Re: [PATCH] block: virtio_blk: don't hold spin lock during world switch

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, May 30, 2014 at 10:49:29AM +0800, Ming Lei wrote:
> Firstly, it isn't necessary to hold lock of vblk->vq_lock
> when notifying hypervisor about queued I/O.
> 
> Secondly, virtqueue_notify() will cause world switch and
> it may take long time on some hypervisors(such as, qemu-arm),
> so it isn't good to hold the lock and block other vCPUs.
> 
> On arm64 quad core VM(qemu-kvm), the patch can increase I/O
> performance a lot with VIRTIO_RING_F_EVENT_IDX enabled:
> 	- without the patch: 14K IOPS
> 	- with the patch: 34K IOPS
> 
> fio script:
> 	[global]
> 	direct=1
> 	bsrange=4k-4k
> 	timeout=10
> 	numjobs=4
> 	ioengine=libaio
> 	iodepth=64
> 
> 	filename=/dev/vdc
> 	group_reporting=1
> 
> 	[f1]
> 	rw=randread
> 
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: virtualization@lists.linux-foundation.org
> Signed-off-by: Ming Lei <ming.lei@canonical.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/block/virtio_blk.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 9f340fa..a6f5424 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -162,6 +162,7 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
>  	unsigned int num;
>  	const bool last = (req->cmd_flags & REQ_END) != 0;
>  	int err;
> +	bool notify = false;
>  
>  	BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
>  
> @@ -214,10 +215,12 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
>  		return BLK_MQ_RQ_QUEUE_ERROR;
>  	}
>  
> -	if (last)
> -		virtqueue_kick(vblk->vq);
> -
> +	if (last && virtqueue_kick_prepare(vblk->vq))
> +		notify = true;
>  	spin_unlock_irqrestore(&vblk->vq_lock, flags);
> +
> +	if (notify)
> +		virtqueue_notify(vblk->vq);
>  	return BLK_MQ_RQ_QUEUE_OK;
>  }
>  
> -- 
> 1.7.9.5