netdev - Intro into qdisc writing?

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Message-ID: <1e2625bd-f0e5-b5cf-8f57-c58968a0d1e5@tarent.de>
Date:   Tue, 10 Aug 2021 05:17:23 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     netdev@...r.kernel.org
Subject: Intro into qdisc writing?

Hi,

I hope this is the right place to ask this kind of questions,
and not just send patches ☺

I’m currently working with a… network simulator of sorts, which
has so far mostly used htb, netem, dualpi2 and fq_codel to do the
various tricks needed for whatever they do, but now I have rather
specific change requests (one of which I already implemented).

The next things on my list basically involve delaying all traffic
or a subset of traffic for a certain amount of time (in the one‑ to
two-digit millisecond ballpark, so rather long, in CPU time). I’ve
seen the netem source use qdisc_watchdog_schedule_ns for this, but,
unlike the functions I used in my earlier module changes, I cannot
find any documentation for this.

Similarily, is there an intro of sorts for qdisc writing, the things
to know, concepts, locking, whatever is needed?

My background is multi-decade low-level programmer, but so far only
userland, libc variants and bootloaders, not kernel, and what bit of
kernel I touched so far was in BSD land so any pointers welcome.

If it helps: while this is for a customer project, so far everything
coming out of it is published under OSS licences; mostly at
https://github.com/tarent/sch_jens/tree/master/sch_jens as regards
the kernel module (and ../jens/ for the relayfs client example) but
https://github.com/tarent/ECN-Bits has a related userspace project.

Thanks in advance,
//mirabilos
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