Re: [RFC] csum experts, csum_replace2() is too expensive

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2014-03-21 at 06:32 -0700, Eric Dumazet wrote:
> On Fri, 2014-03-21 at 05:50 -0700, Eric Dumazet wrote:
> 
> > Or the fact that we mix 16 bit stores and 32bit loads ?
> > 
> > iph->tot_len = newlen;
> > iph->check = 0;
> > iph->check = ip_fast_csum(iph, 5);
> 
> Yep definitely. Using 16 bit loads in ip_fast_csum() totally removes the
> stall. I no longer see inet_gro_complete() in perf top...
> 
> +       if (__builtin_constant_p(ihl) && ihl == 5) {
> +               asm("  movw (%[iph]), %w[sum]\n"        /* ihl/version/tos */
> +                   "  addw 2(%[iph]), %w[sum]\n"       /* tot_len      */
> +                   "  adcw 8(%[iph]), %w[sum]\n"       /* ttl/protocol */
> +                   "  adcw 10(%[iph]), %w[sum]\n"      /* check        */
> +                   "  adcl 4(%[iph]), %[sum]\n"        /* id/frag_off  */
> +                   "  adcl 12(%[iph]), %[sum]\n"       /* saddr        */
> +                   "  adcl 16(%[iph]), %[sum]\n"       /* daddr        */
> +                   "  adcl $0, %[sum]\n"
> +                   : [sum] "=r" (sum)
> +                   : [iph] "r" (iph)
> +                   );
> +               return csum_fold(sum);
> 

Another idea would be to move the ip_fast_csum() call at the end of
inet_gro_complete()

I'll try this :

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 8c54870db792..0ca8f350a532 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1434,8 +1434,8 @@ static int inet_gro_complete(struct sk_buff *skb, int nhoff)
        int proto = iph->protocol;
        int err = -ENOSYS;
 
-       csum_replace2(&iph->check, iph->tot_len, newlen);
        iph->tot_len = newlen;
+       iph->check = 0;
 
        rcu_read_lock();
        ops = rcu_dereference(inet_offloads[proto]);
@@ -1447,6 +1447,7 @@ static int inet_gro_complete(struct sk_buff *skb, int nhoff)
         * inet_gro_receive().
         */
        err = ops->callbacks.gro_complete(skb, nhoff + sizeof(*iph));
+       iph->check = ip_fast_csum((u8 *)iph, 5);
 
 out_unlock:
        rcu_read_unlock();