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

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Date:	Mon, 21 May 2012 09:12:46 -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 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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