netdev - Re: [PATCH v2] sctp: be more restrictive in transport selection on bundled sacks

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Message-ID: <20120628183635.GD29277@hmsreliant.think-freely.org>
Date:	Thu, 28 Jun 2012 14:36:35 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	linux-sctp@...r.kernel.org
Subject: Re: [PATCH v2] sctp: be more restrictive in transport selection on
 bundled sacks

On Thu, Jun 28, 2012 at 02:22:07PM -0400, Vlad Yasevich wrote:
> On 06/28/2012 02:07 PM, Neil Horman wrote:
> >On Thu, Jun 28, 2012 at 11:58:56AM -0400, Vlad Yasevich wrote:
> >>On 06/28/2012 11:33 AM, Neil Horman wrote:
> >>>On Wed, Jun 27, 2012 at 03:44:22PM -0400, Vlad Yasevich wrote:
> >>>>On 06/27/2012 01:28 PM, Neil Horman wrote:
> >>>>>On Wed, Jun 27, 2012 at 11:10:26AM -0400, Vlad Yasevich wrote:
> >>>>>>
> >>>>>>I didn't think of this yesterday, but I think it would be much
> >>>>>>better to use pkt->transport here since you are adding the chunk to
> >>>>>>the packet and it will go out on the transport of the packet.  You
> >>>>>>are also guaranteed that pkt->transport is set.
> >>>>>>
> >>>>>I don't think it really matters, as the chunk transport is used to lookup the
> >>>>>packet that we append to, and if the chunk transport is unset, its somewhat
> >>>>>questionable as to weather we should bundle, but if packet->transport is set,
> >>>>>its probably worth it to avoid the extra conditional.
> >>>>>
> >>>>
> >>>>Just looked at the code flow.  chunk->transport may not be set until
> >>>>the end of sctp_packet_append_chunk.  For new data, transport may
> >>>>not be set.  For retransmitted data, transport is set to last
> >>>>transport data was sent on.  So, we could be looking at the wrong
> >>>>transport.  What you are trying to decided is if the current
> >>>>transport we will be used can take the SACK, but you may not be
> >>>>looking at the current transport. Looking at packet->transport is
> >>>>the correct thing to do.
> >>>>
> >>>>-vlad
> >>>>
> >>>So, I agree after what you said above, that this is the right thing to do.  That
> >>>said, I just tested the change with the SCTP_RR test in netperf, and it wound up
> >>>giving me horrid performance (Its reporting about 5 transactions per second).
> >>>It appears that whats happening is that, because the test alternates which
> >>>transports it sends out, and because it waits for a sack of teh prior packet
> >>>before it sends out the next transaction, we're always missing the bundle
> >>>opportunity, and always waiting for the 200ms timeout for the sack to occur.
> >>>While I know this is a pessimal case, it really seems bad to me.  It seems that
> >>>because I was using chunk->transport previously, I luckily got the transport
> >>>wrong sometimes, and it managed to bundle more often.
> >>>
> >>>So I'm not sure what to do here.  I had really wanted to avoid adding a sysctl
> >>>here, but given that this is likely a corner cases, it seems that might be the
> >>>best approach.  Do you have any thoughts?
> >>>
> >>>Neil
> >>>
> >>
> >>that's strange.  did you modify the SCTP_RR to alternate transports?
> >>Seems like responses in the RR test need to go the address of the
> >>sender so that we don't see things like:
> >>Request (t) --->
> >>             <--- Response (t2)
> >>
> >>Should be:
> >>Request (t1) --->
> >>              <--- Response (t1)
> >>
> >>
> >>-vlad
> >That would seem to me to be the case too....
> >
> >However, having looked at this some more, it seems I just jumped the gun on
> >this. Its happening because sctp_eat_data variants are issuing a SCTP_GEN_SACK
> >command at the end of every received packet, which causes the moved_ctsn value
> >to get cleared.
> 
> Ok, that should only happen the very first time as we are supposed
> to ack the first data immediately.  On subsequent packets it should
> just start a timer because we are following the 2pkt/200ms rule.
> Then, when the response happens, we should bundle the SACK as long
> as the data is leaving on the transport that moved the CTSN.
> 
> So we might be using the wrong transport and as result you send data
> and then end up waiting for a SACK.
> 
Thats what I'm looking into now.  I'll let you know when I've figured it out
Neil

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