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net: add tx_packets/tx_bytes/tx_dropped counters in struct netdev_queue

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offsetof(struct net_device, features)=0x44
offsetof(struct net_device, stats.tx_packets)=0x54
offsetof(struct net_device, stats.tx_bytes)=0x5c
offsetof(struct net_device, stats.tx_dropped)=0x6c

Network drivers that touch dev->stats.tx_packets/stats.tx_bytes in their
tx path can slow down SMP operations, since they dirty a cache line
that should stay shared (dev->features is needed in rx and tx paths)

We could move away stats field in net_device but it wont help that much.
(Two cache lines dirtied in tx path, we can do one only)

Better solution is to add tx_packets/tx_bytes/tx_dropped in struct
netdev_queue because this structure is already touched in tx path and
counters updates will then be free (no increase in size)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2009-05-18 00:34:33 +00:00 • committed by David S. Miller
parent ba98898eb3
commit 7004bf252c
2 changed files with 23 additions and 3 deletions
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3
include/linux/netdevice.h
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@ -474,6 +474,9 @@ struct netdev_queue {
* please use this field instead of dev->trans_start
*/
unsigned long trans_start;
unsigned long tx_bytes;
unsigned long tx_packets;
unsigned long tx_dropped;
} ____cacheline_aligned_in_smp;