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net: add a comment on netdev->last_rx

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As some driver authors seem to reintroduce dev->last_rx use,
add a comment to strongly discourage this.

Since commit 6cf3f41e6c (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>
This commit is contained in:
Eric Dumazet 2010-08-31 07:40:16 +00:00 committed by David S. Miller
parent ba4fd9d828
commit 4dc89133f4
1 changed files with 8 additions and 1 deletions
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9
include/linux/netdevice.h
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@ -953,7 +953,14 @@ struct net_device {
/* /*
* Cache line mostly used on receive path (including eth_type_trans()) * 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() */ /* Interface address info used in eth_type_trans() */
unsigned char *dev_addr; /* hw address, (before bcast unsigned char *dev_addr; /* hw address, (before bcast
because most packets are because most packets are