sch_red: generalize accurate MAX_P support to RED/GRED/CHOKE

Now RED uses a Q0.32 number to store max_p (max probability), allow
RED/GRED/CHOKE to use/report full resolution at config/dump time.

Old tc binaries are non aware of new attributes, and still set/get Plog.

New tc binary set/get both Plog and max_p for backward compatibility,
they display "probability value" if they get max_p from new kernels.

# tc -d  qdisc show dev ...
...
qdisc red 10: parent 1:1 limit 360Kb min 30Kb max 90Kb ecn ewma 5
probability 0.09 Scell_log 15

Make sure we avoid potential divides by 0 in reciprocal_value(), if
(max_th - min_th) is big.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 2 deletions

diff --git a/net/sched/sch_red.c b/net/sched/sch_red.c
index 8f5a85bf9d10..ce2256a17d7e 100644
--- a/net/sched/sch_red.c
+++ b/net/sched/sch_red.c
@@ -170,6 +170,7 @@ static void red_destroy(struct Qdisc *sch)
 static const struct nla_policy red_policy[TCA_RED_MAX + 1] = {
         [TCA_RED_PARMS] = { .len = sizeof(struct tc_red_qopt) },
         [TCA_RED_STAB]  = { .len = RED_STAB_SIZE },
+        [TCA_RED_MAX_P] = { .type = NLA_U32 },
 };
 
 static int red_change(struct Qdisc *sch, struct nlattr *opt)
@@ -179,6 +180,7 @@ static int red_change(struct Qdisc *sch, struct nlattr *opt)
         struct tc_red_qopt *ctl;
         struct Qdisc *child = NULL;
         int err;
+        u32 max_P;
 
         if (opt == NULL)
                 return -EINVAL;
@@ -191,6 +193,8 @@ static int red_change(struct Qdisc *sch, struct nlattr *opt)
             tb[TCA_RED_STAB] == NULL)
                 return -EINVAL;
 
+        max_P = tb[TCA_RED_MAX_P] ? nla_get_u32(tb[TCA_RED_MAX_P]) : 0;
+
         ctl = nla_data(tb[TCA_RED_PARMS]);
 
         if (ctl->limit > 0) {
@@ -209,8 +213,9 @@ static int red_change(struct Qdisc *sch, struct nlattr *opt)
         }
 
         red_set_parms(&q->parms, ctl->qth_min, ctl->qth_max, ctl->Wlog,
-                                 ctl->Plog, ctl->Scell_log,
-                                 nla_data(tb[TCA_RED_STAB]));
+                      ctl->Plog, ctl->Scell_log,
+                      nla_data(tb[TCA_RED_STAB]),
+                      max_P);
 
         del_timer(&q->adapt_timer);
         if (ctl->flags & TC_RED_ADAPTATIVE)