ppp: Don't stop and restart queue on every TX packet

For every transmitted packet, ppp_start_xmit() will stop the netdev
queue and then, if appropriate, restart it. This causes the TX softirq
to run, entirely gratuitously.

This is "only" a waste of CPU time in the normal case, but it's actively
harmful when the PPP device is a TEQL slave — the wakeup will cause the
offending device to receive the next TX packet from the TEQL queue, when
it *should* have gone to the next slave in the list. We end up seeing
large bursts of packets on just *one* slave device, rather than using
the full available bandwidth over all slaves.

This patch fixes the problem by *not* unconditionally stopping the queue
in ppp_start_xmit(). It adds a return value from ppp_xmit_process()
which indicates whether the queue should be stopped or not.

It *doesn't* remove the call to netif_wake_queue() from
ppp_xmit_process(), because other code paths (especially from
ppp_output_wakeup()) need it there and it's messy to push it out to the
other callers to do it based on the return value. So we leave it in
place — it's a no-op in the case where the queue wasn't stopped, so it's
harmless in the TX path.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -235,7 +235,7 @@ struct ppp_net {
 /* Prototypes. */
 static int ppp_unattached_ioctl(struct net *net, struct ppp_file *pf,
 			struct file *file, unsigned int cmd, unsigned long arg);
-static void ppp_xmit_process(struct ppp *ppp);
+static int ppp_xmit_process(struct ppp *ppp);
 static void ppp_send_frame(struct ppp *ppp, struct sk_buff *skb);
 static void ppp_push(struct ppp *ppp);
 static void ppp_channel_push(struct channel *pch);
@@ -968,9 +968,9 @@ ppp_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	proto = npindex_to_proto[npi];
 	put_unaligned_be16(proto, pp);
 
-	netif_stop_queue(dev);
 	skb_queue_tail(&ppp->file.xq, skb);
-	ppp_xmit_process(ppp);
+	if (!ppp_xmit_process(ppp))
+		netif_stop_queue(dev);
 	return NETDEV_TX_OK;
 
  outf:
@@ -1048,10 +1048,11 @@ static void ppp_setup(struct net_device *dev)
  * Called to do any work queued up on the transmit side
  * that can now be done.
  */
-static void
+static int
 ppp_xmit_process(struct ppp *ppp)
 {
 	struct sk_buff *skb;
+	int ret = 0;
 
 	ppp_xmit_lock(ppp);
 	if (!ppp->closing) {
@@ -1061,10 +1062,13 @@ ppp_xmit_process(struct ppp *ppp)
 			ppp_send_frame(ppp, skb);
 		/* If there's no work left to do, tell the core net
 		   code that we can accept some more. */
-		if (!ppp->xmit_pending && !skb_peek(&ppp->file.xq))
+		if (!ppp->xmit_pending && !skb_peek(&ppp->file.xq)) {
 			netif_wake_queue(ppp->dev);
+			ret = 1;
+		}
 	}
 	ppp_xmit_unlock(ppp);
+	return ret;
 }
 
 static inline struct sk_buff *