Add specific drop reasons to SFQ qdisc to improve packet drop observability and monitoring capabilities. This change replaces generic qdisc_drop() calls with qdisc_drop_reason() to provide granular metrics about different drop scenarios in production environments. Two new drop reasons are introduced: - SKB_DROP_REASON_QDISC_SFQ_MAXFLOWS: Used when a new flow cannot be created because the maximum number of flows (flows parameter) has been reached and no free flow slots are available. - SKB_DROP_REASON_QDISC_SFQ_MAXDEPTH: Used when a flow's queue length exceeds the per-flow depth limit (depth parameter), triggering either tail drop or head drop depending on headdrop configuration. The existing SKB_DROP_REASON_QDISC_OVERLIMIT is used in sfq_drop() when the overall qdisc limit is exceeded and packets are dropped from the longest queue. The naming uses qdisc-specific drop reasons tied to SFQ tunables, following the pattern established by Eric's FQ commit 5765c7f6e317 ("net_sched: sch_fq: add three drop_reason") which introduced FQ_BAND_LIMIT, FQ_HORIZON_LIMIT, and FQ_FLOW_LIMIT. The new drop reasons are inserted in the middle of the enum after SKB_DROP_REASON_QDISC_CONGESTED to group all qdisc-related reasons together. While drop reason enum values are not UAPI, the enum names implicitly become UAPI as userspace tools rely on BTF to resolve names to values. This makes middle-insertion safe as long as names remain stable. These detailed drop reasons enable production monitoring systems to distinguish between different SFQ drop scenarios and generate specific metrics for: - Flow table exhaustion (flows exceeded) - Per-flow congestion (depth limit exceeded) - Global qdisc congestion (overall limit exceeded) This granular visibility allows operators to identify capacity planning needs, detect traffic patterns, and optimize SFQ configuration based on real-world drop patterns. Signed-off-by: Jesper Dangaard Brouer --- v3: - Removed SFQ_DR() macro per upstream feedback - Use full drop reason names (SKB_DROP_REASON_QDISC_SFQ_*) for easier grepping - Split long lines to stay under 80 characters v2: - Changed to hierarchical naming: QDISC_SFQ_MAXFLOWS and QDISC_SFQ_MAXDEPTH - Added QDISC_ prefix for pattern matching across qdisc subsystems - Added rationale paragraph explaining userspace monitoring benefits v1: - Initial submission with QDISC_MAXFLOWS and QDISC_MAXDEPTH --- include/net/dropreason-core.h | 12 ++++++++++++ net/sched/sch_sfq.c | 11 +++++++---- 2 files changed, 19 insertions(+), 4 deletions(-) diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h index a7b7abd66e21..92c99169bb97 100644 --- a/include/net/dropreason-core.h +++ b/include/net/dropreason-core.h @@ -70,6 +70,8 @@ FN(QDISC_BURST_DROP) \ FN(QDISC_OVERLIMIT) \ FN(QDISC_CONGESTED) \ + FN(QDISC_SFQ_MAXFLOWS) \ + FN(QDISC_SFQ_MAXDEPTH) \ FN(CAKE_FLOOD) \ FN(FQ_BAND_LIMIT) \ FN(FQ_HORIZON_LIMIT) \ @@ -390,6 +392,16 @@ enum skb_drop_reason { * due to congestion. */ SKB_DROP_REASON_QDISC_CONGESTED, + /** + * @SKB_DROP_REASON_QDISC_SFQ_MAXFLOWS: dropped by SFQ qdisc when the + * maximum number of flows is exceeded. + */ + SKB_DROP_REASON_QDISC_SFQ_MAXFLOWS, + /** + * @SKB_DROP_REASON_QDISC_SFQ_MAXDEPTH: dropped by SFQ qdisc when a flow + * exceeds its maximum queue depth limit. + */ + SKB_DROP_REASON_QDISC_SFQ_MAXDEPTH, /** * @SKB_DROP_REASON_CAKE_FLOOD: dropped by the flood protection part of * CAKE qdisc AQM algorithm (BLUE). diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c index 96eb2f122973..ecc1028bdac8 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, SKB_DROP_REASON_QDISC_OVERLIMIT); return len; } @@ -363,7 +363,8 @@ 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, + SKB_DROP_REASON_QDISC_SFQ_MAXFLOWS); q->ht[hash] = x; slot = &q->slots[x]; slot->hash = hash; @@ -420,14 +421,16 @@ 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, + SKB_DROP_REASON_QDISC_SFQ_MAXDEPTH); /* 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, + SKB_DROP_REASON_QDISC_SFQ_MAXDEPTH); slot_queue_add(slot, skb); qdisc_tree_reduce_backlog(sch, 0, delta);