netdev - Re: [net-next v1 1/2] net: sched: use queue

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Date:   Mon, 6 Dec 2021 18:33:01 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Lobakin <alobakin@...me>,
        Paolo Abeni <pabeni@...hat.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Kevin Hao <haokexin@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Antoine Tenart <atenart@...nel.org>,
        Wei Wang <weiwan@...gle.com>, Arnd Bergmann <arnd@...db.de>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net-next v1 1/2] net: sched: use queue_mapping to pick tx
 queue

On Tue, 7 Dec 2021 10:10:22 +0800 Tonghao Zhang wrote:
> > In general recording the decision in the skb seems a little heavy
> > handed. We just need to carry the information from the egress hook
> > to the queue selection a few lines below. Or in fact maybe egress  
> Yes, we can refactor netdev_core_pick_tx to
> 1. select queue_index and invoke skb_set_queue_mapping, but don't
> return the txq.
> 2. after egress hook, use skb_get_queue_mapping/netdev_get_tx_queue to get txq.

I'm not sure that's what I meant, I meant the information you need to
store does not need to be stored in the skb, you can pass a pointer to
a stack variable to both egress handling and pick_tx.

> > hook shouldn't be used for this in the first place, and we need
> > a more appropriate root qdisc than simple mq?  
> I have no idea about mq, I think clsact may make the things more flexible.
> and act_bpf can also support to change sk queue_mapping. queue_mapping
> was included in __sk_buff.

Qdiscs can run a classifier to select a sub-queue. The advantage of
the classifier run by the Qdisc is that it runs after pick_tx.