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

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Date:	Mon, 21 May 2012 08:42:52 -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 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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