netdev - Re: Is EDT now expected to work with any qdisc?

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20230619134746.vthfosooxoqwuwil@zenon.in.qult.net>
Date: Mon, 19 Jun 2023 15:47:46 +0200
From: Ignacy Gawedzki <ignacy.gawedzki@...en-communications.fr>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org
Subject: Re: Is EDT now expected to work with any qdisc?

On Sun, Jun 18, 2023 at 03:52:35PM -0700, thus spake Cong Wang:
> On Fri, Jun 16, 2023 at 07:31:38PM +0200, Ignacy Gawedzki wrote:
> > I tried very hard to find a confirmation of my hypothesis in the
> > kernel sources, but after three days of continuous searching for
> > answers, I'm starting to feel I'm missing something here.
> > 
> > So is it so that this requested delivery time is honored before the
> > packet is handed over to the qdisc or the driver?  Or maybe nowadays
> > pretty much every driver (including veth) honors that delivery time
> > itself?
> 
> It depends. Some NIC (and its driver) can schedule packets based on
> tstamp too, otherwise we have to rely on the software layer (Qdisc
> layer) to do so.
> 
> Different Qdisc's have different logics to schedule packets, not all
> of them use tstamp to order and schedule packets. This is why you have
> to pick a particular one, like fq, to get the logic you expect.

This is what I understand from reading both the sources and any
documentation I can get hold of.  But empirical tests seem to suggest
otherwise, as I have yet to find a driver where this
scheduling-according-to-tstamp doesn't actually happen.  I've even
tested with both a tun and a tap device, with noqueue on root and
clsact and by BPF code as a filter.  Here again, the packets are
getting through to the userspace fd according to the pacing enforced
by setting the tstamp in the BPF filter code.

I suspect that pacing is happening somewhere around the clsact
mini-qdisc, before the packet is handed over to the actual qdisc, but
I'd rather have a confirmation from the actual code, before I can rely
on that feature.

Thanks anyway,

Ignacy