fq_codel: fix NET_XMIT_CN behavior

My prior attempt to fix the backlogs of parents failed.

If we return NET_XMIT_CN, our parents wont increase their backlog,
so our qdisc_tree_reduce_backlog() should take this into account.

v2: Florian Westphal pointed out that we could drop the packet,
so we need to save qdisc_pkt_len(skb) in a temp variable before
calling fq_codel_drop()

Fixes: 9d18562a2278 ("fq_codel: add batch ability to fq_codel_drop()")
Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Reported-by: Stas Nichiporovich <stasn77@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 6 deletions

diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index 6883a8971562..fff7867f4a4f 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -199,6 +199,7 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)
         unsigned int idx, prev_backlog, prev_qlen;
         struct fq_codel_flow *flow;
         int uninitialized_var(ret);
+        unsigned int pkt_len;
         bool memory_limited;
 
         idx = fq_codel_classify(skb, sch, &ret);
@@ -230,6 +231,8 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)
         prev_backlog = sch->qstats.backlog;
         prev_qlen = sch->q.qlen;
 
+        /* save this packet length as it might be dropped by fq_codel_drop() */
+        pkt_len = qdisc_pkt_len(skb);
         /* fq_codel_drop() is quite expensive, as it performs a linear search
          * in q->backlogs[] to find a fat flow.
          * So instead of dropping a single packet, drop half of its backlog
@@ -237,14 +240,23 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)
          */
         ret = fq_codel_drop(sch, q->drop_batch_size);
 
-        q->drop_overlimit += prev_qlen - sch->q.qlen;
+        prev_qlen -= sch->q.qlen;
+        prev_backlog -= sch->qstats.backlog;
+        q->drop_overlimit += prev_qlen;
         if (memory_limited)
-                q->drop_overmemory += prev_qlen - sch->q.qlen;
-        /* As we dropped packet(s), better let upper stack know this */
-        qdisc_tree_reduce_backlog(sch, prev_qlen - sch->q.qlen,
-                                  prev_backlog - sch->qstats.backlog);
+                q->drop_overmemory += prev_qlen;
 
-        return ret == idx ? NET_XMIT_CN : NET_XMIT_SUCCESS;
+        /* As we dropped packet(s), better let upper stack know this.
+         * If we dropped a packet for this flow, return NET_XMIT_CN,
+         * but in this case, our parents wont increase their backlogs.
+         */
+        if (ret == idx) {
+                qdisc_tree_reduce_backlog(sch, prev_qlen - 1,
+                                          prev_backlog - pkt_len);
+                return NET_XMIT_CN;
+        }
+        qdisc_tree_reduce_backlog(sch, prev_qlen, prev_backlog);
+        return NET_XMIT_SUCCESS;
 }
 
 /* This is the specific function called from codel_dequeue()