On Wednesday, November 28, 2012 08:53:05 AM Stephen Hemminger wrote:

On Wed, 28 Nov 2012 14:48:52 +0800

Jason Wang [off-list ref] wrote:

quoted

On 11/28/2012 12:49 AM, Stephen Hemminger wrote:

quoted

On Tue, 27 Nov 2012 14:45:13 +0800

Jason Wang [off-list ref] wrote:

quoted

On 11/27/2012 01:37 AM, Stephen Hemminger wrote:

quoted

On Mon, 26 Nov 2012 15:56:52 +0800

Jason Wang [off-list ref] wrote:

quoted

Some deivces do not free the old tx skbs immediately after it has
been sent
(usually in tx interrupt). One such example is virtio-net which
optimizes for virt and only free the possible old tx skbs during the
next packet sending. This would lead the pktgen to wait forever in
the refcount of the skb if no other pakcet will be sent afterwards.

Solving this issue by introducing a new flag IFF_TX_SKB_FREE_DELAY
which could notify the pktgen that the device does not free skb
immediately after it has been sent and let it not to wait for the
refcount to be one.

Signed-off-by: Jason Wang <jasowang@redhat.com>

Another alternative would be using skb_orphan() and skb->destructor.
There are other cases where skb's are not freed right away.
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Hi Stephen:

Do you mean registering a skb->destructor for pktgen then set and check
bits in skb->tx_flag?

Yes. Register a destructor that does something like update a counter
(number of packets pending), then just spin while number of packets
pending is over threshold.
--

Not sure this is the best method, since pktgen was used to test the tx
process of the device driver and NIC. If we use skb_orhpan(), we would
miss the test of tx completion part.

There are other places that delay freeing and your solution would mean
finding and fixing all those. Code that does that already has to use
skb_orphan() to work, and I was looking for a way that could use that.
Introducing another flag value seems like a long term burden.

Get the point, will draft another version.

Alternatively, virtio could do cleanup more aggressively. Maybe in response
to ring getting half full, or add a cleanup timer or something to avoid the
problem.

May worth to try. Another method is that virtio has a feature to notify guest 
when tx ring is empty, we could free the old tx skbs there. But it may brings 
extra overhead. If we could let virtio_net free the old tx skb timely, it 
would be easier to bring BQL support to virtio_net also.

Thanks

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