linux-kernel - Re: [PATCH 1/2] net: sched: make default fifo qdiscs appear in the dump

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Message-Id: <20170306.103555.1351381782582641603.davem@davemloft.net>
Date:   Mon, 06 Mar 2017 10:35:55 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     jikos@...nel.org
Cc:     stephen@...workplumber.org, eric.dumazet@...il.com,
        jhs@...atatu.com, phil@....cc, xiyou.wangcong@...il.com,
        daniel@...earbox.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] net: sched: make default fifo qdiscs appear in the
 dump

From: Jiri Kosina <jikos@...nel.org>
Date: Mon, 6 Mar 2017 12:03:38 +0100 (CET)

> Ah, right you are, thanks. The complete fix is not super trivial, as it 
> needs some more surgery to tc_dump_qdisc_root(), tc_dump_tclass_root() and 
> qdisc_match_from_root() (see 69012ae42 for some details).
> 
> There are two options:
> 
> - this gets fixed in two phases, in first everything *but* noop qdisc gets 
>   dumped (in the "give me everything" dump) and later we finalize it by
>   teaching the above functions about noop_qdisc as well
> 
> - I extend this patchset to handle noop qdisc from the very beginning; 
>   I am unlikely to find time for this during coming weeks though. But OTOH
>   this whole thing is very low priority anyway
> 
> What do you think?

I'm not too hot on this whole idea because the only way you can emit
the noop_qdisc is to "dup" it by allocating a new qdisc so that you
can link it in.  This has two downsides:

1) Extra overhead and memory usage

2) All of the simple checks against &noop_qdisc might not be
   so simply any more.