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Date: Mon, 11 Mar 2013 10:26:24 +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 03:33:16PM +0800, Jason Wang wrote:
> On 03/11/2013 03:09 PM, 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.
>
> Or for zerocopy, looks like we can in fact just depends on
> ubuf->callback in stead of tx polling to wakeup tx thread to do tx
> completion, so we can even drop the num_pends and VHOST_MAX_PEND stuffs.
I think we still need to limit the number of outstanding buffers so
VHOST_MAX_PEND needs to stay. I'm not against switching to callback
for this but let's make this a separate change.
> > 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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