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

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>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index cd547d04a8ce..f8574e76b743 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -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;