linux-kernel - Re: Deleting child qdisc doesn't reset parent to default qdisc?

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Date:	Tue, 28 Jun 2016 19:33:09 +0200 (CEST)
From:	Jiri Kosina <jikos@...nel.org>
To:	Cong Wang <xiyou.wangcong@...il.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Phil Sutter <phil@....cc>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Deleting child qdisc doesn't reset parent to default qdisc?

On Tue, 28 Jun 2016, Cong Wang wrote:

> > BTW, I've started to actually work on fixing this, and I've noticed that
> > TBF behavior actually violates what's stated in pfifo_fast manpage:
> >
> > ==========
> >         Whenever  an  interface is created, the pfifo_fast qdisc is
> >         automatically used as a queue. If another qdisc is
> >         attached, it preempts the default pfifo_fast, which automatically
> >         returns to function when an  existing  qdisc is detached.
> >
> >         In this sense this qdisc is magic, and unlike other qdiscs.
> > ==========
> 
> It is out of date, now default qdisc can be set to any other qdisc
> via /proc. Also, probably due to historical reasons, we don't have
> a unified default default qdisc, some uses bfifo, some uses pfifo,
> we may break some existing script if we change that.

While I do understand that reasoning, I'd argue that unpredictable and 
unexpected behavior of TBF causing systems with non-working networking is 
much more likely than any userspace having hard dependency on the fact 
that default (*) qdisc for TBF is noop.

(*) where 'default upon creation' != 'default when reset'

Thanks,

-- 
Jiri Kosina
SUSE Labs