[PATCH 0/3] Add BLK_MQ_F_NO_SCHED flag to admin tag set

[Israel Rukshin]

On 10/18/2017 3:50 PM, Sagi Grimberg wrote:
>
>> Since commit b86dd81
>> "block: get rid of blk-mq default scheduler choice Kconfig entries",
>> when setting nr_hw_queues to 1 the admin tag set uses mq-deadline 
>> scheduler.
>> This flag is useful for admin queues that aren't used for normal IO.
>> We already use this flag at nvme-pci.
>
> How is the fact that we set a single hw queue has anything to do with
> the admin tagset?

Setting nr_hw_queues attribute of admin tag set to 1 influence the value
of nr_hw_queues attribute of the admin queue.
At /block/elevator.c
elevator_init() selects the scheduler according to the value of 
nr_hw_queues
attribute of the admin queue.

if (q->mq_ops) {
     if (q->nr_hw_queues == 1)
         e = elevator_get("mq-deadline", false);
     if (!e)
         return 0;
...

Israel