netdev - Re: [Question] What's the noop_qdisc introduced for in the kernel?

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Message-ID: <CAHA+R7O_ypxh4DjQ1eeKZyGFpYbKQAgKkEjrmk400Ud+zLeEow@mail.gmail.com>
Date:	Tue, 30 Dec 2014 12:15:01 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Dennis Chen <kernel.org.gnu@...il.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: [Question] What's the noop_qdisc introduced for in the kernel?

On Tue, Dec 30, 2014 at 1:23 AM, Dennis Chen <kernel.org.gnu@...il.com> 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.
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