[PKT_SCHED]: Reduce branch mispredictions in pfifo_fast_dequeue

The current call to __qdisc_dequeue_head leads to a branch
misprediction for every loop iteration, the fact that the
most common priority is 2 makes this even worse.  This issue
has been brought up by Eric Dumazet <dada1@cosmosbay.com>
but unlike his solution which was to manually unroll the loop,
this approach preserves the possibility to increase the number
of bands at compile time.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 73e218e646acea7a0c998831cf1e8564dd42ef29..8edefd5d095d5019bae647ccc845314de65c5264 100644 (file)
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -331,11 +331,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc* qdisc)
        int prio;
        struct sk_buff_head *list = qdisc_priv(qdisc);
 
-       for (prio = 0; prio < PFIFO_FAST_BANDS; prio++, list++) {
-               struct sk_buff *skb = __qdisc_dequeue_head(qdisc, list);
-               if (skb) {
+       for (prio = 0; prio < PFIFO_FAST_BANDS; prio++) {
+               if (!skb_queue_empty(list + prio)) {
                        qdisc->q.qlen--;
-                       return skb;
+                       return __qdisc_dequeue_head(qdisc, list + prio);
                }
        }