Subject: Re: [RFC PATCH net-next] net: pktgen: packet bursting via skb->xmit_more

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 26 Sep 2014 10:05:42 +0200

On Thu, 25 Sep 2014 17:46:22 -0700
Alexei Starovoitov <[email-masked]> wrote:

> This patch demonstrates the effect of delaying update of HW tailptr.
> (based on earlier patch by Jesper)
> 
> burst=1 is a default. It sends one packet with xmit_more=false
> burst=2 sends one packet with xmit_more=true and
>         2nd copy of the same packet with xmit_more=false
> burst=3 sends two copies of the same packet with xmit_more=true and
>         3rd copy with xmit_more=false
> 
> Performance with ixgbe:
> 
> usec 30:
> burst=1  tx:9.2 Mpps
> burst=2  tx:13.6 Mpps
> burst=3  tx:14.5 Mpps full 10G line rate

Perfect, full wirespeed! :-)

> usec 1 (default):
> burst=1,4,100 tx:3.9 Mpps

Here you are being limited by the TX ring queue cleanup, being too slow.
As desc here:
 http://netoptimizer.blogspot.dk/2014/06/pktgen-for-network-overload-testing.html

> usec 0:
> burst=1  tx:4.9 Mpps
> burst=2  tx:6.6 Mpps
> burst=3  tx:7.9 Mpps
> burst=4  tx:8.7 Mpps
> burst=8  tx:10.3 Mpps
> burst=128  tx:12.4 Mpps
> 
> Cc: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Alexei Starovoitov <[email-masked]>
> ---

Acked-by: Jesper Dangaard Brouer <[email-masked]>

> tx queue size, irq affinity left in default.
> pause frames are off.
> 
> Nice to finally see line rate generated by one cpu

Yes, 
 
> Comparing to Jesper patch this one amortizes the cost
> of spin_lock and atomic_inc by doing HARD_TX_LOCK and
> atomic_add(N) once across N packets.

Nice additional optimizations :-)

>  net/core/pktgen.c |   33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 5c728aa..47557ba 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -387,6 +387,7 @@ struct pktgen_dev {
>  	u16 queue_map_min;
>  	u16 queue_map_max;
>  	__u32 skb_priority;	/* skb priority field */
> +	int burst;		/* number of duplicated packets to burst */
>  	int node;               /* Memory node */
>  
>  #ifdef CONFIG_XFRM
[...]
> @@ -3299,7 +3313,8 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev)
>  {
>  	struct net_device *odev = pkt_dev->odev;
>  	struct netdev_queue *txq;
> -	int ret;
> +	int burst_cnt, ret;
> +	bool more;
>  
>  	/* If device is offline, then don't send */
>  	if (unlikely(!netif_running(odev) || !netif_carrier_ok(odev))) {
> @@ -3347,8 +3362,14 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev)
>  		pkt_dev->last_ok = 0;
>  		goto unlock;
>  	}
> -	atomic_inc(&(pkt_dev->skb->users));
> -	ret = netdev_start_xmit(pkt_dev->skb, odev, txq, false);
> +	atomic_add(pkt_dev->burst, &pkt_dev->skb->users);
> +
> +	burst_cnt = 0;
> +
> +xmit_more:
> +	more = ++burst_cnt < pkt_dev->burst;
> +
> +	ret = netdev_start_xmit(pkt_dev->skb, odev, txq, more);
>  
>  	switch (ret) {
>  	case NETDEV_TX_OK:
> @@ -3356,6 +3377,8 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev)
>  		pkt_dev->sofar++;
>  		pkt_dev->seq_num++;
>  		pkt_dev->tx_bytes += pkt_dev->last_pkt_size;
> +		if (more)
> +			goto xmit_more;

I think this will break my VLAN hack mode, that allows me to shoot
pktgen after the qdisc layer, but I'm okay with that, as I can just
avoid using this new burst mode and then it will still work for me.

>  		break;
>  	case NET_XMIT_DROP:
>  	case NET_XMIT_CN:
> @@ -3374,6 +3397,9 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev)
>  		atomic_dec(&(pkt_dev->skb->users));
>  		pkt_dev->last_ok = 0;
>  	}
> +
> +	if (unlikely(pkt_dev->burst - burst_cnt > 0))
> +		atomic_sub(pkt_dev->burst - burst_cnt, &pkt_dev->skb->users);
>  unlock:
>  	HARD_TX_UNLOCK(odev, txq);
>  

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer