During Netfilter Workshop 2016 Eric Dumazet pointed out that qdisc schedulers use doubly-linked lists, even though single-linked list would be enough.
The double-linked skb lists incur one extra write on enqueue/dequeue operations (to change ->prev pointer of next list elem). This series converts qdiscs to single-linked version, listhead maintains pointers to first (for dequeue) and last skb (for enqueue). Most qdiscs don't queue at all and instead use a leaf qdisc (typically pfifo_fast) so only a few schedulers needed changes. I briefly tested netem and htb and they seemed fine. UDP_STREAM netperf with 64 byte packets via veth+pfifo_fast shows a small (~2%) improvement. Florian Westphal (5): pie: use qdisc_dequeue_head wrapper sched: don't use skb queue helpers sched: remove qdisc arg from __qdisc_dequeue_head sched: replace __skb_dequeue with __qdisc_dequeue_head sched: add and use qdisc_skb_head helpers include/net/sch_generic.h | 72 +++++++++++++++++++++++++++++++++++----------- net/sched/sch_codel.c | 4 +- net/sched/sch_fifo.c | 4 +- net/sched/sch_generic.c | 30 +++++++++++-------- net/sched/sch_htb.c | 24 ++++++++++++--- net/sched/sch_netem.c | 20 +++++++++--- net/sched/sch_pie.c | 4 +- 7 files changed, 115 insertions(+), 43 deletions(-)