net: sched: introduce qdisc-specific drop reason tracing

Create new enum qdisc_drop_reason and trace_qdisc_drop tracepoint
for qdisc layer drop diagnostics with direct qdisc context visibility.

The new tracepoint includes qdisc handle, parent, kind (name), and
device information. Existing SKB_DROP_REASON_QDISC_DROP is retained
for backwards compatibility via kfree_skb_reason().

Convert qdiscs with drop reasons to use the new infrastructure.

Change CAKE's cobalt_should_drop() return type from enum skb_drop_reason
to enum qdisc_drop_reason to fix implicit enum conversion warnings.
Use QDISC_DROP_UNSPEC as the 'not dropped' sentinel instead of
SKB_NOT_DROPPED_YET. Both have the same compiled value (0), so the
comparison logic remains semantically equivalent.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/177211345275.3011628.1974310302645218067.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

5 files changed, 185 insertions, 51 deletions

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index a7b7abd66e21..3d8d284e05c8 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -68,12 +68,6 @@
 	FN(SECURITY_HOOK)		\
 	FN(QDISC_DROP)			\
 	FN(QDISC_BURST_DROP)		\
-	FN(QDISC_OVERLIMIT)		\
-	FN(QDISC_CONGESTED)		\
-	FN(CAKE_FLOOD)			\
-	FN(FQ_BAND_LIMIT)		\
-	FN(FQ_HORIZON_LIMIT)		\
-	FN(FQ_FLOW_LIMIT)		\
 	FN(CPU_BACKLOG)			\
 	FN(XDP)				\
 	FN(TC_INGRESS)			\
@@ -371,8 +365,10 @@ enum skb_drop_reason {
 	/** @SKB_DROP_REASON_SECURITY_HOOK: dropped due to security HOOK */
 	SKB_DROP_REASON_SECURITY_HOOK,
 	/**
-	 * @SKB_DROP_REASON_QDISC_DROP: dropped by qdisc when packet outputting (
-	 * failed to enqueue to current qdisc)
+	 * @SKB_DROP_REASON_QDISC_DROP: dropped by qdisc during enqueue or
+	 * dequeue. More specific drop reasons are available via the
+	 * qdisc:qdisc_drop tracepoint, which also provides qdisc handle
+	 * and name for identifying the source.
 	 */
 	SKB_DROP_REASON_QDISC_DROP,
 	/**
@@ -381,36 +377,6 @@ enum skb_drop_reason {
 	 */
 	SKB_DROP_REASON_QDISC_BURST_DROP,
 	/**
-	 * @SKB_DROP_REASON_QDISC_OVERLIMIT: dropped by qdisc when a qdisc
-	 * instance exceeds its total buffer size limit.
-	 */
-	SKB_DROP_REASON_QDISC_OVERLIMIT,
-	/**
-	 * @SKB_DROP_REASON_QDISC_CONGESTED: dropped by a qdisc AQM algorithm
-	 * due to congestion.
-	 */
-	SKB_DROP_REASON_QDISC_CONGESTED,
-	/**
-	 * @SKB_DROP_REASON_CAKE_FLOOD: dropped by the flood protection part of
-	 * CAKE qdisc AQM algorithm (BLUE).
-	 */
-	SKB_DROP_REASON_CAKE_FLOOD,
-	/**
-	 * @SKB_DROP_REASON_FQ_BAND_LIMIT: dropped by fq qdisc when per band
-	 * limit is reached.
-	 */
-	SKB_DROP_REASON_FQ_BAND_LIMIT,
-	/**
-	 * @SKB_DROP_REASON_FQ_HORIZON_LIMIT: dropped by fq qdisc when packet
-	 * timestamp is too far in the future.
-	 */
-	SKB_DROP_REASON_FQ_HORIZON_LIMIT,
-	/**
-	 * @SKB_DROP_REASON_FQ_FLOW_LIMIT: dropped by fq qdisc when a flow
-	 * exceeds its limits.
-	 */
-	SKB_DROP_REASON_FQ_FLOW_LIMIT,
-	/**
 	 * @SKB_DROP_REASON_CPU_BACKLOG: failed to enqueue the skb to the per CPU
 	 * backlog queue. This can be caused by backlog queue full (see
 	 * netdev_max_backlog in net.rst) or RPS flow limit
diff --git a/include/net/dropreason-qdisc.h b/include/net/dropreason-qdisc.h
new file mode 100644
index 000000000000..80a2d557e5f7
--- /dev/null
