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Date:   Sun, 03 Jan 2021 13:34:48 -0500
From:   "Valdis Klētnieks" <valdis.kletnieks@...edu>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Kconfig, DEFAULT_NETSCH, and shooting yourself in the foot..

On Sun, 03 Jan 2021 10:20:11 -0800, Stephen Hemminger said:
> You can use a qdisc that is a module, it just has to be available when device
> is loaded. Typically that means putting it in initramfs.

Apparently, that's not *quite* true regarding the default qdisc, because I
hit this situation (copying from another email):

---
[/proc/sys/net] cat core/default_qdisc
fq_codel

[/proc/sys/net] tc -s qdisc show
qdisc noqueue 0: dev lo root refcnt 2
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
qdisc pfifo_fast 0: dev eth0 root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
qdisc noqueue 0: dev wlan0 root refcnt 2
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0

But if I give it a kick in the head...

[/proc/sys/net] tc qdisc add dev wlan0 root fq_codel
[/proc/sys/net] tc -s qdisc show dev wlan0
qdisc fq_codel 8001: root refcnt 2 limit 10240p flows 1024 quantum 1514 target 5ms interval 100ms memory_limit 32Mb ecn
drop_batch 64
 Sent 812 bytes 12 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
  maxpacket 0 drop_overlimit 0 new_flow_count 0 ecn_mark 0
  new_flows_len 0 old_flows_len 0
---

Note that wlan0 doesn't actually get plumbed up until after the initramfs
has done its thing and we're quite late in the boot process and the /lib/modules
on the production system is accessible, so it doesn't look like an implicit modprobe
gets done in that case - fq_codel.ko was very much available when the device
was brought up.

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