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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 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.

    Thanks
    Shirley



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