netdev - [PATCH net-next V5 1/2] net: sched: generalize check for no-queue qdisc on TX queue

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Message-ID: <174489810897.355490.12073714761899580608.stgit@firesoul>
Date: Thu, 17 Apr 2025 15:55:09 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: 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>,
 Toke Høiland-Jørgensen <toke@...e.dk>,
 dsahern@...nel.org, makita.toshiaki@....ntt.co.jp,
 kernel-team@...udflare.com, phil@....cc
Subject: [PATCH net-next V5 1/2] net: sched: generalize check for no-queue
 qdisc on TX queue

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.

Reviewed-by: David Ahern <dsahern@...nel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>
---
 drivers/net/vrf.c         |    4 +---
 include/net/sch_generic.h |    8 ++++++++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
index 7168b33adadb..9a4beea6ee0c 100644
--- a/drivers/net/vrf.c
+++ b/drivers/net/vrf.c
@@ -343,15 +343,13 @@ static int vrf_ifindex_lookup_by_table_id(struct net *net, u32 table_id)
 static bool qdisc_tx_is_default(const struct net_device *dev)
 {
 	struct netdev_queue *txq;
-	struct Qdisc *qdisc;

 	if (dev->num_tx_queues > 1)
 		return false;

 	txq = netdev_get_tx_queue(dev, 0);
-	qdisc = rcu_access_pointer(txq->qdisc);

-	return !qdisc->enqueue;
+	return qdisc_txq_has_no_queue(txq);
 }

 /* Local traffic destined to local address. Reinsert the packet to rx
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index d48c657191cd..b6c177f7141c 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -803,6 +803,14 @@ static inline bool qdisc_tx_changing(const struct net_device *dev)
 	return false;
 }

+/* "noqueue" qdisc identified by not having any enqueue, see noqueue_init() */
+static inline bool qdisc_txq_has_no_queue(const struct netdev_queue *txq)
+{
+	struct Qdisc *qdisc = rcu_access_pointer(txq->qdisc);
+
+	return qdisc->enqueue == NULL;
+}
+
 /* Is the device using the noop qdisc on all queues?  */
 static inline bool qdisc_tx_is_noop(const struct net_device *dev)
 {