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

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Message-ID: <1460648680.10638.47.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Thu, 14 Apr 2016 08:44:40 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Kosina <jikos@...nel.org>
Cc:	Phil Sutter <phil@....cc>, Jamal Hadi Salim <jhs@...atatu.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Deleting child qdisc doesn't reset parent to default qdisc?

On Thu, 2016-04-14 at 17:34 +0200, Jiri Kosina wrote:
> On Thu, 14 Apr 2016, Phil Sutter wrote:
> 
> > OTOH some qdiscs (CBQ, DRR, DSMARK, HFSC, HTB, QFQ) assign the default
> > one upon deletion instead of noop_qdisc, hence I would describe
> > the situation using the words 'inconsistent' and 'accident' rather than
> > 'expected'. :)
> 
> Exactly. I'd again like to stress the fact that this configuration works:
> 
> 	jikos:~ # tc qdisc show
> 	qdisc tbf 10: dev eth0 root refcnt 2 rate 800Mbit burst 131000b lat 1.0ms 
> 
> and this (after performing add/delete operation) doesn't:
> 
> 	jikos:~ # tc qdisc show
> 	qdisc tbf 10: dev eth0 root refcnt 2 rate 800Mbit burst 131000b lat 1.0ms 
> 
> It's hard to spot a difference (hint: there is none).

This is because some qdisc are not visible in the dump.


qdisc_list_add() uses a single list, so adding too much stuff in it
could slow down fast path (qdisc_lookup(), called from
qdisc_tree_reduce_backlog())