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    SubjectRe: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback
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    On Mon, 2012-05-21 at 08:42 -0700, Shirley Ma wrote:
    > On Mon, 2012-05-21 at 14:05 +0800, Jason Wang wrote:
    > > >> - tx polling depends on skb_orphan() which is often called by
    > > device
    > > >> driver when it place the packet into the queue of the devices
    > > instead
    > > >> of when the packets were sent. So it was too early for vhost to
    > be
    > > >> notified.
    > > > Then do you think it's better to replace with vhost_poll_queue
    > here
    > > > instead?
    > >
    > > Just like what does this patch do - calling vhost_poll_queue() in
    > > vhost_zerocopy_callback().
    > > >> - it only works when the pending DMAs exceeds VHOST_MAX_PEND,
    > it's
    > > >> highly possible that guest needs to be notified when the pending
    > > >> packets
    > > >> isn't so much.
    > > > In which situation the guest needs to be notified when there is no
    > > TX
    > > > besides buffers run out?
    > >
    > > Consider guest call virtqueue_enable_cb_delayed() which means it
    > only
    > > need to be notified when 3/4 of pending buffers ( about 178 buffers
    > > (256-MAX_SKB_FRAGS-2)*3/4 ) were sent by host. So vhost_net would
    > > notify
    > > guest when about 60 buffers were pending. Since tx polling is only
    > > enabled when pending packets exceeds VHOST_MAX_PEND 128, so tx work
    > > would not be notified to run and guest would never get the interrupt
    > > it
    > > expected to re-enable the queue.
    >
    > So it seems we still need vhost_enable_notify() in handle_tx when there
    > is no tx in zerocopy case.
    >
    > Do you know which one is more expensive: the cost of
    > vhost_poll_queue()
    > in each zerocopy callback or calling vhost_enable_notify()?
    >
    > Have you compared the results by removing below code in handle_tx()?
    >
    > - if (unlikely(num_pends > VHOST_MAX_PEND)) {
    > - tx_poll_start(net, sock);
    > - set_bit(SOCK_ASYNC_NOSPACE,
    > &sock->flags);
    > - break;
    > - }
    > >
    > > And just like what we've discussed, tx polling based adding and
    > > signaling is too early for vhost.
    >

    Then it could be too early for vhost to notify guest anywhere in
    handle_tx for zerocopy. Then we might need to remove any notification in
    handle_tx for zerocopy to vhost zerocopy callback instead.

    Adding vhost_poll_queue in vhost zerocopy callback unconditionally would
    consume unnecessary cpu.

    We need to think about a better solution here.

    Thanks
    Shirley





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