
> The issue I'm trying to highlight here is the following scenario:
> 
> 1. We set shost->can_queue, then call scsi_add_host, which allocates the tag set.
> 
> 2. On our NPIV login response from the VIOS, we might get a lower value than we
> initially set in shost->can_queue, so we update it, but nobody ever looks at it
> again, and we don't have any protection against sending too many commands to the host.
> 
> 
> Basically, we no longer have any code that ensures we don't send more
> commands to the VIOS than we are told it supports. According to the architecture,
> if we actually do this, the VIOS will do an h_free_crq, which would be a bit
> of a bug on our part.
> 
> I don't think it was ever clearly defined in the API that a driver can
> change shost->can_queue after calling scsi_add_host, but up until
> commit 6eb045e092efefafc6687409a6fa6d1dabf0fb69, this worked and now
> it doesn't. 

Actually it isn't related with commit 6eb045e092ef, because blk_mq_alloc_tag_set()
uses .can_queue to create driver tag sbitmap and request pool.

So even thought without 6eb045e092ef, the updated .can_queue can't work
as expected because the max driver tag depth has been fixed by blk-mq already.

What 6eb045e092ef does is just to remove the double check on max
host-wide allowed commands because that has been respected by blk-mq
driver tag allocation already.

> 
> I started looking through drivers that do this, and so far, it looks like the
> following drivers do: ibmvfc, lpfc, aix94xx, libfc, BusLogic, and likely others...
> 
> We probably need an API that lets us change shost->can_queue dynamically.

I'd suggest to confirm changing .can_queue is one real usecase.


Thanks,
Ming