Re: Broken sack processing

['Marcelo Ricardo Leitner' <marcelo.leitner@gmail.com>]

On Wed, Nov 07, 2018 at 02:37:07PM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 07 November 2018 12:53
> > On Wed, Nov 07, 2018 at 10:01:48AM +0000, David Laight wrote:
> > > I've a customer trace from a very old system (RHEL 5.7) that shows the
> > 
> > Ouch. That's really old. (and unsupported)
> 
> Indeed, but I doubt 5.11 (on extended support until 2021) or 6.x would much better!
> Even RHEL 7 is based on an old kernel.

Yep but that "base" was also a long time ago. The kernel is heavily
patched and its SCTP stack is not that far from what we have upstream
today.

> I wonder if they are paying RH for support?

I don't know which customer we are talking about. Not sure if I could
share it though. ;-) What I can tell is that so far we don't have any
similar reports of this. (And thus info is pretty much limited to what
you can share)

> 
> > > kernel failing to respond to some SACK packets like:
> > >
> > > SACK chunk (Cumulative TSN: 3327915808, a_rwnd: 224400, gaps: 12, duplicate TSNs: 0)
> > > Chunk type: SACK (3)
> > > Chunk flags: 0x00
> > > Chunk length: 64
> > > Cumulative TSN ACK: 3327915808
> > > Advertised receiver window credit (a_rwnd): 224400
> > > Number of gap acknowledgement blocks: 12
> > > Number of duplicated TSNs: 0
> > > Gap Acknowledgement for TSN 3327915813 to 3327915814
> > > Gap Acknowledgement for TSN 3327915818 to 3327915818
> > > Gap Acknowledgement for TSN 3327915822 to 3327915838
> > > Gap Acknowledgement for TSN 3327915842 to 3327915852
> > > Gap Acknowledgement for TSN 3327915856 to 3327915858
> > > Gap Acknowledgement for TSN 3327915860 to 3327915864
> > > Gap Acknowledgement for TSN 3327915866 to 3327915866
> > > Gap Acknowledgement for TSN 3327915868 to 3327915869
> > > Gap Acknowledgement for TSN 3327915873 to 3327915877
> > > Gap Acknowledgement for TSN 3327915881 to 3327915892
> > > Gap Acknowledgement for TSN 3327915894 to 3327916102
> > > Gap Acknowledgement for TSN 3327916104 to 3327916172
> > > [Number of TSNs in gap acknowledgement blocks: 337]
> > >
> > > Does this ring a bell, any idea how long ago it was fixed?
> > 
> > I don't follow. What is broken in this SACK? And what does it mean
> > "kenrel fails to repond some SACK", like, is it not retransmitting?
> 
> There are no outbound data chunks at all.
> There are a lot on inbound ones all of which are acked.
> The above SACK is repeated every few ms (maybe after every inbound data chunk).

Every few ms, so that's not RTO. If RTO was kicking in, SACK flow
would also be reduced. Seems fast rtx is kicking in then and new data
is allowed to continue flowing.

> 
> Fortunately most of the several million packets queued for transmit
> are in userspace!
> 
> And yes, it is pretty reproducible in the one system.
> Most of the 16 SCTP connections are hosed.

If the receiving host is (severily) CPU overloaded, it is possible
that the socket is loosing packets in the socket backlog. This
wouldn't reflect on a_rwnd (because backlog is like a network buffer
and is just invisible to a_rwnd) but will generate a SACK full of
holes such as this one.

As SCTP SNMP stats are not a recent thing, these drops should reflect
on /proc/net/sctp/snmp. Mainly SCTP_MIB_IN_PKT_BACKLOG and
SCTP_MIB_IN_PKT_DISCARDS (from sctp_rcv()).

This being the case, you must be seeing a ton of retransmissions too
and even if do a capture on the receiving side, you would see the
original packets on it, even though the SACK reports these gaps.
(as the network tap happens way before the socket backlog)

If you have the 16 associations under the same socket (1-to-many), the
probability of being this backlog issue gets a boost. As the backlog
is used whenever the socket (and not just the asoc) is in use, with 16
busy associations odds of overflowing the backlog are higher.

> 
> > > Not sure why the connection isn't dropped because of the unacked packets?
> > 
> > Whenever a new delivery is confirmed, the error count is zeroed. But,
> > once it enters RTO, it won't do new deliveries.
> > After a SACK like this I would expect some fast rtx, then RTO and then
> > a possible abort.
> > But if TSN 3327915809 (next after cumack) gets delivered, it will zero
> > the error count (again).
> 
> I had a look through the changes 'git log net/sctp' unfortunately it
> misses everything that happens during the merge window.
> Nothing leaps out, but there a lot of comments about the error count
> being zeroed - which you never want to do unless progress is made.
> 
> There are also all the problems that you can't disconnect connections
> with unacked data - they appear in traces for other connections.
> I've a workaround for that (disconnect with ABORT) but they don't
> have that version of our software either.
> 
> > Is this connection triggering zero windows, by any chance? Doesn't
> > seem so, as per
> >   Advertised receiver window credit (a_rwnd): 224400
> 
> I think sends are being limited by a local packet/byte count.
> Although the M3UA loadsharing (in our driver) will stop txing on
> both connections when one gets blocked like this, so the data chunks
> for the M3UA 'inactivate' do get sent.
> 
> No idea what triggers the problem.
> My guess is something to do with lost packets.

Mine too, and by the infos we have so far, I'm guessing the buffer/cpu
issue as above.

> 
> There is the commit a3007446e53af07c53bdb4cabad7b3ea60859da4
>     sctp: fix the handling of SACK Gap Ack blocks
> which went into 4.8 (dunno if it got back ported).

Only to RHEL7.

> But that wasn't thought to have a real effect.

Yep..

  Marcelo