Re: [RFC PATCH 09/14] SCSI: create admin queue for each host

["jianchao.wang" <jianchao.w.wang@oracle.com>]

On 08/08/2018 03:11 PM, Ming Lei wrote:
> On Wed, Aug 8, 2018 at 1:57 PM, jianchao.wang
> <jianchao.w.wang@oracle.com> wrote:
>> Hi Ming
>>
>> On 08/08/2018 01:44 AM, Ming Lei wrote:
>>>
>>> +static struct request_queue *scsi_mq_alloc_admin_queue(struct Scsi_Host *shost)
>>> +{
>>> +     struct request_queue *q = __blk_mq_init_queue(&shost->tag_set,
>>> +                     QUEUE_FLAG_MQ_NO_SCHED_DEFAULT);
>>> +     if (IS_ERR(q))
>>> +             return NULL;
>>> +
>>> +     q->mq_ops = &scsi_mq_admin_ops;
>>> +
>>> +     __scsi_init_queue(shost, q);
>>> +
>>> +     return q;
>>> +}
>>
>> In your patch set, the logical adminq per host is standalone request_queue which share
>> the tagset with other request_queue.
>>
>> Due to the hctx_may_queue,
>> If only one LUN, the adminq will take away half of the driver tags when
>> the adminq is active.
> 
> Most of times, the admin queue is inactive, so it shouldn't be a big deal.
> 
>> And when multiple LUNs, all of the LUNs have to share the limited budget of tags
>> of the adminq.
>> This is unacceptable.
> 
> That can be solved easily, such as, let __blk_mq_tag_busy() not take
> account of admin queue, will do it in V2.
> 
>>
>> And also, not all the admin request is send out through scsi_execute, maybe SG_IO.
>> So the admin queue here looks more like the pm queue ?
> 
> SG_IO should be covered by IO queue in which SCSI_PASSTHROUGH
> is enabled. It is reasonable too since SG_IO can be normal IO from
> userspace.
> 

I just concern too much complexity would be introduced by this logical admin queue. ;)

Thanks
Jianchao