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

[Tyrel Datwyler]

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@xxxxxxxxxxxxx>
>> ---
>>  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 complaint before was not using shared tags increases the tag memory
allocation because they are statically allocated for each queue. The question is
what claims more memory our event pool allocation or the tagset per queue
allocation.

If we chose to not use the shared tagset then the queue depth for each hw queue
becomes (can_queue / nr_hw_queues).

-Tyrel

> 
> Thanks,
> 
> Brian
> 
>