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>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1dc157c85a83888a209db9cc1e9c9de7b16f280a..89bd9cb9defcb7786cdeef555d2726d40243884e 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -568,6 +568,20 @@ static void __blk_mq_complete_request(struct request *rq)
        if (!blk_mq_mark_complete(rq))
                return;
 
+       /*
+        * 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 15c1f5e12eb89460bc42eb7f5807eaa03254e51d..e47a2f751884ddb7821fededef643693db752393 100644 (file)
--- 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