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

[Brian King <brking@linux.vnet.ibm.com>]

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. 

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.

Thanks,

Brian

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center