From 090dde0780266eb80126fa970870bafdb4e937c3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jonas=20K=C3=B6ppeler?= <j.koeppeler@tu-berlin.de>
Date: Fri, 9 Jan 2026 14:15:34 +0100
Subject: [PATCH 5/7] net/sched: sch_cake: share shaper state across
 sub-instances of cake_mq
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This commit adds shared shaper state across the cake instances beneath a
cake_mq qdisc. It works by periodically tracking the number of active
instances, and scaling the configured rate by the number of active
queues.

The scan is lockless and simply reads the qlen and the last_active state
variable of each of the instances configured beneath the parent cake_mq
instance. Locking is not required since the values are only updated by
the owning instance, and eventual consistency is sufficient for the
purpose of estimating the number of active queues.

The interval for scanning the number of active queues is set to 200 us.
We found this to be a good tradeoff between overhead and response time.
For a detailed analysis of this aspect see the Netdevconf talk:

https://netdevconf.info/0x19/docs/netdev-0x19-paper16-talk-paper.pdf

Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20260109-mq-cake-sub-qdisc-v8-5-8d613fece5d8@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 Documentation/netlink/specs/tc.yaml |  3 ++
 include/uapi/linux/pkt_sched.h      |  1 +
 net/sched/sch_cake.c                | 51 +++++++++++++++++++++++++++++
 3 files changed, 55 insertions(+)

--- a/Documentation/netlink/specs/tc.yaml
+++ b/Documentation/netlink/specs/tc.yaml
@@ -2161,6 +2161,9 @@ attribute-sets:
       -
         name: blue-timer-us
         type: s32
+      -
+        name: active-queues
+        type: u32
   -
     name: tc-cake-tin-stats-attrs
     attributes:
--- a/include/uapi/linux/pkt_sched.h
+++ b/include/uapi/linux/pkt_sched.h
@@ -1034,6 +1034,7 @@ enum {
 	TCA_CAKE_STATS_DROP_NEXT_US,
 	TCA_CAKE_STATS_P_DROP,
 	TCA_CAKE_STATS_BLUE_TIMER_US,
+	TCA_CAKE_STATS_ACTIVE_QUEUES,
 	__TCA_CAKE_STATS_MAX
 };
 #define TCA_CAKE_STATS_MAX (__TCA_CAKE_STATS_MAX - 1)
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -202,6 +202,7 @@ struct cake_sched_config {
 	u64		rate_bps;
 	u64		interval;
 	u64		target;
+	u64		sync_time;
 	u32		buffer_config_limit;
 	u32		fwmark_mask;
 	u16		fwmark_shft;
@@ -258,6 +259,11 @@ struct cake_sched_data {
 	u16		max_adjlen;
 	u16		min_netlen;
 	u16		min_adjlen;
+
+	/* mq sync state */
+	u64		last_checked_active;
+	u64		last_active;
+	u32		active_queues;
 };
 
 enum {
@@ -384,6 +390,8 @@ static const u32 inv_sqrt_cache[REC_INV_
 	1239850263, 1191209601, 1147878294, 1108955788
 };
 
+static void cake_set_rate(struct cake_tin_data *b, u64 rate, u32 mtu,
+			  u64 target_ns, u64 rtt_est_ns);
 /* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots
  * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
  *
@@ -2003,6 +2011,40 @@ static struct sk_buff *cake_dequeue(stru
 	u64 delay;
 	u32 len;
 
+	if (q->config->is_shared && now - q->last_checked_active >= q->config->sync_time) {
+		struct net_device *dev = qdisc_dev(sch);
+		struct cake_sched_data *other_priv;
+		u64 new_rate = q->config->rate_bps;
+		u64 other_qlen, other_last_active;
+		struct Qdisc *other_sch;
+		u32 num_active_qs = 1;
+		unsigned int ntx;
+
+		for (ntx = 0; ntx < dev->num_tx_queues; ntx++) {
+			other_sch = rcu_dereference(netdev_get_tx_queue(dev, ntx)->qdisc_sleeping);
+			other_priv = qdisc_priv(other_sch);
+
+			if (other_priv == q)
+				continue;
+
+			other_qlen = READ_ONCE(other_sch->q.qlen);
+			other_last_active = READ_ONCE(other_priv->last_active);
+
+			if (other_qlen || other_last_active > q->last_checked_active)
+				num_active_qs++;
+		}
+
+		if (num_active_qs > 1)
+			new_rate = div64_u64(q->config->rate_bps, num_active_qs);
+
+		/* mtu = 0 is used to only update the rate and not mess with cobalt params */
+		cake_set_rate(b, new_rate, 0, 0, 0);
+		q->last_checked_active = now;
+		q->active_queues = num_active_qs;
+		q->rate_ns = b->tin_rate_ns;
+		q->rate_shft = b->tin_rate_shft;
+	}
+
 begin:
 	if (!sch->q.qlen)
 		return NULL;
@@ -2202,6 +2244,7 @@ retry:
 
 	b->tin_ecn_mark += !!flow->cvars.ecn_marked;
 	qdisc_bstats_update(sch, skb);
+	WRITE_ONCE(q->last_active, now);
 
 	/* collect delay stats */
 	delay = ktime_to_ns(ktime_sub(now, cobalt_get_enqueue_time(skb)));
@@ -2302,6 +2345,9 @@ static void cake_set_rate(struct cake_ti
 	b->tin_rate_ns   = rate_ns;
 	b->tin_rate_shft = rate_shft;
 
+	if (mtu == 0)
+		return;
+
 	byte_target_ns = (byte_target * rate_ns) >> rate_shft;
 
 	WRITE_ONCE(b->cparams.target,
@@ -2769,6 +2815,7 @@ static void cake_config_init(struct cake
 			       */
 	q->rate_flags |= CAKE_FLAG_SPLIT_GSO;
 	q->is_shared = is_shared;
+	q->sync_time = 200 * NSEC_PER_USEC;
 }
 
 static int cake_init(struct Qdisc *sch, struct nlattr *opt,
@@ -2840,6 +2887,9 @@ static int cake_init(struct Qdisc *sch,
 	qd->avg_peak_bandwidth = q->rate_bps;
 	qd->min_netlen = ~0;
 	qd->min_adjlen = ~0;
+	qd->active_queues = 0;
+	qd->last_checked_active = 0;
+
 	return 0;
 err:
 	kvfree(qd->config);
@@ -2972,6 +3022,7 @@ static int cake_dump_stats(struct Qdisc
 	PUT_STAT_U32(MAX_ADJLEN, q->max_adjlen);
 	PUT_STAT_U32(MIN_NETLEN, q->min_netlen);
 	PUT_STAT_U32(MIN_ADJLEN, q->min_adjlen);
+	PUT_STAT_U32(ACTIVE_QUEUES, q->active_queues);
 
 #undef PUT_STAT_U32
 #undef PUT_STAT_U64
