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Date:	Wed, 31 Dec 2014 15:31:42 +0800
From:	Dennis Chen <>
To:	Cong Wang <>
Cc:	netdev <>
Subject: Re: [Question] What's the noop_qdisc introduced for in the kernel?

On Wed, Dec 31, 2014 at 4:15 AM, Cong Wang <> wrote:
> On Tue, Dec 30, 2014 at 1:23 AM, Dennis Chen <> wrote:
>> After google and the code reading, seems this Qdisc instance is only
>> used for the initialization purpose, I can't find the reason that this
>> object introduced in the kernel. Does anybody know what the historical
>> reason is for this invention? the purpose or the benefit for this
>> Qdisc object?
> Not just for initialization, it is kinda a null qdisc when
> the previous qdisc gets removed:
>         /* ... and graft new one */
>         if (qdisc == NULL)
>                 qdisc = &noop_qdisc;
> or the entire device is not activated yet. It guarantees no
> packets can be sent out via this qdisc.

Thanks Cong, got it. The device will use the noop_qdisc when it's not
activated yet...

BTW, do you have some methods to recommend if I want to find the
initial codes that introduce the noop_qdisc first in the kernel? I
tried to search the LKML archives and google, but seems it's not easy
to do that

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