In case of BLK_MQ_F_BLOCKING, blk-mq uses SRCU to mark read critical
section during dispatching request, then request queue quiesce is based on
SRCU. What we want to get is low cost added in fast path.
However, from srcu_read_lock/srcu_read_unlock implementation, not see
it is quicker than percpu refcount, so use percpu_ref to implement
queue quiesce. This usage is cleaner and simpler & enough for implementing
queue quiesce. The main requirement is to make sure all read sections to observe
QUEUE_FLAG_QUIESCED once blk_mq_quiesce_queue() returns.
Also it becomes much easier to add interface of async queue quiesce.
BTW, no obvious IOPS difference is observed with this patch applied when running
io on null_blk(blocking, submit_queues=32) in one dual-socket, 32cores system.
Thanks Ming, can you test for non-blocking on the same setup?
I can test some reset storms during traffic.