+++ b/include/net/dropreason-qdisc.h
@@ -0,0 +1,94 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#ifndef _LINUX_DROPREASON_QDISC_H
+#define _LINUX_DROPREASON_QDISC_H
+#include <net/dropreason.h>
+
+#define DEFINE_QDISC_DROP_REASON(FN, FNe)	\
+	FN(UNSPEC)			\
+	FN(GENERIC)			\
+	FN(OVERLIMIT)			\
+	FN(CONGESTED)			\
+	FN(CAKE_FLOOD)			\
+	FN(FQ_BAND_LIMIT)		\
+	FN(FQ_HORIZON_LIMIT)		\
+	FN(FQ_FLOW_LIMIT)		\
+	FNe(MAX)
+
+#undef FN
+#undef FNe
+#define FN(reason)	QDISC_DROP_##reason,
+#define FNe(reason)	QDISC_DROP_##reason
+
+/**
+ * enum qdisc_drop_reason - reason why a qdisc dropped a packet
+ *
+ * Qdisc-specific drop reasons for packet drops that occur within the
+ * traffic control (TC) queueing discipline layer. These reasons provide
+ * detailed diagnostics about why packets were dropped by various qdisc
+ * algorithms, enabling fine-grained monitoring and troubleshooting of
+ * queue behavior.
+ */
+enum qdisc_drop_reason {
+	/**
+	 * @QDISC_DROP_UNSPEC: unspecified/invalid qdisc drop reason.
+	 * Value 0 serves as analogous to SKB_NOT_DROPPED_YET for enum skb_drop_reason.
+	 * Used for catching zero-initialized drop_reason fields.
+	 */
+	QDISC_DROP_UNSPEC = 0,
+	/**
+	 * @__QDISC_DROP_REASON: subsystem base value for qdisc drop reasons
+	 */
+	__QDISC_DROP_REASON = SKB_DROP_REASON_SUBSYS_QDISC <<
+				SKB_DROP_REASON_SUBSYS_SHIFT,
+	/**
+	 * @QDISC_DROP_GENERIC: generic/default qdisc drop, used when no
+	 * more specific reason applies
+	 */
+	QDISC_DROP_GENERIC,
+	/**
+	 * @QDISC_DROP_OVERLIMIT: packet dropped because the qdisc queue
+	 * length exceeded its configured limit (sch->limit). This typically
+	 * indicates the queue is full and cannot accept more packets.
+	 */
+	QDISC_DROP_OVERLIMIT,
+	/**
+	 * @QDISC_DROP_CONGESTED: packet dropped due to active congestion
+	 * control algorithms (e.g., CoDel, PIE, RED) detecting network
+	 * congestion. The qdisc proactively dropped the packet to signal
+	 * congestion to the sender and prevent bufferbloat.
+	 */
+	QDISC_DROP_CONGESTED,
+	/**
+	 * @QDISC_DROP_CAKE_FLOOD: CAKE qdisc dropped packet due to flood
+	 * protection mechanism (BLUE algorithm). This indicates potential
+	 * DoS/flood attack or unresponsive flow behavior.
+	 */
+	QDISC_DROP_CAKE_FLOOD,
+	/**
+	 * @QDISC_DROP_FQ_BAND_LIMIT: FQ (Fair Queue) dropped packet because
+	 * the priority band's packet limit was reached. Each priority band
+	 * in FQ has its own limit.
+	 */
+	QDISC_DROP_FQ_BAND_LIMIT,
+	/**
+	 * @QDISC_DROP_FQ_HORIZON_LIMIT: FQ dropped packet because its
+	 * timestamp is too far in the future (beyond the configured horizon).
+	 */
+	QDISC_DROP_FQ_HORIZON_LIMIT,
+	/**
+	 * @QDISC_DROP_FQ_FLOW_LIMIT: FQ dropped packet because an individual
+	 * flow exceeded its per-flow packet limit.
+	 */
+	QDISC_DROP_FQ_FLOW_LIMIT,
+	/**
+	 * @QDISC_DROP_MAX: the maximum of qdisc drop reasons, which
+	 * shouldn't be used as a real 'reason' - only for tracing code gen
+	 */
+	QDISC_DROP_MAX,
+};
+
+#undef FN
+#undef FNe
+
+#endif
diff --git a/include/net/dropreason.h b/include/net/dropreason.h
index 7d3b1a2a6fec..1df60645fb27 100644
--- a/include/net/dropreason.h
+++ b/include/net/dropreason.h
@@ -23,6 +23,12 @@ enum skb_drop_reason_subsys {
 	 */
 	SKB_DROP_REASON_SUBSYS_OPENVSWITCH,
 
