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Message-ID: <20120830141413.GD21132@redhat.com>
Date: Thu, 30 Aug 2012 17:14:13 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Sasha Levin <levinsasha928@...il.com>
Cc: rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH v3 1/2] virtio-ring: Use threshold for switching to
indirect descriptors
On Thu, Aug 30, 2012 at 01:21:57PM +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@...il.com>
Some minor comments here and for the next patch.
And I think I see a bug. Otherwise looks good to me.
Let's add MODULE_PARM_DESC clearly explaining
that this is a debugging option and users should
not touch it.
We can also add this text:
Min. number of free space in the ring to trigger direct
descriptor use. Set to 1 to always use direct
descriptors.
Also maybe direct_thresh is a better name.
> ---
> 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..7c63065 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..3a644f1 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..da2e44c 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..949c89e 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 = 16;
> +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..ca5ae7a 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..4063e03 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.
> + */
Fix comments to mention threshold.
> + if ((out + in) > 1 && vq->num_free &&
> + (vq->num_free < vq->indirect_thresh)) {
Please do not add () around conditions like this.
> 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;
> +
cleaner
else
` vq->indirect_thresh = 0;
So when indirect_thresh is 0 (default) this
disables indirect completely. Likely not what you wanted.
Maybe the right thing to do is something like
if (!indirect_thresh)
vq->indirect_thresh = vq->vring.num + 1
?
> 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..418f933 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
--
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