Lot of controllers may have only one irq vector for completing IO
request. And usually affinity of the only irq vector is all possible
CPUs, however, on most of ARCH, there may be only one specific CPU
for handling this interrupt.

So if all IOs are completed in hardirq context, it is inevitable to
degrade IO performance because of increased irq latency.

This patch tries to address this issue by allowing to complete request
in softirq context, like the legacy IO path.

IOPS is observed as ~13%+ in the following randread test on raid0 over
virtio-scsi.

mdadm --create --verbose /dev/md0 --level=0 --chunk=1024 --raid-devices=8 /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf /dev/sdg /dev/sdh /dev/sdi

fio --time_based --name=benchmark --runtime=30 --filename=/dev/md0 --nrfiles=1 --ioengine=libaio --iodepth=32 --direct=1 --invalidate=1 --verify=0 --verify_fatal=0 --numjobs=32 --rw=randread --blocksize=4k

Cc: Zach Marano <redacted>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Jianchao Wang <redacted>
Signed-off-by: Ming Lei <redacted>
---
 block/blk-mq.c      | 14 ++++++++++++++
 block/blk-softirq.c |  7 +++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 85a1c1a59c72..d4792c3ac983 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -565,6 +565,20 @@ static void __blk_mq_complete_request(struct request *rq)
 	if (rq->internal_tag != -1)
 		blk_mq_sched_completed_request(rq);
 
+	/*
+	 * Most of single queue controllers, there is only one irq vector
+	 * for handling IO completion, and the only irq's affinity is set
+	 * as all possible CPUs. On most of ARCHs, this affinity means the
+	 * irq is handled on one specific CPU.
+	 *
+	 * So complete IO reqeust in softirq context in case of single queue
+	 * for not degrading IO performance by irqsoff latency.
+	 */
+	if (rq->q->nr_hw_queues == 1) {
+		__blk_complete_request(rq);
+		return;
+	}
+
 	if (!test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags)) {
 		rq->q->softirq_done_fn(rq);
 		return;

diff --git a/block/blk-softirq.c b/block/blk-softirq.c
index 15c1f5e12eb8..b1df9b6c1731 100644
--- a/block/blk-softirq.c
+++ b/block/blk-softirq.c

@@ -101,17 +101,20 @@ void __blk_complete_request(struct request *req)
 	struct request_queue *q = req->q;
 	unsigned long flags;
 	bool shared = false;
+	int rq_cpu;
 
 	BUG_ON(!q->softirq_done_fn);
 
+	rq_cpu = q->mq_ops ? req->mq_ctx->cpu : req->cpu;
+
 	local_irq_save(flags);
 	cpu = smp_processor_id();
 
 	/*
 	 * Select completion CPU
 	 */
-	if (req->cpu != -1) {
-		ccpu = req->cpu;
+	if (rq_cpu != -1) {
+		ccpu = q->mq_ops ? req->mq_ctx->cpu : req->cpu;
 		if (!test_bit(QUEUE_FLAG_SAME_FORCE, &q->queue_flags))
 			shared = cpus_share_cache(cpu, ccpu);
 	} else