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

From: Hannes Reinecke <hare@suse.de>
To: Brian King <brking@linux.vnet.ibm.com>,
	Tyrel Datwyler <tyreld@linux.ibm.com>,
	james.bottomley@hansenpartnership.com
Cc: brking@linux.ibm.com, linuxppc-dev@lists.ozlabs.org,
	linux-scsi@vger.kernel.org, martin.petersen@oracle.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 01/17] ibmvfc: add vhost fields and defaults for MQ enablement
Date: Mon, 7 Dec 2020 12:56:31 +0100	[thread overview]
Message-ID: <6ce79011-d288-7a49-3d51-262da58d8486@suse.de> (raw)
In-Reply-To: <efbfe9e9-c692-80a1-f5b4-55473d8193e4@linux.vnet.ibm.com>

On 12/4/20 3:26 PM, Brian King wrote:
> 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@linux.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.
> 
Before doing that I'd rather use the host-wide shared tagset.
Increasing the number of requests will increase the memory footprint of 
the driver (as each request will be statically allocated).

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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