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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>