netdev - Re: VLAN I/F's and TX queue.

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Message-ID: <OF803EE436.793B95D8-ONC125771C.003295C9-C125771C.00341AFF@transmode.se>
Date:	Fri, 7 May 2010 11:29:07 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: VLAN I/F's and TX queue.

Eric Dumazet <eric.dumazet@...il.com> wrote on 2010/05/07 10:53:23:
>
> Le vendredi 07 mai 2010 à 10:04 +0200, Joakim Tjernlund a écrit :
> > Joakim Tjernlund/Transmode wrote on 2010/05/03 13:34:28:
> > >
> > > We noted dropped pkgs on our VLAN interfaces and i stated to look
> > > for a cause. Here is a ifconfig example:
> > >
> > > eth0      Link encap:Ethernet  HWaddr 00:AA:BB:CC:DD:EE
> > >           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> > >           RX packets:8886910 errors:0 dropped:0 overruns:0 frame:0
> > >           TX packets:8880219 errors:0 dropped:0 overruns:0 carrier:0
> > >           collisions:0 txqueuelen:100
> > >           RX bytes:1626842951 (1.5 GiB)  TX bytes:1555540810 (1.4 GiB)
> > >
> > > eth0.1    Link encap:Ethernet  HWaddr 00:AA:BB:CC:DD:EE
> > >           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> > >           RX packets:2163164 errors:0 dropped:0 overruns:0 frame:0
> > >           TX packets:2161943 errors:0 dropped:98 overruns:0 carrier:0
> > >           collisions:0 txqueuelen:0
> > >           RX bytes:2467090557 (2.2 GiB)  TX bytes:2480246455 (2.3 GiB)
> > >
> > > eth0.1.1  Link encap:Ethernet  HWaddr 00:AA:BB:CC:DD:EE
> > >           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> > >           RX packets:2163164 errors:0 dropped:0 overruns:0 frame:0
> > >           TX packets:2161943 errors:0 dropped:98 overruns:0 carrier:0
> > >           collisions:0 txqueuelen:0
> > >           RX bytes:2458437901 (2.2 GiB)  TX bytes:2471598683 (2.3 GiB)
> > >
> > > Here I note that txqueuelen is 0 for eth0.1/eth0.1.1 and 100 for eth0 and
> > > that it is only eth0.1 and eth0.1.1 that drops pkgs. It feels as if eth0.1
> > > bypasses eth0's tx queue and passes pkgs directly to the HW driver. Is that so?
> > > If so, that feels a bit strange and I am not sure how to best
> > > fix this. Any ides?
> > >
> > > Using kernel 2.6.33
> >
> > So I did some more testing
> > two nodes A and B connected over a slow link.
> > Create two VLAN's as above and start pinging from A to B
> > with pkg size 9600, start a few(4-10) parallel ping processes.
> >
> > Now I see dropped packages on B, the receiver of pings, and no
> > pkg loss on A.
> >
>
> dropped on RX path or TX path ?

On TX path(see the ifconfig listing above)

>
> > 1) since the link is symmetrical, why do I only see pkg loss
> >    at B?
> >
> > 2) pkg loss in B only manifests on the VLAN's interfaces and
> >    always in pair as if one lost pkg is counted twice?
> >
>
> Congestion notifications can be stacked since commit cbbef5e183079
> (vlan/macvlan: propagate transmission state to upper layers)

I see.

>
> > 3) I would expect lost pkgs to be accounted on eth0 instead of
> >    the VLAN interface(s) since that is where the pkg is lost, why
> >    isn't it so?
>
> You try to send packets on eth0.XXX, some are dropped, and accounted for
> on eth0.XXX stats. What is wrong with this ?

In this case one lost pkg is accounted for twice, once on eth0.1 and
once more on eth0.1.1. Note that eth0.1.1 is stacked on
top of eth0.1

I would at least expect eth0 to also account lost pkgs too.
I was confused by the current accounting as I knew that
the underlying HW I/F should be the only I/F that could
drop pkgs.

>
> If you want to avoid this, just add queues to your vlans
>
> ip link add link eth0 eth0.103 txqueuelen 100 type vlan id 103

>From memory now, but that didn't help. Still accounts pgks
as described. Why would where to account pkgs be affected by
queue or no queue?

  Jocke

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