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Date: Mon, 11 Mar 2013 10:29:23 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost_net: remove tx polling state
On Mon, Mar 11, 2013 at 03:09:10PM +0800, Jason Wang wrote:
> On 03/11/2013 12:50 AM, Michael S. Tsirkin wrote:
> > On Thu, Mar 07, 2013 at 12:31:56PM +0800, Jason Wang wrote:
> >> After commit 2b8b328b61c799957a456a5a8dab8cc7dea68575 (vhost_net: handle polling
> >> errors when setting backend), we in fact track the polling state through
> >> poll->wqh, so there's no need to duplicate the work with an extra
> >> vhost_net_polling_state. So this patch removes this and make the code simpler.
> >>
> >> Signed-off-by: Jason Wang <jasowang@...hat.com>
> > I'd prefer a more radical approach, since I think it can be even
> > simpler: tap and macvtap backends both only send events when tx queue
> > overruns which should almost never happen.
> >
> > So let's just start polling when VQ is enabled
> > drop all poll_start/stop calls on data path.
>
> I think then it may damage the performance at least for tx. We
> conditionally enable the tx polling in the past to reduce the
> unnecessary wakeups of vhost thead when zerocopy or sndbuf is used. If
> we don't stop the polling, after each packet were transmitted,
> tap/macvtap will try to wakeup the vhost thread.
It won't actually.
static void macvtap_sock_write_space(struct sock *sk)
{
wait_queue_head_t *wqueue;
if (!sock_writeable(sk) ||
!test_and_clear_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags))
return;
wqueue = sk_sleep(sk);
if (wqueue && waitqueue_active(wqueue))
wake_up_interruptible_poll(wqueue, POLLOUT | POLLWRNORM | POLLWRBAND);
}
As long as SOCK_ASYNC_NOSPACE is not set, there's no wakeup.
> Actually, I'm considering the reverse direction. Looks like we can
> disable the rx polling like what we do for tx in handle_tx() to reduce
> the uncessary wakeups.
> > The optimization was written for packet socket backend but I know of no
> > one caring about performance of that one that much.
> > Needs a bit of perf testing to make sure I didn't miss anything though
> > but it's not 3.9 material anyway so there's no rush.
> >
> >> ---
> >> drivers/vhost/net.c | 60 ++++++++----------------------------------------
> >> drivers/vhost/vhost.c | 3 ++
> >> 2 files changed, 13 insertions(+), 50 deletions(-)
> >>
> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> >> index 959b1cd..d1a03dd 100644
> >> --- a/drivers/vhost/net.c
> >> +++ b/drivers/vhost/net.c
> >> @@ -64,20 +64,10 @@ enum {
> >> VHOST_NET_VQ_MAX = 2,
> >> };
> >>
> >> -enum vhost_net_poll_state {
> >> - VHOST_NET_POLL_DISABLED = 0,
> >> - VHOST_NET_POLL_STARTED = 1,
> >> - VHOST_NET_POLL_STOPPED = 2,
> >> -};
> >> -
> >> struct vhost_net {
> >> struct vhost_dev dev;
> >> struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
> >> struct vhost_poll poll[VHOST_NET_VQ_MAX];
> >> - /* Tells us whether we are polling a socket for TX.
> >> - * We only do this when socket buffer fills up.
> >> - * Protected by tx vq lock. */
> >> - enum vhost_net_poll_state tx_poll_state;
> >> /* Number of TX recently submitted.
> >> * Protected by tx vq lock. */
> >> unsigned tx_packets;
> >> @@ -155,28 +145,6 @@ static void copy_iovec_hdr(const struct iovec *from, struct iovec *to,
> >> }
> >> }
> >>
> >> -/* Caller must have TX VQ lock */
> >> -static void tx_poll_stop(struct vhost_net *net)
> >> -{
> >> - if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> >> - return;
> >> - vhost_poll_stop(net->poll + VHOST_NET_VQ_TX);
> >> - net->tx_poll_state = VHOST_NET_POLL_STOPPED;
> >> -}
> >> -
> >> -/* Caller must have TX VQ lock */
> >> -static int tx_poll_start(struct vhost_net *net, struct socket *sock)
> >> -{
> >> - int ret;
> >> -
> >> - if (unlikely(net->tx_poll_state != VHOST_NET_POLL_STOPPED))
> >> - return 0;
> >> - ret = vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
> >> - if (!ret)
> >> - net->tx_poll_state = VHOST_NET_POLL_STARTED;
> >> - return ret;
> >> -}
> >> -
> >> /* In case of DMA done not in order in lower device driver for some reason.
