Subject: Re: [PATCH] Optimize loopback stats

From: Nivedita Singhvi <[email-masked]>
Date: Tue, 15 Mar 2005 12:14:54 -0800

Chuck Ebbert wrote:

>   This patch optimizes the loopback driver's statistics by using a single
> counter for rx and tx stats instead of one for rx and one for tx.  It also
> adds unlikely() to the test for TSO since it's no longer supported by default.
> (Maybe the TSO code should be bracketed by "#if 0" ?)

Hmm, some of us want those counters separate - if this is really
needed, could it be a configurable option, please?

thanks,
Nivedita

>   o saves 84 bytes per CPU on 32bit and 168 bytes on 64 bit
>     (should save 84K data on 512-way ia64)
> 
>   o AFAICT the driver is ~2.5% faster sending PF_PACKET data
> 
>   o applies on top of Christoph's patch in -mm that removes update
>     of the device's last_rx field
> 
> Signed-off-by: Chuck Ebbert <[email-masked]>
> 
> --- 2.6.11-mm/drivers/net/loopback.c    2005-03-15 14:23:30.180677000 -0500
> +++ 2.6.11-ce/drivers/net/loopback.c    2005-03-15 14:26:23.700677000 -0500
> @@ -58,7 +58,12 @@
>  #include <linux/tcp.h>
>  #include <linux/percpu.h>
>  
> -static DEFINE_PER_CPU(struct net_device_stats, loopback_stats);
> +struct loopback_device_stats {
> +       unsigned long rx_tx_bytes;
> +       unsigned long rx_tx_packets;
> +};
> +
> +static DEFINE_PER_CPU(struct loopback_device_stats, loopback_stats);
>  
>  #define LOOPBACK_OVERHEAD (128 + MAX_HEADER + 16 + 16)
>  
> @@ -126,7 +131,7 @@ static void emulate_large_send_offload(s
>   */
>  static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
> -       struct net_device_stats *lb_stats;
> +       struct loopback_device_stats *lb_stats;
>  
>         skb_orphan(skb);
>  
> @@ -136,7 +141,7 @@ static int loopback_xmit(struct sk_buff 
>         skb->ip_summed = CHECKSUM_UNNECESSARY;
>  #endif
>  
> -       if (skb_shinfo(skb)->tso_size) {
> +       if (unlikely(skb_shinfo(skb)->tso_size)) {
>                 BUG_ON(skb->protocol != htons(ETH_P_IP));
>                 BUG_ON(skb->nh.iph->protocol != IPPROTO_TCP);
>  
> @@ -145,10 +150,8 @@ static int loopback_xmit(struct sk_buff 
>         }
>  
>         lb_stats = &per_cpu(loopback_stats, get_cpu());
> -       lb_stats->rx_bytes += skb->len;
> -       lb_stats->tx_bytes += skb->len;
> -       lb_stats->rx_packets++;
> -       lb_stats->tx_packets++;
> +       lb_stats->rx_tx_bytes += skb->len;
> +       lb_stats->rx_tx_packets++;
>         put_cpu();
>  
>         netif_rx(skb);
> @@ -168,15 +171,15 @@ static struct net_device_stats *get_stat
>         memset(stats, 0, sizeof(struct net_device_stats));
>  
>         for (i=0; i < NR_CPUS; i++) {
> -               struct net_device_stats *lb_stats;
> +               struct loopback_device_stats *lb_stats;
>  
>                 if (!cpu_possible(i)) 
>                         continue;
>                 lb_stats = &per_cpu(loopback_stats, i);
> -               stats->rx_bytes   += lb_stats->rx_bytes;
> -               stats->tx_bytes   += lb_stats->tx_bytes;
> -               stats->rx_packets += lb_stats->rx_packets;
> -               stats->tx_packets += lb_stats->tx_packets;
> +               stats->rx_bytes   += lb_stats->rx_tx_bytes;
> +               stats->tx_bytes    = stats->rx_bytes;
> +               stats->rx_packets += lb_stats->rx_tx_packets;
> +               stats->tx_packets  = stats->rx_packets;
>         }
>                                 
>         return stats;
> _
> 
> --
> Chuck
> 
>