Skip to content


Browse files Browse the repository at this point in the history
net: dev: introduce support for sch BYPASS for lockless qdisc
With commit c5ad119 ("net: sched: pfifo_fast use skb_array")
pfifo_fast no longer benefit from the TCQ_F_CAN_BYPASS optimization.
Due to retpolines the cost of the enqueue()/dequeue() pair has become
relevant and we observe measurable regression for the uncontended
scenario when the packet-rate is below line rate.

After commit 46b1c18 ("net: sched: put back q.qlen into a
single location") we can check for empty qdisc with a reasonably
fast operation even for nolock qdiscs.

This change extends TCQ_F_CAN_BYPASS support to nolock qdisc.
The new chunk of code mirrors closely the existing one for traditional
qdisc, leveraging a newly introduced helper to read atomically the
qdisc length.

Tested with pktgen in queue xmit mode, with pfifo_fast, a MQ
device, and MQ root qdisc:

threads         vanilla         patched
                kpps            kpps
1               2465            2889
2               4304            5188
4               7898            9589

Same as above, but with a single queue device:

threads         vanilla         patched
                kpps            kpps
1               2556            2827
2               2900            2900
4               5000            5000
8               4700            4700

No mesaurable changes in the contended scenarios, and more 10%
improvement in the uncontended ones.

 v1 -> v2:
  - rebased after flag name change

Signed-off-by: Paolo Abeni <>
Tested-by: Ivan Vecera <>
Reviewed-by: Eric Dumazet <>
Reviewed-by: Ivan Vecera <>
Signed-off-by: David S. Miller <>
  • Loading branch information
Paolo Abeni authored and davem330 committed Mar 24, 2019
1 parent 28cff53 commit ba27b4c
Showing 1 changed file with 9 additions and 0 deletions.
9 changes: 9 additions & 0 deletions net/core/dev.c
Expand Up @@ -3468,6 +3468,15 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
__qdisc_drop(skb, &to_free);
} else if ((q->flags & TCQ_F_CAN_BYPASS) && q->empty &&
qdisc_run_begin(q)) {
qdisc_bstats_cpu_update(q, skb);

if (sch_direct_xmit(skb, q, dev, txq, NULL, true))

} else {
rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
Expand Down

0 comments on commit ba27b4c

Please sign in to comment.