net: add a comment on netdev->last_rx
As some driver authors seem to reintroduce dev->last_rx use,
add a comment to strongly discourage this.
Since commit 6cf3f41e6c0 (bonding, net: Move last_rx update into bonding
recv logic), network drivers dont need to update last_rx themselves,
unless they use this field to implement a timeout.
Not updating last_rx helps not dirtying a cache line, improving
performance in SMP.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 0cf9448..c82220a 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -953,7 +953,14 @@
/*
* Cache line mostly used on receive path (including eth_type_trans())
*/
- unsigned long last_rx; /* Time of last Rx */
+ unsigned long last_rx; /* Time of last Rx
+ * This should not be set in
+ * drivers, unless really needed,
+ * because network stack (bonding)
+ * use it if/when necessary, to
+ * avoid dirtying this cache line.
+ */
+
/* Interface address info used in eth_type_trans() */
unsigned char *dev_addr; /* hw address, (before bcast
because most packets are