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Date:   Sun, 8 Mar 2020 20:15:14 -0600
From:   David Ahern <>
To:     Alarig Le Lay <>
        Vincent Bernat <>
Subject: Re: IPv6 regression introduced by commit

On 3/8/20 4:57 AM, Alarig Le Lay wrote:
> On sam.  7 mars 17:52:10 2020, David Ahern wrote:
>> On 3/5/20 1:17 AM, Alarig Le Lay wrote:
>> Kernel version?
> I’ve seen this from 4.19 on my experience, it works at least until 4.15.
>> you are monitoring neighbor states with 'ip monitor' or something else?
> Yes, 'ip -ts monitor neigh' to be exact.
>> The above does not reproduce for me on 5.6 or 4.19, and I would have
>> been really surprised if it had, so I have to question the git bisect
>> result.
> My personal experience is that, while routing is activated (and having a
> full-view, I don’t have any soft router without it), the neighbors are
> flapping, thus causing a blackhole.
> It doesn’t happen with a limit traffic processing. The limit is around
> 20 Mbps from what I can see.

If you are using x86 based CPU you can do this:
    perf probe ip6_dst_alloc%return ret=%ax

    perf record -e probe:* -a -g -- sleep 10
    --> run this during the flapping

    perf script

this will show if the flapping is due to dst alloc failures.

Other things to try:
    perf probe ip6_dst_gc
    perf stat -e probe:* -a -I 1000
    --> will show calls/sec to running dst gc

    perf probe __ip6_rt_update_pmtu
    perf stat -e probe:* -a -I 1000
    --> will show calls/sec to mtu updating

    perf probe rt6_insert_exception
    perf state -e probe:* -a -I 1000
    --> shows calls/sec to inserting exceptions

(in each you can remove the previous probe using 'perf probe -d <name>'
or use -e <exact name> to only see data for the one event).

> I have the problem with 5.3 (proxmox 6), so unless FIB handling has been
> changed since then, I doubt that it will works, but I will try on
> Monday.

a fair amount of changes went in through 5.4 including improvements to
neighbor handling. 5.4 (I think) also had changes around dumping the
route cache.

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