+	/**
+	 * @SKB_DROP_REASON_SUBSYS_QDISC: TC qdisc drop reasons,
+	 * see include/net/dropreason-qdisc.h
+	 */
+	SKB_DROP_REASON_SUBSYS_QDISC,
+
 	/** @SKB_DROP_REASON_SUBSYS_NUM: number of subsystems defined */
 	SKB_DROP_REASON_SUBSYS_NUM
 };
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index c3a7268b567e..31c25a6d6acc 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -20,12 +20,15 @@
 #include <net/rtnetlink.h>
 #include <net/flow_offload.h>
 #include <linux/xarray.h>
+#include <net/dropreason-qdisc.h>
 
 struct Qdisc_ops;
 struct qdisc_walker;
 struct tcf_walker;
 struct module;
 struct bpf_flow_keys;
+struct Qdisc;
+struct netdev_queue;
 
 struct qdisc_rate_table {
 	struct tc_ratespec rate;
@@ -1106,36 +1109,50 @@ static inline struct tc_skb_cb *tc_skb_cb(const struct sk_buff *skb)
 	return cb;
 }
 
+/* TC classifier accessors - use enum skb_drop_reason */
 static inline enum skb_drop_reason
 tcf_get_drop_reason(const struct sk_buff *skb)
 {
-	return tc_skb_cb(skb)->drop_reason;
+	return (enum skb_drop_reason)tc_skb_cb(skb)->drop_reason;
 }
 
 static inline void tcf_set_drop_reason(const struct sk_buff *skb,
 				       enum skb_drop_reason reason)
 {
-	tc_skb_cb(skb)->drop_reason = reason;
+	tc_skb_cb(skb)->drop_reason = (enum qdisc_drop_reason)reason;
 }
 
-static inline void tcf_kfree_skb_list(struct sk_buff *skb)
+/* Qdisc accessors - use enum qdisc_drop_reason */
+static inline enum qdisc_drop_reason
+tcf_get_qdisc_drop_reason(const struct sk_buff *skb)
 {
-	while (unlikely(skb)) {
-		struct sk_buff *next = skb->next;
+	return tc_skb_cb(skb)->drop_reason;
+}
 
-		prefetch(next);
-		kfree_skb_reason(skb, tcf_get_drop_reason(skb));
-		skb = next;
-	}
+static inline void tcf_set_qdisc_drop_reason(const struct sk_buff *skb,
+					     enum qdisc_drop_reason reason)
+{
+	tc_skb_cb(skb)->drop_reason = reason;
+}
+
+void __tcf_kfree_skb_list(struct sk_buff *skb, struct Qdisc *q,
+			  struct netdev_queue *txq, struct net_device *dev);
+
+static inline void tcf_kfree_skb_list(struct sk_buff *skb, struct Qdisc *q,
+				      struct netdev_queue *txq,
+				      struct net_device *dev)
+{
+	if (unlikely(skb))
+		__tcf_kfree_skb_list(skb, q, txq, dev);
 }
 
