net_sched: drr: warn when qdisc is not work conserving

The DRR scheduler requires that items on the active list are work
conserving, i.e. do not hold on to skbs for throttling purposes, etc.
Attaching e.g. tbf renders DRR useless because all other classes on the
active list are delayed as well.

So, warn users that this configuration won't work as expected; we
already do this in couple of other qdiscs, see e.g.

commit b00355db3f88d96810a60011a30cfb2c3469409d
('pkt_sched: sch_hfsc: sch_htb: Add non-work-conserving warning handler')

The 'const' change is needed to avoid compiler warning ("discards 'const'
qualifier from pointer target type").

tested with:
drr_hier() {
        parent=$1
        classes=$2
        for i in  $(seq 1 $classes); do
                classid=$parent$(printf %x $i)
                tc class add dev eth0 parent $parent classid $classid drr
		tc qdisc add dev eth0 parent $classid tbf rate 64kbit burst 256kbit limit 64kbit
        done
}
tc qdisc add dev eth0 root handle 1: drr
drr_hier 1: 32
tc filter add dev eth0 protocol all pref 1 parent 1: handle 1 flow hash keys dst perturb 1 divisor 32

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/pkt_sched.h

@@ -96,7 +96,7 @@ struct qdisc_rate_table *qdisc_get_rtab(struct tc_ratespec *r,
 					struct nlattr *tab);
 void qdisc_put_rtab(struct qdisc_rate_table *tab);
 void qdisc_put_stab(struct qdisc_size_table *tab);
-void qdisc_warn_nonwc(char *txt, struct Qdisc *qdisc);
+void qdisc_warn_nonwc(const char *txt, struct Qdisc *qdisc);
 int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
 		    struct net_device *dev, struct netdev_queue *txq,
 		    spinlock_t *root_lock);

net/sched/sch_api.c

@@ -563,7 +563,7 @@ out:
 }
 EXPORT_SYMBOL(__qdisc_calculate_pkt_len);
 
-void qdisc_warn_nonwc(char *txt, struct Qdisc *qdisc)
+void qdisc_warn_nonwc(const char *txt, struct Qdisc *qdisc)
 {
 	if (!(qdisc->flags & TCQ_F_WARN_NONWC)) {
 		pr_warn("%s: %s qdisc %X: is non-work-conserving?\n",

net/sched/sch_drr.c

@@ -391,8 +391,10 @@ static struct sk_buff *drr_dequeue(struct Qdisc *sch)
 	while (1) {
 		cl = list_first_entry(&q->active, struct drr_class, alist);
 		skb = cl->qdisc->ops->peek(cl->qdisc);
-		if (skb == NULL)
+		if (skb == NULL) {
+			qdisc_warn_nonwc(__func__, cl->qdisc);
 			goto out;
+		}
 
 		len = qdisc_pkt_len(skb);
 		if (len <= cl->deficit) {