sch_cake: don't call diffserv parsing code when it is not needed

As a further optimisation of the diffserv parsing codepath, we can skip it
entirely if CAKE is configured to neither use diffserv-based
classification, nor to zero out the diffserv bits.

Fixes: c87b4ecd ("sch_cake: Make sure we can write the IP header before changing DSCP bits")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_cake.c

@@ -1551,7 +1551,7 @@ static unsigned int cake_drop(struct Qdisc *sch, struct sk_buff **to_free)
 	return idx + (tin << 16);
 }
 
-static u8 cake_handle_diffserv(struct sk_buff *skb, u16 wash)
+static u8 cake_handle_diffserv(struct sk_buff *skb, bool wash)
 {
 	const int offset = skb_network_offset(skb);
 	u16 *buf, buf_;
@@ -1612,14 +1612,17 @@ static struct cake_tin_data *cake_select_tin(struct Qdisc *sch,
 {
 	struct cake_sched_data *q = qdisc_priv(sch);
 	u32 tin, mark;
+	bool wash;
 	u8 dscp;
 
 	/* Tin selection: Default to diffserv-based selection, allow overriding
-	 * using firewall marks or skb->priority.
+	 * using firewall marks or skb->priority. Call DSCP parsing early if
+	 * wash is enabled, otherwise defer to below to skip unneeded parsing.
 	 */
-	dscp = cake_handle_diffserv(skb,
-				    q->rate_flags & CAKE_FLAG_WASH);
 	mark = (skb->mark & q->fwmark_mask) >> q->fwmark_shft;
+	wash = !!(q->rate_flags & CAKE_FLAG_WASH);
+	if (wash)
+		dscp = cake_handle_diffserv(skb, wash);
 
 	if (q->tin_mode == CAKE_DIFFSERV_BESTEFFORT)
 		tin = 0;
@@ -1633,6 +1636,8 @@ static struct cake_tin_data *cake_select_tin(struct Qdisc *sch,
 		tin = q->tin_order[TC_H_MIN(skb->priority) - 1];
 
 	else {
+		if (!wash)
+			dscp = cake_handle_diffserv(skb, wash);
 		tin = q->tin_index[dscp];
 
 		if (unlikely(tin >= q->tin_cnt))