Re: [net-next RFC] pktgen: don't wait for the device who doesn't free skb immediately after sent

On Mon, 03 Dec 2012 14:45:46 +0800
Jason Wang [off-list ref] wrote:

On Tuesday, November 27, 2012 08:49:19 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 <redacted>

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.

Have some experiments on this, looks like it does not work weel when clone_skb 
is used. For driver that call skb_orphan() in ndo_start_xmit, the destructor 
is only called when the first packet were sent, but what we need to know is 
when the last were sent. Any thoughts on this or we can just introduce another 
flag (anyway we have something like IFF_TX_SKB_SHARING) ?

The SKB_SHARING flag looks like the best solution then.
Surprisingly, transmit buffer completion is a major bottleneck for 10G
devices, and I suspect more changes will come.