Subject: Re: [RFC PATCH net-next 1/3] ixgbe: support netdev_ops->ndo_xmit_flush()

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 25 Aug 2014 14:07:21 +0200

On Sun, 24 Aug 2014 15:42:16 +0200
Daniel Borkmann <[email-masked]> wrote:

> This implements the deferred tail pointer flush API for the ixgbe
> driver. Similar version also proposed longer time ago by Alexander Duyck.

I've run some benchmarks with this patch only, which actually shows a
performance regression.

Using trafgen with QDISC_BYPASS and mmap mode, via cmdline:
 trafgen --cpp  --dev eth5 --conf udp_example01.trafgen -V --cpus 1

BASELINE(no-patch): trafgen QDISC_BYPASS and mmap:
 - tx:1562539 pps

(This patch only): ixgbe use of .ndo_xmit_flush.
 - tx:1532299 pps

Regression: -30240 pps
 * In nanosec: (1/1562539*10^9)-(1/1532299*10^9) = -12.63 ns

As DaveM points out, me might not need the mmiowb().
Result when not performing the mmiowb():
 - tx:1548352 pps

Still a small regression: -14187 pps
 * In nanosec: (1/1562539*10^9)-(1/1548352*10^9) = -5.86 ns

I was not expecting this "slowdown", with this rather simple use of the
new ndo_xmit_flush API.  Can anyone explain why this is happening?

> Signed-off-by: Daniel Borkmann <[email-masked]>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 28 +++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index 87bd53f..4e073cf 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -6957,10 +6957,6 @@ static void ixgbe_tx_map(struct ixgbe_ring *tx_ring,
>  		i = 0;
>  
>  	tx_ring->next_to_use = i;
> -
> -	/* notify HW of packet */
> -	ixgbe_write_tail(tx_ring, i);
> -
>  	return;
>  dma_error:
>  	dev_err(tx_ring->dev, "TX DMA map failed\n");
> @@ -7301,6 +7297,29 @@ static netdev_tx_t ixgbe_xmit_frame(struct sk_buff *skb,
>  	return __ixgbe_xmit_frame(skb, netdev, NULL);
>  }
>  
> +static inline struct ixgbe_ring *
> +__ixgb_tx_queue_mapping(struct ixgbe_adapter *adapter, u16 queue)
> +{
> +	if (unlikely(queue >= adapter->num_tx_queues))
> +		queue = queue % adapter->num_tx_queues;
> +
> +	return adapter->tx_ring[queue];
> +}
> +
> +static void ixgbe_xmit_flush(struct net_device *netdev, u16 queue)
> +{
> +	struct ixgbe_adapter *adapter = netdev_priv(netdev);
> +	struct ixgbe_ring *tx_ring;
> +
> +	tx_ring = __ixgb_tx_queue_mapping(adapter, queue);
> +	ixgbe_write_tail(tx_ring, tx_ring->next_to_use);
> +
> +	/* we need this if more than one processor can write to our tail
> +	 * at a time, it synchronizes IO on IA64/Altix systems
> +	 */
> +	mmiowb();

This is the mmiowb() which is avoided in above measurement.

> +}
> +
>  /**
>   * ixgbe_set_mac - Change the Ethernet Address of the NIC
>   * @netdev: network interface device structure
> @@ -7914,6 +7933,7 @@ static const struct net_device_ops ixgbe_netdev_ops = {
>  	.ndo_open		= ixgbe_open,
>  	.ndo_stop		= ixgbe_close,
>  	.ndo_start_xmit		= ixgbe_xmit_frame,
> +	.ndo_xmit_flush		= ixgbe_xmit_flush,
>  	.ndo_select_queue	= ixgbe_select_queue,
>  	.ndo_set_rx_mode	= ixgbe_set_rx_mode,
>  	.ndo_validate_addr	= eth_validate_addr,

-- 
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