netdev - Re: Intro into qdisc writing?

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Date:   Tue, 10 Aug 2021 10:34:32 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Thorsten Glaser <t.glaser@...ent.de>, netdev@...r.kernel.org
Subject: Re: Intro into qdisc writing?



On 8/10/21 5:17 AM, Thorsten Glaser wrote:
> 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
> 

Instead of writing a new qdisc, you could simply use FQ packet scheduler,
and a eBPF program adjusting skb->tstamp depending on your needs.

https://legacy.netdevconf.info/0x14/session.html?talk-replacing-HTB-with-EDT-and-BPF