[064/146] sch_sfq: avoid giving spurious NET_XMIT_CN signals

Greg KH
2011-06-01T08:12:06Z

From: Eric Dumazet <eric.dumazet@gmail.com>

[ Upstream commit 8efa885406359af300d46910642b50ca82c0fe47 ]

While chasing a possible net_sched bug, I found that IP fragments have
litle chance to pass a congestioned SFQ qdisc :

- Say SFQ qdisc is full because one flow is non responsive.
- ip_fragment() wants to send two fragments belonging to an idle flow.
- sfq_enqueue() queues first packet, but see queue limit reached :
- sfq_enqueue() drops one packet from 'big consumer', and returns
NET_XMIT_CN.
- ip_fragment() cancel remaining fragments.

This patch restores fairness, making sure we return NET_XMIT_CN only if
we dropped a packet from the same flow.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
CC: Jarek Poplawski <jarkao2@gmail.com>
CC: Jamal Hadi Salim <hadi@cyberus.ca>
CC: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
net/sched/sch_sfq.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -361,7 +361,7 @@ sfq_enqueue(struct sk_buff *skb, struct
{
struct sfq_sched_data *q = qdisc_priv(sch);
unsigned int hash;
- sfq_index x;
+ sfq_index x, qlen;
struct sfq_slot *slot;
int uninitialized_var(ret);

@@ -405,8 +405,12 @@ sfq_enqueue(struct sk_buff *skb, struct
if (++sch->q.qlen <= q->limit)
return NET_XMIT_SUCCESS;

+ qlen = slot->qlen;
sfq_drop(sch);
- return NET_XMIT_CN;
+ /* Return Congestion Notification only if we dropped a packet
+ * from this flow.
+ */
+ return (qlen != slot->qlen) ? NET_XMIT_CN : NET_XMIT_SUCCESS;
}

static struct sk_buff *