netdev - Re: Weird behavior (bug?) with mq qdisc

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Message-ID: <CAM_iQpWOn=Y+f-2F-uAGC16eKVKjwE1JtngRRyeATt9Xrw2ELg@mail.gmail.com>
Date:   Wed, 8 Apr 2020 12:30:28 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Maxim Mikityanskiy <maximmi@...lanox.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Yossi Kuperman <yossiku@...lanox.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: Re: Weird behavior (bug?) with mq qdisc

On Wed, Apr 8, 2020 at 8:13 AM Maxim Mikityanskiy <maximmi@...lanox.com> wrote:
> I would like to hear more opinions on these two issues (1. qdiscs are
> not shown when the number of queues grows, 2. tc qdisc del for a queue
> reverts to noop, rather than to some sane default). Any ideas about
> fixing them, especially issue 1? Some kind of notification mechanism
> from netif_set_real_num_tx_queues to mq or even complete reattachment of
> mq when the number of queues change...

For 1) we don't update qdisc's when changing tx queues, it should not
be hard to call ->attach() in netif_set_real_num_tx_queues(). But it is
not easy either, because mq_attach() merely attaches default qdisc's,
let's say if you already have 4 non-default qdisc's in your case, you
probably want to just duplicate 4 more when growing to 8. IMHO, this
is not a bug, it is just inconvenient.

For 2), I do not think it is a bug, as you can think noop as a deleted
qdisc.

Thanks.