s390/qeth: stop/wake TX queues based on their fill level

Current xmit code only stops the txq after attempting to fill an
IO buffer that hasn't been TX-completed yet. In many-connection
scenarios, this can result in frequent rejected TX attempts, requeuing
of skbs with NETDEV_TX_BUSY and extra overhead.

Now that we have a proper 1-to-1 relation between stack-side txqs and
our HW Queues, overhaul the stop/wake logic so that the xmit code
stops the txq as needed.
Given that we might map multiple skbs into a single buffer, it's crucial
to ensure that the queue always provides an _entirely_ empty IO buffer.
Otherwise large skbs (eg TSO) might not fit into the last available
buffer. So whenever qeth_do_send_packet() first utilizes an _empty_
buffer, it updates & checks the used_buffers count.

This now ensures that an skb passed to qeth_xmit() can always be mapped
into an IO buffer, so remove all of the -EBUSY roll-back handling in the
TX path. We preserve the minimal safety-checks ("Is this IO buffer
really available?"), just in case some nasty future bug ever attempts to
corrupt an in-use buffer.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 10 deletions

diff --git a/drivers/s390/net/qeth_l2_main.c b/drivers/s390/net/qeth_l2_main.c
index 7381917f76dd..e26a6dff286f 100644
--- a/drivers/s390/net/qeth_l2_main.c
+++ b/drivers/s390/net/qeth_l2_main.c
@@ -607,11 +607,8 @@ static netdev_tx_t qeth_l2_hard_start_xmit(struct sk_buff *skb,
 	int rc;
 
 	if (IS_IQD(card))
-		queue = card->qdio.out_qs[qeth_iqd_translate_txq(dev, txq)];
-	else
-		queue = card->qdio.out_qs[txq];
-
-	netif_stop_subqueue(dev, txq);
+		txq = qeth_iqd_translate_txq(dev, txq);
+	queue = card->qdio.out_qs[txq];
 
 	if (IS_OSN(card))
 		rc = qeth_l2_xmit_osn(card, skb, queue);
@@ -622,15 +619,11 @@ static netdev_tx_t qeth_l2_hard_start_xmit(struct sk_buff *skb,
 	if (!rc) {
 		QETH_TXQ_STAT_INC(queue, tx_packets);
 		QETH_TXQ_STAT_ADD(queue, tx_bytes, tx_bytes);
-		netif_wake_subqueue(dev, txq);
 		return NETDEV_TX_OK;
-	} else if (rc == -EBUSY) {
-		return NETDEV_TX_BUSY;
-	} /* else fall through */
+	}
 
 	QETH_TXQ_STAT_INC(queue, tx_dropped);
 	kfree_skb(skb);
-	netif_wake_subqueue(dev, txq);
 	return NETDEV_TX_OK;
 }