Re: [PATCH V2 3/3] vhost_net: basic polling support

From: Michael S. Tsirkin
Date: Wed Jan 20 2016 - 09:35:35 EST

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On Tue, Dec 01, 2015 at 02:39:45PM +0800, Jason Wang wrote:
> This patch tries to poll for new added tx buffer or socket receive
> queue for a while at the end of tx/rx processing. The maximum time
> spent on polling were specified through a new kind of vring ioctl.
>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> drivers/vhost/net.c | 72 ++++++++++++++++++++++++++++++++++++++++++----
> drivers/vhost/vhost.c | 15 ++++++++++
> drivers/vhost/vhost.h | 1 +
> include/uapi/linux/vhost.h | 11 +++++++
> 4 files changed, 94 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9eda69e..ce6da77 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -287,6 +287,41 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
> rcu_read_unlock_bh();
> }
>
> +static inline unsigned long busy_clock(void)
> +{
> + return local_clock() >> 10;
> +}
> +
> +static bool vhost_can_busy_poll(struct vhost_dev *dev,
> + unsigned long endtime)
> +{
> + return likely(!need_resched()) &&
> + likely(!time_after(busy_clock(), endtime)) &&
> + likely(!signal_pending(current)) &&
> + !vhost_has_work(dev) &&
> + single_task_running();
> +}
> +
> +static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
> + struct vhost_virtqueue *vq,
> + struct iovec iov[], unsigned int iov_size,
> + unsigned int *out_num, unsigned int *in_num)
> +{
> + unsigned long uninitialized_var(endtime);
> +
> + if (vq->busyloop_timeout) {
> + preempt_disable();
> + endtime = busy_clock() + vq->busyloop_timeout;
> + while (vhost_can_busy_poll(vq->dev, endtime) &&
> + !vhost_vq_more_avail(vq->dev, vq))
> + cpu_relax();
> + preempt_enable();
> + }

Isn't there a way to call all this after vhost_get_vq_desc?
First, this will reduce the good path overhead as you
won't have to play with timers and preemption.

Second, this will reduce the chance of a pagefault on avail ring read.

> +
> + return vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
> + out_num, in_num, NULL, NULL);
> +}
> +
> /* Expects to be always run from workqueue - which acts as
> * read-size critical section for our kind of RCU. */
> static void handle_tx(struct vhost_net *net)
> @@ -331,10 +366,9 @@ static void handle_tx(struct vhost_net *net)
> % UIO_MAXIOV == nvq->done_idx))
> break;
>
> - head = vhost_get_vq_desc(vq, vq->iov,
> - ARRAY_SIZE(vq->iov),
> - &out, &in,
> - NULL, NULL);
> + head = vhost_net_tx_get_vq_desc(net, vq, vq->iov,
> + ARRAY_SIZE(vq->iov),
> + &out, &in);
> /* On error, stop handling until the next kick. */
> if (unlikely(head < 0))
> break;
> @@ -435,6 +469,34 @@ static int peek_head_len(struct sock *sk)
> return len;
> }
>
> +static int vhost_net_peek_head_len(struct vhost_net *net, struct sock *sk)

Need a hint that it's rx related in the name.

> +{
> + struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
> + struct vhost_virtqueue *vq = &nvq->vq;
> + unsigned long uninitialized_var(endtime);
> +
> + if (vq->busyloop_timeout) {
> + mutex_lock(&vq->mutex);

This appears to be called under vq mutex in handle_rx.
So how does this work then?

> + vhost_disable_notify(&net->dev, vq);

This appears to be called after disable notify
in handle_rx - so why disable here again?

> +
> + preempt_disable();
> + endtime = busy_clock() + vq->busyloop_timeout;
> +
> + while (vhost_can_busy_poll(&net->dev, endtime) &&
> + skb_queue_empty(&sk->sk_receive_queue) &&
> + !vhost_vq_more_avail(&net->dev, vq))
> + cpu_relax();

This seems to mix in several items.
RX queue is normally not empty. I don't think
we need to poll for that.
So IMHO we only need to poll for sk_receive_queue really.

