netdev - Re: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback

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Date:	Tue, 22 May 2012 08:55:45 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, eric.dumazet@...il.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, davem@...emloft.net
Subject: Re: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback

On Tue, 2012-05-22 at 18:05 +0800, Jason Wang wrote:
> On 05/21/2012 11:42 PM, 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()?
> 
> Didn't follow here, do you mean vhost_signal() here? 

I meant removing the code in handle_tx for zerocopy as below:

+	if (zcopy) {
                        /* If more outstanding DMAs, queue the work.
                         * Handle upend_idx wrap around
                         */
                        num_pends = likely(vq->upend_idx >= vq->done_idx) ?
                                    (vq->upend_idx - vq->done_idx) :
                                    (vq->upend_idx + UIO_MAXIOV - vq->done_idx);
+			/* zerocopy vhost_enable_notify is under zerocopy callback
+			 * since it could be too early to notify here */
+			break;
+	}
-                       if (unlikely(num_pends > VHOST_MAX_PEND)) {
-                                tx_poll_start(net, sock);
-                                set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
-                                break;
-                        }
                        if (unlikely(vhost_enable_notify(&net->dev, vq))) {
                                vhost_disable_notify(&net->dev, vq);
                                continue;
                        }
                        break;

Thanks
Shirley

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