net: tx scalability works : trans_start

struct net_device trans_start field is a hot spot on SMP and high performance
devices, particularly multi queues ones, because every transmitter dirties
it. Is main use is tx watchdog and bonding alive checks.

But as most devices dont use NETIF_F_LLTX, we have to lock
a netdev_queue before calling their ndo_start_xmit(). So it makes
sense to move trans_start from net_device to netdev_queue. Its update
will occur on a already present (and in exclusive state) cache line, for
free.

We can do this transition smoothly. An old driver continue to
update dev->trans_start, while an updated one updates txq->trans_start.

Further patches could also put tx_bytes/tx_packets counters in 
netdev_queue to avoid dirtying dev->stats (vlan device comes to mind)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 2af89b662ca..cd547d04a8c 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -470,6 +470,10 @@ struct netdev_queue {
  */
         spinlock_t              _xmit_lock ____cacheline_aligned_in_smp;
         int                     xmit_lock_owner;
+        /*
+         * please use this field instead of dev->trans_start
+         */
+        unsigned long           trans_start;
 } ____cacheline_aligned_in_smp;
 
 
@@ -819,6 +823,11 @@ struct net_device
  * One part is mostly used on xmit path (device)
  */
         /* These may be needed for future network-power-down code. */
+
+        /*
+         * trans_start here is expensive for high speed devices on SMP,
+         * please use netdev_queue->trans_start instead.
+         */
         unsigned long           trans_start;    /* Time (in jiffies) of last Tx */
 
         int                     watchdog_timeo; /* used by dev_watchdog() */
@@ -1541,6 +1550,8 @@ static inline int netif_carrier_ok(const struct net_device *dev)
         return !test_bit(__LINK_STATE_NOCARRIER, &dev->state);
 }
 
+extern unsigned long dev_trans_start(struct net_device *dev);
+
 extern void __netdev_watchdog_up(struct net_device *dev);
 
 extern void netif_carrier_on(struct net_device *dev);