Linux Virtualization - May 2014 - [PATCH] block: virtio_blk: don't hold spin lock during world switch

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 at rustcorp.com.au>
Cc: "Michael S. Tsirkin" <mst at redhat.com>
Cc: virtualization at lists.linux-foundation.org
Signed-off-by: Ming Lei <ming.lei at canonical.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

On 2014-05-29 20:49, 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
Patch looks good to me. I don't see a hit on my qemu-kvm testing, but it 
definitely makes sense and I can see it hurting in other places.

-- 
Jens Axboe

On Fri, May 30, 2014 at 11:19 AM, Jens Axboe <axboe at kernel.dk>
wrote:
> On 2014-05-29 20:49, 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
>
>
> Patch looks good to me. I don't see a hit on my qemu-kvm testing, but
it
> definitely makes sense and I can see it hurting in other places.
It isn't easy to observe the improvement on x86 VM, especially
with few vCPUs, because qemu-system-x86_64 only takes
several microseconds to handle the notification, but on arm64, it
may take hundreds of microseconds, so the improvement is
obvious on arm VM.

I hope this patch can be merged, at least arm VM can benefit
from it.


Thanks,
--
Ming Lei

Ming Lei <ming.lei at canonical.com> writes:
> 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 at rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst at redhat.com>
> Cc: virtualization at lists.linux-foundation.org
> Signed-off-by: Ming Lei <ming.lei at canonical.com>
Acked-by: Rusty Russell <rusty at rustcorp.com.au>

Thanks!
Rusty.

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 at rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst at redhat.com>
> Cc: virtualization at lists.linux-foundation.org
> Signed-off-by: Ming Lei <ming.lei at canonical.com>
Acked-by: Michael S. Tsirkin <mst at 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