> +
> + preempt_enable();
> +
> + if (vhost_enable_notify(&net->dev, vq))
> + vhost_poll_queue(&vq->poll);

But vhost_enable_notify returns true on queue not empty.
So in fact this will requeue on good path -
does not make sense to me.

> + mutex_unlock(&vq->mutex);
> + }
> +

Same comment as for get vq desc here: don't slow
down the good path.

> + return peek_head_len(sk);
> +}
> +
> /* This is a multi-buffer version of vhost_get_desc, that works if
> * vq has read descriptors only.
> * @vq - the relevant virtqueue
> @@ -553,7 +615,7 @@ static void handle_rx(struct vhost_net *net)
> vq->log : NULL;
> mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
>
> - while ((sock_len = peek_head_len(sock->sk))) {
> + while ((sock_len = vhost_net_peek_head_len(net, sock->sk))) {
> sock_len += sock_hlen;
> vhost_len = sock_len + vhost_hlen;
> headcount = get_rx_bufs(vq, vq->heads, vhost_len,
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 4f45a03..b8ca873 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -285,6 +285,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> vq->memory = NULL;
> vq->is_le = virtio_legacy_is_little_endian();
> vhost_vq_reset_user_be(vq);
> + vq->busyloop_timeout = 0;
> }
>
> static int vhost_worker(void *data)
> @@ -747,6 +748,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> struct vhost_vring_state s;
> struct vhost_vring_file f;
> struct vhost_vring_addr a;
> + struct vhost_vring_busyloop_timeout t;
> u32 idx;
> long r;
>
> @@ -919,6 +921,19 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
> case VHOST_GET_VRING_ENDIAN:
> r = vhost_get_vring_endian(vq, idx, argp);
> break;
> + case VHOST_SET_VRING_BUSYLOOP_TIMEOUT:
> + if (copy_from_user(&t, argp, sizeof(t))) {
> + r = -EFAULT;
> + break;
> + }
> + vq->busyloop_timeout = t.timeout;
> + break;
> + case VHOST_GET_VRING_BUSYLOOP_TIMEOUT:
> + t.index = idx;
> + t.timeout = vq->busyloop_timeout;
> + if (copy_to_user(argp, &t, sizeof(t)))
> + r = -EFAULT;
> + break;
> default:
> r = -ENOIOCTLCMD;
> }
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 2f3c57c..4b7d4fa 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -115,6 +115,7 @@ struct vhost_virtqueue {
> /* Ring endianness requested by userspace for cross-endian support. */
> bool user_be;
> #endif
> + u32 busyloop_timeout;
> };
>
> struct vhost_dev {
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index ab373191..eaf6c33 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -27,6 +27,11 @@ struct vhost_vring_file {
>
> };
>
> +struct vhost_vring_busyloop_timeout {
> + unsigned int index;
> + unsigned int timeout;
> +};
> +

So why not reuse vhost_vring_state then?

> struct vhost_vring_addr {
> unsigned int index;
> /* Option flags. */
> @@ -126,6 +131,12 @@ struct vhost_memory {
> #define VHOST_SET_VRING_CALL _IOW(VHOST_VIRTIO, 0x21, struct vhost_vring_file)
> /* Set eventfd to signal an error */
> #define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
> +/* Set busy loop timeout */

Units?

> +#define VHOST_SET_VRING_BUSYLOOP_TIMEOUT _IOW(VHOST_VIRTIO, 0x23, \
> + struct vhost_vring_busyloop_timeout)
> +/* Get busy loop timeout */
> +#define VHOST_GET_VRING_BUSYLOOP_TIMEOUT _IOW(VHOST_VIRTIO, 0x24, \
> + struct vhost_vring_busyloop_timeout)
>
> /* VHOST_NET specific defines */
>
> --
> 2.5.0

Next message: Wan, Kaike: "RE: net: GPF in __netlink_ns_capable"
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