netdev - Re: RFH, where did I go wrong?

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Date:   Tue, 13 Sep 2022 20:37:53 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     Haye.Haehne@...ekom.de
cc:     netdev@...r.kernel.org
Subject: Re: RFH, where did I go wrong?

Hi Haye,

> I could retest the crash scenario of the qdisc, it occurs in the
> context of tc change /reconfiguration. When I define a static qdisc
> setup, the iperf udp stream with 50M (qdisc rate 20M) is stable.

thanks. (By the way, gstscream/scripts/sysctl.sh from SCReAM indeed
allows me to fill the queue fully with locally originating traffic.)

> So you should indeed take a deeper look into the processing during
> reconfiguration.

I did have a look. I could not reproduce the crash just with changing
qdisc things, not even a lot of times (100 Hz).

I tried:

$ sudo mksh -c 'qq=0; while sleep 0.01; do
	if (( ++qq == 500 )); then lim=50; qq=0; else lim=10240; fi
	tc qdisc change dev eth1 handle 1: janz rate 20mbit limit $lim
    done'

(changing the qdisc 100 times a second; every five seconds the limit
goes WAY down, to show the dropping code isn’t the cause either)

3x $ iperf -u -c 10.82.222.129 -t 600 -b 40M -S 0x01

(for some reason, IPv6 did not work with iperf for me)

$ sudo mksh -c 'while sleep 0.25; do tc -s qdisc show dev eth1 | fgrep backlog; done'

(for monitoring)

Evidently, what I’m changing, or how often, isn’t sufficient to trigger
the issue. Let’s phone tomorrow to try to figure out a good reproducer,
I might need to use jenscli with the data rate pattern you use, which is
less self-contained than I would normally prefer in a reproducer…

bye,
//mirabilos
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