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

Ming Lei
2018-08-08T07:11:51Z

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.

thanks,
Ming Lei