netdev - [PATCH net-next V2 2/2] net: sched: generalize check for no-op qdisc

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Message-ID: <174412628464.3702169.81132659219041209.stgit@firesoul>
Date: Tue, 08 Apr 2025 17:31:24 +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
Subject: [PATCH net-next V2 2/2] net: sched: generalize check for no-op qdisc

Several drivers (e.g., veth, vrf) contain open-coded checks to determine
whether a TX queue has a real qdisc attached - typically by testing if
qdisc->enqueue is non-NULL.

These checks are functionally equivalent to comparing the queue's qdisc
pointer against &noop_qdisc (qdisc named "noqueue"). This equivalence
stems from noqueue_init(), which explicitly clears the enqueue pointer
for the "noqueue" qdisc. As a result, __dev_queue_xmit() treats the qdisc
as a no-op only when enqueue == NULL.

This patch introduces a common helper, qdisc_txq_is_noop() to standardize
this check. The helper is added in sch_generic.h and replaces open-coded
logic in both the veth and vrf drivers.

This is a non-functional change.

Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>
---
 drivers/net/veth.c        |   14 +-------------
 drivers/net/vrf.c         |    3 +--
 include/net/sch_generic.h |    7 ++++++-
 3 files changed, 8 insertions(+), 16 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index f29a0db2ba36..83c7758534da 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -341,18 +341,6 @@ static bool veth_skb_is_eligible_for_gro(const struct net_device *dev,
 		 rcv->features & (NETIF_F_GRO_FRAGLIST | NETIF_F_GRO_UDP_FWD));
 }

-/* Does specific txq have a real qdisc attached? - see noqueue_init() */
-static inline bool txq_has_qdisc(struct netdev_queue *txq)
-{
-	struct Qdisc *q;
-
-	q = rcu_dereference(txq->qdisc);
-	if (q->enqueue)
-		return true;
-	else
-		return false;
-}
-
 static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct veth_priv *rcv_priv, *priv = netdev_priv(dev);
@@ -399,7 +387,7 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 		 */
 		txq = netdev_get_tx_queue(dev, rxq);

-		if (!txq_has_qdisc(txq)) {
+		if (qdisc_txq_is_noop(txq)) {
 			dev_kfree_skb_any(skb);
 			goto drop;
 		}
diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
index 7168b33adadb..d4fe36c55f29 100644
--- a/drivers/net/vrf.c
+++ b/drivers/net/vrf.c
@@ -349,9 +349,8 @@ static bool qdisc_tx_is_default(const struct net_device *dev)
 		return false;

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

-	return !qdisc->enqueue;
+	return qdisc_txq_is_noop(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..eb90d5103371 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -803,6 +803,11 @@ static inline bool qdisc_tx_changing(const struct net_device *dev)
 	return false;
 }

+static inline bool qdisc_txq_is_noop(const struct netdev_queue *txq)
+{
+	return (rcu_access_pointer(txq->qdisc) == &noop_qdisc);
+}
+
 /* Is the device using the noop qdisc on all queues?  */
 static inline bool qdisc_tx_is_noop(const struct net_device *dev)
 {
@@ -810,7 +815,7 @@ static inline bool qdisc_tx_is_noop(const struct net_device *dev)

 	for (i = 0; i < dev->num_tx_queues; i++) {
 		struct netdev_queue *txq = netdev_get_tx_queue(dev, i);
-		if (rcu_access_pointer(txq->qdisc) != &noop_qdisc)
+		if (!qdisc_txq_is_noop(txq))
 			return false;
 	}
 	return true;