Multicast routing + sch_fq not working since 4.20 (bisected)

[Andre Tomt <andre@tomt.net>]

TLDR; Multicast routing (at least IPv4) in combination with sch_fq is 
not working since kernel 4.20-rc1 and up to and including 5.12-rc1. 
Other tested qdisc schedulers work fine (pfifo_fast, fq_codel, cake)

Hello all

I've been chasing a issue with multicast routing the past few days where 
nothing went out on the physical egress port even though:
* the multicast routes were registered and resolved with the correct 
interfaces in ip mroute show
* the reverse path was OK
* data was flowing in on the ingress side
* forwarding / mc_forwarding enabled
* registered fine in a nftables log rule in forward (which was accepting 
all)
* packets showed up in (local) tcpdump on egress vlan virtual interface

After some digging, tracing, a bisect, and a kprint to verify, it seems 
as the multicast routing code is using a different clock than fq and 
setting skb->tstamp to something sch_fq considers far, far into the 
future, failing the beyond horizon check.


Things immediately starts to work if I do a tc qdisc replace with a 
different scheduler, and stops when changing back to fq.

This stopped working when fq changed to CLOCK_MONOTONIC in 
fb420d5d91c1274d5966917725e71f27ed092a85 tcp/fq: move back to 
CLOCK_MONOTONIC
Reverting it on top of 4.20-rc1 restores multicast routing with fq.

from debug printk in fq_enqueue when horizon check fails:
tstamp skb 1614615921893669854 ktime 59949897819

tstamp skb 1614615921968395652 ktime 60024624355

tstamp skb 1614615922043160089 ktime 60099388127


The setup is a Linux router running FRR bgpd + pimd for multicast 
routing. The multicast source is some TV broadcast equipment one more 
hop away sending a mpeg transport streams on IPv4, using 1316 byte TS 
datagrams (not fragmented, jumbos or anything otherwise funny.)


git bisect start

# bad: [993f0b0510dad98b4e6e39506834dab0d13fd539] sched/topology: Fix 
off by one bug

git bisect bad 993f0b0510dad98b4e6e39506834dab0d13fd539

# good: [84df9525b0c27f3ebc2ebb1864fa62a97fdedb7d] Linux 4.19

git bisect good 2241b8bcf2b5f1b01ebb1cbd1231bbbb72230064

# bad: [50b825d7e87f4cff7070df6eb26390152bb29537] Merge 
git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next

git bisect bad 50b825d7e87f4cff7070df6eb26390152bb29537

# bad: [99e9acd85ccbdc8f5785f9e961d4956e96bd6aa5] Merge tag 
'mlx5-updates-2018-10-17' of 
git://git.kernel.org/pub/scm/linux/kernel/git/saeed/linux

git bisect bad 99e9acd85ccbdc8f5785f9e961d4956e96bd6aa5

# bad: [d793fb46822ff7408a1767313ef6b12e811baa55] Merge tag 
'wireless-drivers-next-for-davem-2018-10-02' of 
git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/wireless-drivers-next

git bisect bad d793fb46822ff7408a1767313ef6b12e811baa55

# good: [72b0094f918294e6cb8cf5c3b4520d928fbb1a57] tcp: switch 
tcp_clock_ns() to CLOCK_TAI base

git bisect good 72b0094f918294e6cb8cf5c3b4520d928fbb1a57

# bad: [d5486377b8c526e4f373ec0506c4c5398c99082e] Merge branch '100GbE' 
of git://git.kernel.org/pub/scm/linux/kernel/git/jkirsher/next-queue

git bisect bad d5486377b8c526e4f373ec0506c4c5398c99082e

# good: [d888f39666774c7debfa34e4e20ba33cf61a6d71] net-ipv4: remove 2 
always zero parameters from ipv4_update_pmtu()

git bisect good d888f39666774c7debfa34e4e20ba33cf61a6d71

# good: [041a14d2671573611ffd6412bc16e2f64469f7fb] tcp: start receiver 
buffer autotuning sooner

git bisect good 041a14d2671573611ffd6412bc16e2f64469f7fb

# good: [6871af29b3abe6d6ae3a0e28b8bdf44bd4cb8d30] net: hns3: Add reset 
handle for flow director

git bisect good 6871af29b3abe6d6ae3a0e28b8bdf44bd4cb8d30

# bad: [024926def6ca95819442699fbecc1fe376253fb9] net: phy: Convert to 
using %pOFn instead of device_node.name

git bisect bad 024926def6ca95819442699fbecc1fe376253fb9

# good: [297357d1a165cf23cc85a6a7ec32ffc854cbf13c] net: systemport: 
Utilize bcm_sysport_set_features() during resume/open

git bisect good 297357d1a165cf23cc85a6a7ec32ffc854cbf13c

# good: [a0651d8e2784b189924b4f4f41b901835feef8a4] Merge branch 
'net-systemport-Turn-on-offloads-by-default'

git bisect good a0651d8e2784b189924b4f4f41b901835feef8a4

# good: [e3a9667a5bf7e520a1fa24eadccc6010c135ec53] hv_netvsc: Fix 
rndis_per_packet_info internal field initialization

git bisect good e3a9667a5bf7e520a1fa24eadccc6010c135ec53

# bad: [fb420d5d91c1274d5966917725e71f27ed092a85] tcp/fq: move back to 
CLOCK_MONOTONIC

git bisect bad fb420d5d91c1274d5966917725e71f27ed092a85

# good: [0ed3015c9964dab7a1693b3e40650f329c16691e] selftests/tls: Fix 
recv(MSG_PEEK) & splice() test cases

git bisect good 0ed3015c9964dab7a1693b3e40650f329c16691e

# first bad commit: [fb420d5d91c1274d5966917725e71f27ed092a85] tcp/fq: 
move back to CLOCK_MONOTONIC