Hello all,
I might make a fool out of me, but I think the prio qdisc doesn't work as
advertised in any document I could lay my hands on.
My problem was that the link quality reported by the olsr.org olsrd degraded
depending on the amount of payload traffic transferred through an adhoc/mesh
interface. The LQ is calculated from the packet loss of LQ Hello packets sent
through this interface. To make sure normal traffic does not interfere with
this value, olsrd sets the TOS field to 0x10 (Minimize-Delay) by default. This
should give olsr traffic the highest priority on the link.
Investigating this issue I replaced the default Pfifo_fast with a prio qdisc
and attached a pfifo on each of the bands:
INTERFACE=wifi0
tc qdisc add dev $INTERFACE root handle 1: prio
tc qdisc add dev $INTERFACE parent 1:1 handle 10: pfifo
tc qdisc add dev $INTERFACE parent 1:2 handle 20: pfifo
tc qdisc add dev $INTERFACE parent 1:3 handle 30: pfifo
The I used ping -Q TOSVALUE to send packets with different TOS values through
the interface. tcpdump confirmed the correct TOS values in the outgoing packets.
With "tc -s qdisc ls dev wifi0" I could observe the effects of the different
TOS values. The result: no effect at all! Every single packet used the band
indicated by the first value in the priomap (e.g. band 1 by default, in my case
the pfifo with handle 20:). I can't square this observation with the available
documentation.
Looking at the source code, it seems that sched_prio uses the skb->priority
value to select the outgoing band. According to some documentation I found, an
application can set this value.
Now I'm at a loss. I can work around this problem with filters, but I don't
think that this is the correct solution. Any suggestions?
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