net: piggy back on the memory barrier in bql when waking queues

Drivers call netdev_tx_completed_queue() right before
netif_txq_maybe_wake(). If BQL is enabled netdev_tx_completed_queue()
should issue a memory barrier, so we can depend on that separating
the stop check from the consumer index update, instead of adding
another barrier in netif_txq_maybe_wake().

This matters more than the barriers on the xmit path, because
the wake condition is almost always true. So we issue the
consumer side barrier often.

Wrap netdev_tx_completed_queue() in a local helper to issue
the barrier even if BQL is disabled. Keep the same semantics
as netdev_tx_completed_queue() (barrier only if bytes != 0)
to make it clear that the barrier is conditional.

Plus since macro gets pkt/byte counts as arguments now -
we can skip waking if there were no packets completed.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 7bec9a2be8ef1d..fe355592dfde26 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3532,7 +3532,7 @@ static inline void netdev_tx_completed_queue(struct netdev_queue *dev_queue,
 	 * netdev_tx_sent_queue will miss the update and cause the queue to
 	 * be stopped forever
 	 */
-	smp_mb();
+	smp_mb(); /* NOTE: netdev_txq_completed_mb() assumes this exists */
 
 	if (unlikely(dql_avail(&dev_queue->dql) < 0))
 		return;