linux-kernel - Re: [PATCH v2 01/17] ibmvfc: add vhost fields and defaults for MQ enablement

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Message-ID: <efbfe9e9-c692-80a1-f5b4-55473d8193e4@linux.vnet.ibm.com>
Date:   Fri, 4 Dec 2020 08:26:06 -0600
From:   Brian King <brking@...ux.vnet.ibm.com>
To:     Tyrel Datwyler <tyreld@...ux.ibm.com>,
        james.bottomley@...senpartnership.com
Cc:     martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        brking@...ux.ibm.com
Subject: Re: [PATCH v2 01/17] ibmvfc: add vhost fields and defaults for MQ
 enablement

On 12/2/20 11:27 AM, Tyrel Datwyler wrote:
> On 12/2/20 7:14 AM, Brian King wrote:
>> On 12/1/20 6:53 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@...ux.ibm.com>
>>> ---
>>>  drivers/scsi/ibmvscsi/ibmvfc.c |  9 ++++++++-
>>>  drivers/scsi/ibmvscsi/ibmvfc.h | 13 +++++++++++--
>>>  2 files changed, 19 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/scsi/ibmvscsi/ibmvfc.c b/drivers/scsi/ibmvscsi/ibmvfc.c
>>> index 42e4d35e0d35..f1d677a7423d 100644
>>> --- a/drivers/scsi/ibmvscsi/ibmvfc.c
>>> +++ b/drivers/scsi/ibmvscsi/ibmvfc.c
>>> @@ -5161,12 +5161,13 @@ static int ibmvfc_probe(struct vio_dev *vdev, const struct vio_device_id *id)
>>>  	}
>>>  
>>>  	shost->transportt = ibmvfc_transport_template;
>>> -	shost->can_queue = max_requests;
>>> +	shost->can_queue = (max_requests / IBMVFC_SCSI_HW_QUEUES);
>>
>> This doesn't look right. can_queue is the SCSI host queue depth, not the MQ queue depth.
> 
> Our max_requests is the total number commands allowed across all queues. From
> what I understand is can_queue is the total number of commands in flight allowed
> for each hw queue.
> 
>         /*
>          * In scsi-mq mode, the number of hardware queues supported by the LLD.
>          *
>          * Note: it is assumed that each hardware queue has a queue depth of
>          * can_queue. In other words, the total queue depth per host
>          * is nr_hw_queues * can_queue. However, for when host_tagset is set,
>          * the total queue depth is can_queue.
>          */
> 
> We currently don't use the host wide shared tagset.

Ok. I missed that bit... In that case, since we allocate by default only 100
event structs. If we slice that across IBMVFC_SCSI_HW_QUEUES (16) queues, then
we end up with only about 6 commands that can be outstanding per queue,
which is going to really hurt performance... I'd suggest bumping up
IBMVFC_MAX_REQUESTS_DEFAULT from 100 to 1000 as a starting point.

Thanks,

Brian


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