On Fri, Sep 28, 2018 at 04:42:20PM +0800, Ming Lei wrote:
> 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@xxxxxxxxxx>
> Cc: Zach Marano <zmarano@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Bart Van Assche <bvanassche@xxxxxxx>
> Cc: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> block/blk-mq.c | 14 ++++++++++++++
> block/blk-softirq.c | 5 ++---
> 2 files changed, 16 insertions(+), 3 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..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
> --
> 2.9.5
>
Ping...
Thanks,
Ming
