proposed optimization for network drivers

[Don Cohen]

I notice that netif_rx (net/core/dev.c) discards packets when it's
in throttle mode.  However, before it does this the driver has already
wasted a lot of time allocating the skb, calling eth_type_trans, etc.
That effort could be saved if the driver checked throttle first.

Sample diff:

--- /usr/src/linux-2.4.18-clean/drivers/net/8139too.c   Mon Feb 25 11:37:59 2002
+++ /usr/src/linux-2.4.18/drivers/net/8139too.c Wed Apr 16 23:30:30 2003
@@ -1908,6 +1908,11 @@
                        return;
                }

+               if (softnet_data[smp_processor_id()].throttle) {
+                 dev->last_rx = jiffies;
+                 netdev_rx_stat[smp_processor_id()].dropped++;
+               } else {
+
                /* Malloc up new buffer, compatible with net-2e. */
                /* Omit the four octet CRC from the length. */

@@ -1936,6 +1941,7 @@
                                dev->name);
                        tp->stats.rx_dropped++;
                }
+               }

                cur_rx = (cur_rx + rx_size + 4 + 3) & ~3;
                RTL_W16 (RxBufPtr, cur_rx - 16);

Hmm, now that I notice, there is one small change in semantics here.
Before every packet that was counted by netdev_rx_stat[].dropped had
also been counted in stats.rx_bytes and rx_packets and this is no
longer the case.  If that bothers you then perhaps there should just
be a separate counter for packets dropped like this.

In my limited experimentation this sort of change has significantly
increased the number of packets I could process when they were
arriving too fast. 

Please cc me on replies.