linux-kernel - [ 062/167] [PATCH 02/26] ppp: Dont stop and restart queue on every TX packet

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Date:	Wed, 09 May 2012 06:51:31 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	David Woodhouse <David.Woodhouse@...el.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [ 062/167] [PATCH 02/26] ppp: Dont stop and restart queue on every TX packet

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Woodhouse <David.Woodhouse@...el.com>

[ This combines upstream commit
  e675f0cc9a872fd152edc0c77acfed19bf28b81e and follow-on bug fix
  commit 9a5d2bd99e0dfe9a31b3c160073ac445ba3d773f ]

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@...el.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/net/ppp/ppp_generic.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 486b404..3ed983c 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -968,7 +968,6 @@ 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);
 	return NETDEV_TX_OK;
@@ -1063,6 +1062,8 @@ ppp_xmit_process(struct ppp *ppp)
 		   code that we can accept some more. */
 		if (!ppp->xmit_pending && !skb_peek(&ppp->file.xq))
 			netif_wake_queue(ppp->dev);
+		else
+			netif_stop_queue(ppp->dev);
 	}
 	ppp_xmit_unlock(ppp);
 }
-- 
1.7.10

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