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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>
This commit is contained in:
Jonas Köppeler 2026-01-09 14:15:34 +01:00 • committed by Paolo Abeni
parent 87826c0183
commit 1bddd758ba
3 changed files with 55 additions and 0 deletions
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3
Documentation/netlink/specs/tc.yaml
View file

@ -2207,6 +2207,9 @@ attribute-sets:
-
name: blue-timer-us
type: s32
-
name: active-queues
type: u32
-
name: cake-tin-stats-attrs
name-prefix: tca-cake-tin-stats-

1
include/uapi/linux/pkt_sched.h
View file

@ -1036,6 +1036,7 @@ 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)

51
net/sched/sch_cake.c
View file

@ -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_SQRT_CACHE] = {
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)
*
@ -2004,6 +2012,40 @@ static struct sk_buff *cake_dequeue(struct Qdisc *sch)
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;
@ -2203,6 +2245,7 @@ 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)));
@ -2303,6 +2346,9 @@ static void cake_set_rate(struct cake_tin_data *b, u64 rate, u32 mtu,
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;
b->cparams.target = max((byte_target_ns * 3) / 2, target_ns);
@ -2769,6 +2815,7 @@ static void cake_config_init(struct cake_sched_config *q, bool is_shared)
*/
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,
@ -2842,6 +2889,9 @@ static int cake_init(struct Qdisc *sch, struct nlattr *opt,
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);
@ -2974,6 +3024,7 @@ static int cake_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
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