netdev - Re: [net-next v5 1/2] net: sched: use queue

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Date:   Mon, 20 Dec 2021 09:51:31 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        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>
Subject: Re: [net-next v5 1/2] net: sched: use queue_mapping to pick tx queue

On Mon, Dec 20, 2021 at 4:38 AM <xiangxia.m.yue@...il.com> wrote:
>   In general recording the decision in the skb seems a little heavy handed.
>   This patch introduces a per-CPU variable, suggested by Eric.
>
>   The xmit.skip_txqueue flag is firstly cleared in __dev_queue_xmit().
>   - Tx Qdisc may install that skbedit actions, then xmit.skip_txqueue flag
>     is set in qdisc->enqueue() though tx queue has been selected in
>     netdev_tx_queue_mapping() or netdev_core_pick_tx(). That flag is cleared
>     firstly in __dev_queue_xmit(), is useful:
>   - Avoid picking Tx queue with netdev_tx_queue_mapping() in next netdev
>     in such case: eth0 macvlan - eth0.3 vlan - eth0 ixgbe-phy:
>     For example, eth0, macvlan in pod, which root Qdisc install skbedit
>     queue_mapping, send packets to eth0.3, vlan in host. In __dev_queue_xmit() of
>     eth0.3, clear the flag, does not select tx queue according to skb->queue_mapping
>     because there is no filters in clsact or tx Qdisc of this netdev.
>     Same action taked in eth0, ixgbe in Host.
>   - Avoid picking Tx queue for next packet. If we set xmit.skip_txqueue
>     in tx Qdisc (qdisc->enqueue()), the proper way to clear it is clearing it
>     in __dev_queue_xmit when processing next packets.

Any reason why we can't just move sch_handle_egress() down after
netdev_core_pick_tx() and recalculate txq with skb->queue_mapping?

Thanks.