Re: [PATCH v2 1/2] virtio-ring: Use threshold for switching to indirect descriptors

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

On Tue, Aug 28, 2012 at 03:04:02PM +0200, Sasha Levin wrote:
> Currently if VIRTIO_RING_F_INDIRECT_DESC is enabled we will use indirect
> descriptors even if we have plenty of space in the ring. This means that
> we take a performance hit at all times due to the overhead of creating
> indirect descriptors.
> 
> Instead, use it only after we're below a configurable offset.
> 
> Signed-off-by: Sasha Levin <levinsasha928@gmail.com>

I imagine this helps performance? Any numbers?

> ---
>  drivers/block/virtio_blk.c          |  4 ++++
>  drivers/char/hw_random/virtio-rng.c |  4 ++++
>  drivers/char/virtio_console.c       |  4 ++++
>  drivers/net/virtio_net.c            |  4 ++++
>  drivers/virtio/virtio_balloon.c     |  4 ++++
>  drivers/virtio/virtio_ring.c        | 21 +++++++++++++++------
>  include/linux/virtio.h              |  1 +
>  net/9p/trans_virtio.c               |  4 ++++
>  8 files changed, 40 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 2edfb5c..13b8ae9 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -22,6 +22,9 @@ static DEFINE_IDA(vd_index_ida);
>  
>  struct workqueue_struct *virtblk_wq;
>  
> +static unsigned int indirect_thresh;
> +module_param(indirect_thresh, uint, S_IRUGO);
> +
>  struct virtio_blk
>  {
>  	struct virtio_device *vdev;
> @@ -735,6 +738,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  
>  	INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
>  	vblk->config_enable = true;
> +	vdev->indirect_thresh = indirect_thresh;
>  
>  	err = init_vq(vblk);
>  	if (err)
> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> index 5708299..02d8421 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -25,6 +25,9 @@
>  #include <linux/virtio_rng.h>
>  #include <linux/module.h>
>  
> +static unsigned int indirect_thresh;
> +module_param(indirect_thresh, uint, S_IRUGO);
> +
>  static struct virtqueue *vq;
>  static unsigned int data_avail;
>  static DECLARE_COMPLETION(have_data);
> @@ -93,6 +96,7 @@ static int probe_common(struct virtio_device *vdev)
>  	int err;
>  
>  	/* We expect a single virtqueue. */
> +	vdev->indirect_thresh = indirect_thresh;
>  	vq = virtio_find_single_vq(vdev, random_recv_done, "input");
>  	if (IS_ERR(vq))
>  		return PTR_ERR(vq);
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index e88f843..fc14e7f 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -39,6 +39,9 @@
>  #include <linux/module.h>
>  #include "../tty/hvc/hvc_console.h"
>  
> +static unsigned int indirect_thresh;
> +module_param(indirect_thresh, uint, S_IRUGO);
> +
>  /*
>   * This is a global struct for storing common data for all the devices
>   * this driver handles.
> @@ -1887,6 +1890,7 @@ static int __devinit virtcons_probe(struct virtio_device *vdev)
>  				       max_nr_ports),
>  			      &portdev->config.max_nr_ports) == 0)
>  		multiport = true;
> +	vdev->indirect_thresh = indirect_thresh;
>  
>  	err = init_vqs(portdev);
>  	if (err < 0) {
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index cbf8b06..64a8321 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -34,6 +34,9 @@ static bool csum = true, gso = true;
>  module_param(csum, bool, 0444);
>  module_param(gso, bool, 0444);
>  
> +static unsigned int indirect_thresh;
> +module_param(indirect_thresh, uint, S_IRUGO);
> +
>  /* FIXME: MTU in config. */
>  #define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
>  #define GOOD_COPY_LEN	128
> @@ -1128,6 +1131,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
>  		vi->mergeable_rx_bufs = true;
> +	vdev->indirect_thresh = indirect_thresh;
>  
>  	err = init_vqs(vi);
>  	if (err)
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 0908e60..fce7347 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -35,6 +35,9 @@
>   */
>  #define VIRTIO_BALLOON_PAGES_PER_PAGE (PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
>  
> +static unsigned int indirect_thresh;
> +module_param(indirect_thresh, uint, S_IRUGO);
> +
>  struct virtio_balloon
>  {
>  	struct virtio_device *vdev;
> @@ -356,6 +359,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  	init_waitqueue_head(&vb->acked);
>  	vb->vdev = vdev;
>  	vb->need_stats_update = 0;
> +	vdev->indirect_thresh = indirect_thresh;
>  
>  	err = init_vqs(vb);
>  	if (err)
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 5aa43c3..99a64a7 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -87,8 +87,11 @@ struct vring_virtqueue
>  	/* Other side has made a mess, don't try any more. */
>  	bool broken;
>  
> -	/* Host supports indirect buffers */
> -	bool indirect;
> +	/*
> +	 * Min. number of free space in the ring to trigger direct
> +	 * descriptor use
> +	 */
> +	unsigned int indirect_thresh;
>  
>  	/* Host publishes avail event idx */
>  	bool event;
> @@ -216,9 +219,12 @@ int virtqueue_add_buf(struct virtqueue *_vq,
>  	}
>  #endif
>  
> -	/* If the host supports indirect descriptor tables, and we have multiple
> -	 * buffers, then go indirect. FIXME: tune this threshold */
> -	if (vq->indirect && (out + in) > 1 && vq->num_free) {
> +	/*
> +	 * If the host supports indirect descriptor tables, and we have multiple
> +	 * buffers, then go indirect.
> +	 */
> +	if ((out + in) > 1 && vq->num_free &&
> +		(vq->num_free < vq->indirect_thresh)) {
>  		head = vring_add_indirect(vq, sg, out, in, gfp);
>  		if (likely(head >= 0))
>  			goto add_head;
> @@ -647,13 +653,16 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
>  	vq->broken = false;
>  	vq->last_used_idx = 0;
>  	vq->num_added = 0;
> +	vq->indirect_thresh = 0;
>  	list_add_tail(&vq->vq.list, &vdev->vqs);
>  #ifdef DEBUG
>  	vq->in_use = false;
>  	vq->last_add_time_valid = false;
>  #endif
>  
> -	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
> +	if (virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
> +		vq->indirect_thresh = vdev->indirect_thresh;
> +
>  	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>  
>  	/* No callback?  Tell other side not to bother us. */
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index a1ba8bb..48bc457 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -69,6 +69,7 @@ struct virtio_device {
>  	/* Note that this is a Linux set_bit-style bitmap. */
>  	unsigned long features[1];
>  	void *priv;
> +	unsigned int indirect_thresh;
>  };
>  
>  #define dev_to_virtio(dev) container_of(dev, struct virtio_device, dev)
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 35b8911..fc93962 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -52,6 +52,9 @@
>  
>  #define VIRTQUEUE_NUM	128
>  
> +static unsigned int indirect_thresh;
> +module_param(indirect_thresh, uint, S_IRUGO);
> +
>  /* a single mutex to manage channel initialization and attachment */
>  static DEFINE_MUTEX(virtio_9p_lock);
>  static DECLARE_WAIT_QUEUE_HEAD(vp_wq);
> @@ -501,6 +504,7 @@ static int p9_virtio_probe(struct virtio_device *vdev)
>  	chan->vdev = vdev;
>  
>  	/* We expect one virtqueue, for requests. */
> +	vdev->indirect_thresh = indirect_thresh;
>  	chan->vq = virtio_find_single_vq(vdev, req_done, "requests");
>  	if (IS_ERR(chan->vq)) {
>  		err = PTR_ERR(chan->vq);
> -- 
> 1.7.12