net: Avoid enqueuing skb for default qdiscs

dev_queue_xmit enqueue's a skb and calls qdisc_run which
dequeue's the skb and xmits it. In most cases, the skb that
is enqueue'd is the same one that is dequeue'd (unless the
queue gets stopped or multiple cpu's write to the same queue
and ends in a race with qdisc_run). For default qdiscs, we
can remove the redundant enqueue/dequeue and simply xmit the
skb since the default qdisc is work-conserving.

The patch uses a new flag - TCQ_F_CAN_BYPASS to identify the
default fast queue. The controversial part of the patch is
incrementing qlen when a skb is requeued - this is to avoid
checks like the second line below:

+  } else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
>>         !q->gso_skb &&
+          !test_and_set_bit(__QDISC_STATE_RUNNING, &q->state)) {

Results of a 2 hour testing for multiple netperf sessions (1,
2, 4, 8, 12 sessions on a 4 cpu system-X). The BW numbers are
aggregate Mb/s across iterations tested with this version on
System-X boxes with Chelsio 10gbps cards:

----------------------------------
Size |  ORG BW          NEW BW   |
----------------------------------
128K |  156964          159381   |
256K |  158650          162042   |
----------------------------------

Changes from ver1:

1. Move sch_direct_xmit declaration from sch_generic.h to
   pkt_sched.h
2. Update qdisc basic statistics for direct xmit path.
3. Set qlen to zero in qdisc_reset.
4. Changed some function names to more meaningful ones.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 2 deletions

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 964ffa0d8815..84b3fc2aef0f 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -45,6 +45,7 @@ struct Qdisc
 #define TCQ_F_BUILTIN           1
 #define TCQ_F_THROTTLED         2
 #define TCQ_F_INGRESS           4
+#define TCQ_F_CAN_BYPASS        8
 #define TCQ_F_WARN_NONWC        (1 << 16)
         int                     padded;
         struct Qdisc_ops        *ops;
@@ -182,6 +183,11 @@ struct qdisc_skb_cb {
         char                    data[];
 };
 
+static inline int qdisc_qlen(struct Qdisc *q)
+{
+        return q->q.qlen;
+}
+
 static inline struct qdisc_skb_cb *qdisc_skb_cb(struct sk_buff *skb)
 {
         return (struct qdisc_skb_cb *)skb->cb;
@@ -387,13 +393,18 @@ static inline int qdisc_enqueue_root(struct sk_buff *skb, struct Qdisc *sch)
         return qdisc_enqueue(skb, sch) & NET_XMIT_MASK;
 }
 
+static inline void __qdisc_update_bstats(struct Qdisc *sch, unsigned int len)
+{
+        sch->bstats.bytes += len;
+        sch->bstats.packets++;
+}
+
 static inline int __qdisc_enqueue_tail(struct sk_buff *skb, struct Qdisc *sch,
                                        struct sk_buff_head *list)
 {
         __skb_queue_tail(list, skb);
         sch->qstats.backlog += qdisc_pkt_len(skb);
-        sch->bstats.bytes += qdisc_pkt_len(skb);
-        sch->bstats.packets++;
+        __qdisc_update_bstats(sch, qdisc_pkt_len(skb));
 
         return NET_XMIT_SUCCESS;
 }