netdev - Re: [PATCH net-next V4 1/2] net: sched: generalize check for no-queue qdisc on TX queue

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Message-ID: <87wmbki65m.fsf@toke.dk>
Date: Wed, 16 Apr 2025 15:32:05 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>, netdev@...r.kernel.org, Jakub
 Kicinski <kuba@...nel.org>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, bpf@...r.kernel.org,
 tom@...bertland.com, Eric Dumazet <eric.dumazet@...il.com>, "David S.
 Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
 dsahern@...nel.org, makita.toshiaki@....ntt.co.jp,
 kernel-team@...udflare.com, phil@....cc
Subject: Re: [PATCH net-next V4 1/2] net: sched: generalize check for
 no-queue qdisc on TX queue

Jesper Dangaard Brouer <hawk@...nel.org> writes:

> The "noqueue" qdisc can either be directly attached, or get default
> attached if net_device priv_flags has IFF_NO_QUEUE. In both cases, the
> allocated Qdisc structure gets it's enqueue function pointer reset to
> NULL by noqueue_init() via noqueue_qdisc_ops.
>
> This is a common case for software virtual net_devices. For these devices
> with no-queue, the transmission path in __dev_queue_xmit() will bypass
> the qdisc layer. Directly invoking device drivers ndo_start_xmit (via
> dev_hard_start_xmit).  In this mode the device driver is not allowed to
> ask for packets to be queued (either via returning NETDEV_TX_BUSY or
> stopping the TXQ).
>
> The simplest and most reliable way to identify this no-queue case is by
> checking if enqueue == NULL.
>
> The vrf driver currently open-codes this check (!qdisc->enqueue). While
> functionally correct, this low-level detail is better encapsulated in a
> dedicated helper for clarity and long-term maintainability.
>
> To make this behavior more explicit and reusable, this patch introduce a
> new helper: qdisc_txq_has_no_queue(). Helper will also be used by the
> veth driver in the next patch, which introduces optional qdisc-based
> backpressure.
>
> This is a non-functional change.
>
> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>

Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>