[PATCH net-next v5 2/5] net: sched: sfq: convert to qdisc drop reasons

[Jesper Dangaard Brouer <hawk@kernel.org>]

Convert SFQ to use the new qdisc-specific drop reason infrastructure.

This patch demonstrates how to convert a flow-based qdisc to use the
new enum qdisc_drop_reason. As part of this conversion:

- Add QDISC_DROP_MAXFLOWS for flow table exhaustion
- Rename FQ_FLOW_LIMIT to generic FLOW_LIMIT, now shared by FQ and SFQ
- Use QDISC_DROP_OVERLIMIT for sfq_drop() when overall limit exceeded
- Use QDISC_DROP_FLOW_LIMIT for per-flow depth limit exceeded

The FLOW_LIMIT reason is now a common drop reason for per-flow limits,
applicable to both FQ and SFQ qdiscs.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 include/net/dropreason-qdisc.h |   18 ++++++++++++++----
 net/sched/sch_fq.c             |    2 +-
 net/sched/sch_sfq.c            |    8 ++++----
 3 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/include/net/dropreason-qdisc.h b/include/net/dropreason-qdisc.h
index 80a2d557e5f7..02a9f580411b 100644
--- a/include/net/dropreason-qdisc.h
+++ b/include/net/dropreason-qdisc.h
@@ -9,10 +9,11 @@
 	FN(GENERIC)			\
 	FN(OVERLIMIT)			\
 	FN(CONGESTED)			\
+	FN(MAXFLOWS)			\
 	FN(CAKE_FLOOD)			\
 	FN(FQ_BAND_LIMIT)		\
 	FN(FQ_HORIZON_LIMIT)		\
-	FN(FQ_FLOW_LIMIT)		\
+	FN(FLOW_LIMIT)			\
 	FNe(MAX)
 
 #undef FN
@@ -59,6 +60,13 @@ enum qdisc_drop_reason {
 	 * congestion to the sender and prevent bufferbloat.
 	 */
 	QDISC_DROP_CONGESTED,
+	/**
+	 * @QDISC_DROP_MAXFLOWS: packet dropped because the qdisc's flow
+	 * tracking table is full and no free slots are available to allocate
+	 * for a new flow. This indicates flow table exhaustion in flow-based
+	 * qdiscs that maintain per-flow state (e.g., SFQ).
+	 */
+	QDISC_DROP_MAXFLOWS,
 	/**
 	 * @QDISC_DROP_CAKE_FLOOD: CAKE qdisc dropped packet due to flood
 	 * protection mechanism (BLUE algorithm). This indicates potential
@@ -77,10 +85,12 @@ enum qdisc_drop_reason {
 	 */
 	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_FLOW_LIMIT: packet dropped because an individual flow
+	 * exceeded its per-flow packet/depth limit. Used by FQ and SFQ qdiscs
+	 * to enforce per-flow fairness and prevent a single flow from
+	 * monopolizing queue resources.
 	 */
-	QDISC_DROP_FQ_FLOW_LIMIT,
+	QDISC_DROP_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
diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 81322187bbe2..eb5ae2b15cc0 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -578,7 +578,7 @@ static int fq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		if (unlikely(f->qlen >= q->flow_plimit)) {
 			q->stat_flows_plimit++;
 			return qdisc_drop_reason(skb, sch, to_free,
-						 QDISC_DROP_FQ_FLOW_LIMIT);
+						 QDISC_DROP_FLOW_LIMIT);
 		}
 
 		if (fq_flow_is_detached(f)) {
diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 96eb2f122973..efb796976a5b 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -302,7 +302,7 @@ static unsigned int sfq_drop(struct Qdisc *sch, struct sk_buff **to_free)
 		sfq_dec(q, x);
 		sch->q.qlen--;
 		qdisc_qstats_backlog_dec(sch, skb);
-		qdisc_drop(skb, sch, to_free);
+		qdisc_drop_reason(skb, sch, to_free, QDISC_DROP_OVERLIMIT);
 		return len;
 	}
 
@@ -363,7 +363,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 	if (x == SFQ_EMPTY_SLOT) {
 		x = q->dep[0].next; /* get a free slot */
 		if (x >= SFQ_MAX_FLOWS)
-			return qdisc_drop(skb, sch, to_free);
+			return qdisc_drop_reason(skb, sch, to_free, QDISC_DROP_MAXFLOWS);
 		q->ht[hash] = x;
 		slot = &q->slots[x];
 		slot->hash = hash;
@@ -420,14 +420,14 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 	if (slot->qlen >= q->maxdepth) {
 congestion_drop:
 		if (!sfq_headdrop(q))
-			return qdisc_drop(skb, sch, to_free);
+			return qdisc_drop_reason(skb, sch, to_free, QDISC_DROP_FLOW_LIMIT);
 
 		/* We know we have at least one packet in queue */
 		head = slot_dequeue_head(slot);
 		delta = qdisc_pkt_len(head) - qdisc_pkt_len(skb);
 		sch->qstats.backlog -= delta;
 		slot->backlog -= delta;
-		qdisc_drop(head, sch, to_free);
+		qdisc_drop_reason(head, sch, to_free, QDISC_DROP_FLOW_LIMIT);
 
 		slot_queue_add(slot, skb);
 		qdisc_tree_reduce_backlog(sch, 0, delta);