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

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Date:   Tue, 7 Dec 2021 11:22:28 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
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, Dec 7, 2021 at 10:33 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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.
Thanks, I got it. I think we store the txq index in skb->queue_mapping
better. because in egress hook,
act_skbedit/act_bpf can change the skb queue_mapping. Then we can
pick_tx depending on queue_mapping.

> > > 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.
Yes, we should consider the qdisc lock too. Qdisc lock may affect
performance and latency when running a classifier in Qdisc
and clsact is outside of qdisc.


-- 
Best regards, Tonghao