cbq: incorrectly low bandwidth setting blocks limited traffic

Mainstream commit f0f6ee1f70c4 ("cbq: incorrect processing of high limits")
have side effect: if cbq bandwidth setting is less than real interface
throughput non-limited traffic can delay limited traffic for a very long time.

This happen because of q->now changes incorrectly in cbq_dequeue():
in described scenario L2T is much greater than real time delay,
and q->now gets an extra boost for each transmitted packet.

Accumulated boost prevents update q->now, and blocked class can wait
very long time until (q->now >= cl->undertime) will be true again.

To fix the problem the patch updates q->now on each cbq_update() call.
L2T-related pre-modification q->now was moved to cbq_update().

My testing confirmed that it fixes the problem and did not discover
any side-effects

Fixes: f0f6ee1f70c4 ("cbq: incorrect processing of high limits")

Signed-off-by: Vasily Averin <vvs@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 24 deletions

diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
index ead526467cca..550be9504fff 100644
--- a/net/sched/sch_cbq.c
+++ b/net/sched/sch_cbq.c
@@ -700,8 +700,13 @@ cbq_update(struct cbq_sched_data *q)
         struct cbq_class *this = q->tx_class;
         struct cbq_class *cl = this;
         int len = q->tx_len;
+        psched_time_t now;
 
         q->tx_class = NULL;
+        /* Time integrator. We calculate EOS time
+         * by adding expected packet transmission time.
+         */
+        now = q->now + L2T(&q->link, len);
 
         for ( ; cl; cl = cl->share) {
                 long avgidle = cl->avgidle;
@@ -717,7 +722,7 @@ cbq_update(struct cbq_sched_data *q)
                  *      idle = (now - last) - last_pktlen/rate
                  */
 
-                idle = q->now - cl->last;
+                idle = now - cl->last;
                 if ((unsigned long)idle > 128*1024*1024) {
                         avgidle = cl->maxidle;
                 } else {
@@ -761,7 +766,7 @@ cbq_update(struct cbq_sched_data *q)
                         idle -= L2T(&q->link, len);
                         idle += L2T(cl, len);
 
-                        cl->undertime = q->now + idle;
+                        cl->undertime = now + idle;
                 } else {
                         /* Underlimit */
 
@@ -771,7 +776,8 @@ cbq_update(struct cbq_sched_data *q)
                         else
                                 cl->avgidle = avgidle;
                 }
-                cl->last = q->now;
+                if ((s64)(now - cl->last) > 0)
+                        cl->last = now;
         }
 
         cbq_update_toplevel(q, this, q->tx_borrowed);
@@ -943,30 +949,13 @@ cbq_dequeue(struct Qdisc *sch)
         struct sk_buff *skb;
         struct cbq_sched_data *q = qdisc_priv(sch);
         psched_time_t now;
-        psched_tdiff_t incr;
 
         now = psched_get_time();
-        incr = now - q->now_rt;
-
-        if (q->tx_class) {
-                psched_tdiff_t incr2;
-                /* Time integrator. We calculate EOS time
-                 * by adding expected packet transmission time.
-                 * If real time is greater, we warp artificial clock,
-                 * so that:
-                 *
-                 * cbq_time = max(real_time, work);
-                 */
-                incr2 = L2T(&q->link, q->tx_len);
-                q->now += incr2;
+
+        if (q->tx_class)
                 cbq_update(q);
-                if ((incr -= incr2) < 0)
-                        incr = 0;
-                q->now += incr;
-        } else {
-                if (now > q->now)
-                        q->now = now;
-        }
+
+        q->now = now;
         q->now_rt = now;
 
         for (;;) {