 static inline void qdisc_dequeue_drop(struct Qdisc *q, struct sk_buff *skb,
-				      enum skb_drop_reason reason)
+				      enum qdisc_drop_reason reason)
 {
 	DEBUG_NET_WARN_ON_ONCE(!(q->flags & TCQ_F_DEQUEUE_DROPS));
 	DEBUG_NET_WARN_ON_ONCE(q->flags & TCQ_F_NOLOCK);
 
-	tcf_set_drop_reason(skb, reason);
+	tcf_set_qdisc_drop_reason(skb, reason);
 	skb->next = q->to_free;
 	q->to_free = skb;
 }
@@ -1312,9 +1329,9 @@ static inline int qdisc_drop(struct sk_buff *skb, struct Qdisc *sch,
 
 static inline int qdisc_drop_reason(struct sk_buff *skb, struct Qdisc *sch,
 				    struct sk_buff **to_free,
-				    enum skb_drop_reason reason)
+				    enum qdisc_drop_reason reason)
 {
-	tcf_set_drop_reason(skb, reason);
+	tcf_set_qdisc_drop_reason(skb, reason);
 	return qdisc_drop(skb, sch, to_free);
 }
 
diff --git a/include/trace/events/qdisc.h b/include/trace/events/qdisc.h
index ff33f41a9db7..d8a5c2677470 100644
--- a/include/trace/events/qdisc.h
+++ b/include/trace/events/qdisc.h
@@ -74,6 +74,57 @@ TRACE_EVENT(qdisc_enqueue,
 		  __entry->ifindex, __entry->handle, __entry->parent, __entry->skbaddr)
 );
 
+#undef FN
+#undef FNe
+#define FN(reason)	TRACE_DEFINE_ENUM(QDISC_DROP_##reason);
+#define FNe(reason)	TRACE_DEFINE_ENUM(QDISC_DROP_##reason);
+DEFINE_QDISC_DROP_REASON(FN, FNe)
+
+#undef FN
+#undef FNe
+#define FN(reason)	{ QDISC_DROP_##reason, #reason },
+#define FNe(reason)	{ QDISC_DROP_##reason, #reason }
+
+TRACE_EVENT(qdisc_drop,
+
+	TP_PROTO(struct Qdisc *qdisc, const struct netdev_queue *txq,
+		 struct net_device *dev, struct sk_buff *skb,
+		 enum qdisc_drop_reason reason),
+
+	TP_ARGS(qdisc, txq, dev, skb, reason),
+
+	TP_STRUCT__entry(
+		__field(struct Qdisc *, qdisc)
+		__field(const struct netdev_queue *, txq)
+		__field(void *,	skbaddr)
+		__field(int, ifindex)
+		__field(u32, handle)
+		__field(u32, parent)
+		__field(enum qdisc_drop_reason, reason)
+		__string(kind, qdisc->ops->id)
+	),
+
+	TP_fast_assign(
+		__entry->qdisc = qdisc;
+		__entry->txq	 = txq;
+		__entry->skbaddr = skb;
+		__entry->ifindex = dev ? dev->ifindex : 0;
+		__entry->handle	 = qdisc->handle;
+		__entry->parent	 = qdisc->parent;
+		__entry->reason	 = reason;
+		__assign_str(kind);
+	),
+
+	TP_printk("drop ifindex=%d kind=%s handle=0x%X parent=0x%X skbaddr=%p reason=%s",
+		  __entry->ifindex, __get_str(kind), __entry->handle,
+		  __entry->parent, __entry->skbaddr,
+		  __print_symbolic(__entry->reason,
+				   DEFINE_QDISC_DROP_REASON(FN, FNe)))
+);
+
+#undef FN
+#undef FNe
+
 TRACE_EVENT(qdisc_reset,
 
 	TP_PROTO(struct Qdisc *q),