blk-mq: complete req in softirq context in case of single queue

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: Dongli Zhang <dongli.zhang@oracle.com>
Cc: Zach Marano <zmarano@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 2 insertions, 3 deletions

diff --git a/block/blk-softirq.c b/block/blk-softirq.c
index 15c1f5e12eb8..e47a2f751884 100644
--- a/block/blk-softirq.c
+++ b/block/blk-softirq.c
@@ -97,8 +97,8 @@ static int blk_softirq_cpu_dead(unsigned int cpu)
 
 void __blk_complete_request(struct request *req)
 {
-        int ccpu, cpu;
         struct request_queue *q = req->q;
+        int cpu, ccpu = q->mq_ops ? req->mq_ctx->cpu : req->cpu;
         unsigned long flags;
         bool shared = false;
 
@@ -110,8 +110,7 @@ void __blk_complete_request(struct request *req)
         /*
          * Select completion CPU
          */
-        if (req->cpu != -1) {
-                ccpu = req->cpu;
+        if (test_bit(QUEUE_FLAG_SAME_COMP, &q->queue_flags) && ccpu != -1) {
                 if (!test_bit(QUEUE_FLAG_SAME_FORCE, &q->queue_flags))
                         shared = cpus_share_cache(cpu, ccpu);
         } else