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

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Message-ID: <20121127084919.1587c647@nehalam.linuxnetplumber.net>
Date:	Tue, 27 Nov 2012 08:49:19 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	rusty@...tcorp.com.au, mst@...hat.com, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [net-next RFC] pktgen: don't wait for the device who doesn't
 free skb immediately after sent

On Tue, 27 Nov 2012 14:45:13 +0800
Jason Wang <jasowang@...hat.com> wrote:

> On 11/27/2012 01:37 AM, Stephen Hemminger wrote:
> > On Mon, 26 Nov 2012 15:56:52 +0800
> > Jason Wang <jasowang@...hat.com> wrote:
> >
> >> 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@...hat.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.
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