> >> * upend_idx is used to track end of used idx, done_idx is used to track head
> >> * of used idx. Once lower device DMA done contiguously, we will signal KVM
> >> @@ -231,6 +199,7 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
> >> static void handle_tx(struct vhost_net *net)
> >> {
> >> struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> >> + struct vhost_poll *poll = net->poll + VHOST_NET_VQ_TX;
> >> unsigned out, in, s;
> >> int head;
> >> struct msghdr msg = {
> >> @@ -256,7 +225,7 @@ static void handle_tx(struct vhost_net *net)
> >> wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> >> if (wmem >= sock->sk->sk_sndbuf) {
> >> mutex_lock(&vq->mutex);
> >> - tx_poll_start(net, sock);
> >> + vhost_poll_start(poll, sock->file);
> >> mutex_unlock(&vq->mutex);
> >> return;
> >> }
> >> @@ -265,7 +234,7 @@ static void handle_tx(struct vhost_net *net)
> >> vhost_disable_notify(&net->dev, vq);
> >>
> >> if (wmem < sock->sk->sk_sndbuf / 2)
> >> - tx_poll_stop(net);
> >> + vhost_poll_stop(poll);
> >> hdr_size = vq->vhost_hlen;
> >> zcopy = vq->ubufs;
> >>
> >> @@ -287,7 +256,7 @@ static void handle_tx(struct vhost_net *net)
> >>
> >> wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> >> if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
> >> - tx_poll_start(net, sock);
> >> + vhost_poll_start(poll, sock->file);
> >> set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> >> break;
> >> }
> >> @@ -298,7 +267,7 @@ static void handle_tx(struct vhost_net *net)
> >> (vq->upend_idx - vq->done_idx) :
> >> (vq->upend_idx + UIO_MAXIOV - vq->done_idx);
> >> if (unlikely(num_pends > VHOST_MAX_PEND)) {
> >> - tx_poll_start(net, sock);
> >> + vhost_poll_start(poll, sock->file);
> >> set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> >> break;
> >> }
> >> @@ -364,7 +333,7 @@ static void handle_tx(struct vhost_net *net)
> >> }
> >> vhost_discard_vq_desc(vq, 1);
> >> if (err == -EAGAIN || err == -ENOBUFS)
> >> - tx_poll_start(net, sock);
> >> + vhost_poll_start(poll, sock->file);
> >> break;
> >> }
> >> if (err != len)
> >> @@ -627,7 +596,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
> >>
> >> vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
> >> vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
> >> - n->tx_poll_state = VHOST_NET_POLL_DISABLED;
> >>
> >> f->private_data = n;
> >>
> >> @@ -637,32 +605,24 @@ static int vhost_net_open(struct inode *inode, struct file *f)
> >> static void vhost_net_disable_vq(struct vhost_net *n,
> >> struct vhost_virtqueue *vq)
> >> {
> >> + struct vhost_poll *poll = n->poll + (vq - n->vqs);
> >> if (!vq->private_data)
> >> return;
> >> - if (vq == n->vqs + VHOST_NET_VQ_TX) {
> >> - tx_poll_stop(n);
> >> - n->tx_poll_state = VHOST_NET_POLL_DISABLED;
> >> - } else
> >> - vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
> >> + vhost_poll_stop(poll);
> >> }
> >>
> >> static int vhost_net_enable_vq(struct vhost_net *n,
> >> struct vhost_virtqueue *vq)
> >> {
> >> + struct vhost_poll *poll = n->poll + (vq - n->vqs);
> >> struct socket *sock;
> >> - int ret;
> >>
> >> sock = rcu_dereference_protected(vq->private_data,
> >> lockdep_is_held(&vq->mutex));
> >> if (!sock)
> >> return 0;
> >> - if (vq == n->vqs + VHOST_NET_VQ_TX) {
> >> - n->tx_poll_state = VHOST_NET_POLL_STOPPED;
> >> - ret = tx_poll_start(n, sock);
> >> - } else
> >> - ret = vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
> >>
> >> - return ret;
> >> + return vhost_poll_start(poll, sock->file);
> >> }
> >>
> >> static struct socket *vhost_net_stop_vq(struct vhost_net *n,
> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> >> index 9759249..4eecdb8 100644
> >> --- a/drivers/vhost/vhost.c
> >> +++ b/drivers/vhost/vhost.c
> >> @@ -89,6 +89,9 @@ int vhost_poll_start(struct vhost_poll *poll, struct file *file)
> >> unsigned long mask;
> >> int ret = 0;
> >>
> >> + if (poll->wqh)
> >> + return 0;
> >> +
> >> mask = file->f_op->poll(file, &poll->table);
> >> if (mask)
> >> vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
> >> --
> >> 1.7.1
> > --
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