netdev - Re: [Bugme-new] [Bug 39372] New: Problems with HFSC Scheduler

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Date:	Fri, 29 Jul 2011 16:00:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Soltys <soltys@....info>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, netdev@...r.kernel.org,
	bugme-daemon@...zilla.kernel.org,
	Jamal Hadi Salim <hadi@...erus.ca>, lucas.bocchi@...il.com,
	Patrick McHardy <kaber@...sh.net>, 631945@...s.debian.org,
	00bormoj@...il.com, fdelawarde@...elessmundi.com
Subject: Re: [Bugme-new] [Bug 39372] New: Problems with HFSC Scheduler

Le vendredi 29 juillet 2011 à 15:27 +0200, Eric Dumazet a écrit :
> Le vendredi 29 juillet 2011 à 14:29 +0200, Michal Soltys a écrit :
> > On 11-07-15 00:14, Andrew Morton wrote:
> > > 
> > > (switched to email.  Please respond via emailed reply-to-all, not via
> > > the bugzilla web interface).
> > > 
> > > 
> > > Here: WARN_ON(next_time == 0);
> > > 
> > 
> > From the other thread on netfilter-devel:
> > 
> > > On 11-07-22 11:58, Michal Pokrywka wrote: After bisecting 2.6.39.1 it
> > > turned out that the bug is caused independently by two patches:
> > > 
> > > commit b262a5da755cc6ed0cb4fba230cd9bf4037e1096 sch_sfq: fix peek()
> > > implementation
> > > 
> > > and
> > > 
> > > commit 9df49f2bfe862573911a080c75a6d81113c5c81d sch_sfq: avoid giving
> > > spurious NET_XMIT_CN signals
> > > 
> > > Reverting these patches makes HFSC work again.
> > > 
> > 
> > This one (upstream 8efa885406359af300d46910642b50ca82c0fe47) seems to be
> > the culprit (does reverting only that one cures the problem ?)
> > 
> > It allows SFQ to return success on enqueuing, when the packet really
> > replaced some other packet in some other flow. This confuses outer qdisc
> > (in this particular case HFSC) which thinks new packet was actually
> > added each time such situation happes.
> > 
> 
> Technically speaking, _this_ packet was successfuly enqueued.
> 
> Returning NET_XMIT_CN or NET_XMIT_SUCCESS should not trigger a bug in
> caller.
> 
> > This in turn causes additional dequeues and ends with attempt
> > to schedule non-existent packets, and triggers the warning.
> > 
> 
> Then its probably a bug in HFSC : It doesnt understand SFQ lost a
> packet.
> 
> I'll take a look, thanks for the report.
> 
> 

Oh well, it seems one qdisc_tree_decrease_qlen(sch, 1) is missing

Maybe following patch would help...


diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 4536ee6..2a2d287 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -410,7 +410,12 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	/* Return Congestion Notification only if we dropped a packet
 	 * from this flow.
 	 */
-	return (qlen != slot->qlen) ? NET_XMIT_CN : NET_XMIT_SUCCESS;
+	if (qlen != slot->qlen)
+		return NET_XMIT_CN;
+
+	/* as we dropped a packet, better let upper stack know this */
+	qdisc_tree_decrease_qlen(sch, 1);
+	return NET_XMIT_SUCCESS;
 }
 
 static struct sk_buff *


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