sch_red: Adaptative RED AQM

Adaptative RED AQM for linux, based on paper from Sally FLoyd,
Ramakrishna Gummadi, and Scott Shenker, August 2001 :

http://icir.org/floyd/papers/adaptiveRed.pdf

Goal of Adaptative RED is to make max_p a dynamic value between 1% and
50% to reach the target average queue : (max_th - min_th) / 2

Every 500 ms:
 if (avg > target and max_p <= 0.5)
  increase max_p : max_p += alpha;
 else if (avg < target and max_p >= 0.01)
  decrease max_p : max_p *= beta;

target :[min_th + 0.4*(min_th - max_th),
          min_th + 0.6*(min_th - max_th)].
alpha : min(0.01, max_p / 4)
beta : 0.9
max_P is a Q0.32 fixed point number (unsigned, with 32 bits mantissa)

Changes against our RED implementation are :

max_p is no longer a negative power of two (1/(2^Plog)), but a Q0.32
fixed point number, to allow full range described in Adatative paper.

To deliver a random number, we now use a reciprocal divide (thats really
a multiply), but this operation is done once per marked/droped packet
when in RED_BETWEEN_TRESH window, so added cost (compared to previous
AND operation) is near zero.

dump operation gives current max_p value in a new TCA_RED_MAX_P
attribute.

Example on a 10Mbit link :

tc qdisc add dev $DEV parent 1:1 handle 10: est 1sec 8sec red \
   limit 400000 min 30000 max 90000 avpkt 1000 \
   burst 55 ecn adaptative bandwidth 10Mbit

# tc -s -d qdisc show dev eth3
...
qdisc red 10: parent 1:1 limit 400000b min 30000b max 90000b ecn
adaptative ewma 5 max_p=0.113335 Scell_log 15
 Sent 50414282 bytes 34504 pkt (dropped 35, overlimits 1392 requeues 0)
 rate 9749Kbit 831pps backlog 72056b 16p requeues 0
  marked 1357 early 35 pdrop 0 other 0

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

1 files changed, 84 insertions, 17 deletions

diff --git a/include/net/red.h b/include/net/red.h
index b72a3b833936..24606b22d01e 100644
--- a/include/net/red.h
+++ b/include/net/red.h
@@ -5,6 +5,7 @@
 #include <net/pkt_sched.h>
 #include <net/inet_ecn.h>
 #include <net/dsfield.h>
+#include <linux/reciprocal_div.h>
 
 /*      Random Early Detection (RED) algorithm.
         =======================================
@@ -87,6 +88,29 @@
         etc.
  */
 
+/*
+ * Adaptative RED : An Algorithm for Increasing the Robustness of RED's AQM
+ * (Sally FLoyd, Ramakrishna Gummadi, and Scott Shenker) August 2001
+ *
+ * Every 500 ms:
+ *  if (avg > target and max_p <= 0.5)
+ *   increase max_p : max_p += alpha;
+ *  else if (avg < target and max_p >= 0.01)
+ *   decrease max_p : max_p *= beta;
+ *
+ * target :[qth_min + 0.4*(qth_min - qth_max),
+ *          qth_min + 0.6*(qth_min - qth_max)].
+ * alpha : min(0.01, max_p / 4)
+ * beta : 0.9
+ * max_P is a Q0.32 fixed point number (with 32 bits mantissa)
+ * max_P between 0.01 and 0.5 (1% - 50%) [ Its no longer a negative power of two ]
+ */
+#define RED_ONE_PERCENT ((u32)DIV_ROUND_CLOSEST(1ULL<<32, 100))
+
+#define MAX_P_MIN (1 * RED_ONE_PERCENT)
+#define MAX_P_MAX (50 * RED_ONE_PERCENT)
+#define MAX_P_ALPHA(val) min(MAX_P_MIN, val / 4)
+
 #define RED_STAB_SIZE   256
 #define RED_STAB_MASK   (RED_STAB_SIZE - 1)
 
@@ -101,10 +125,14 @@ struct red_stats {
 
 struct red_parms {
         /* Parameters */
-        u32             qth_min;        /* Min avg length threshold: A scaled */
-        u32             qth_max;        /* Max avg length threshold: A scaled */
+        u32             qth_min;        /* Min avg length threshold: Wlog scaled */
+        u32             qth_max;        /* Max avg length threshold: Wlog scaled */
         u32             Scell_max;
-        u32             Rmask;          /* Cached random mask, see red_rmask */
+        u32             max_P;          /* probability, [0 .. 1.0] 32 scaled */
+        u32             max_P_reciprocal; /* reciprocal_value(max_P / qth_delta) */
+        u32             qth_delta;      /* max_th - min_th */
+        u32             target_min;     /* min_th + 0.4*(max_th - min_th) */
+        u32             target_max;     /* min_th + 0.6*(max_th - min_th) */
         u8              Scell_log;
         u8              Wlog;           /* log(W)               */
         u8              Plog;           /* random number bits   */
@@ -115,19 +143,22 @@ struct red_parms {
                                            number generation */
         u32             qR;             /* Cached random number */
 
-        unsigned long   qavg;           /* Average queue length: A scaled */
+        unsigned long   qavg;           /* Average queue length: Wlog scaled */
         ktime_t         qidlestart;     /* Start of current idle period */
 };
 
-static inline u32 red_rmask(u8 Plog)
+static inline u32 red_maxp(u8 Plog)
 {
-        return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
+        return Plog < 32 ? (~0U >> Plog) : ~0U;
 }
 
+
 static inline void red_set_parms(struct red_parms *p,
                                  u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
                                  u8 Scell_log, u8 *stab)
 {
+        int delta = qth_max - qth_min;
+
         /* Reset average queue length, the value is strictly bound
          * to the parameters below, reseting hurts a bit but leaving
          * it might result in an unreasonable qavg for a while. --TGR
@@ -139,14 +170,29 @@ static inline void red_set_parms(struct red_parms *p,
         p->qth_max      = qth_max << Wlog;
         p->Wlog         = Wlog;
         p->Plog         = Plog;
-        p->Rmask        = red_rmask(Plog);
+        if (delta < 0)
+                delta = 1;
+        p->qth_delta    = delta;
+        p->max_P        = red_maxp(Plog);
+        p->max_P        *= delta; /* max_P = (qth_max-qth_min)/2^Plog */
+
+        p->max_P_reciprocal  = reciprocal_value(p->max_P / delta);
+
+        /* RED Adaptative target :
+         * [min_th + 0.4*(min_th - max_th),
+         *  min_th + 0.6*(min_th - max_th)].
+         */
+        delta /= 5;
+        p->target_min = qth_min + 2*delta;
+        p->target_max = qth_min + 3*delta;
+
         p->Scell_log    = Scell_log;
         p->Scell_max    = (255 << Scell_log);
 
         memcpy(p->Stab, stab, sizeof(p->Stab));
 }
 
-static inline int red_is_idling(struct red_parms *p)
+static inline int red_is_idling(const struct red_parms *p)
 {
         return p->qidlestart.tv64 != 0;
 }
@@ -168,7 +214,7 @@ static inline void red_restart(struct red_parms *p)
         p->qcount = -1;
 }
 
-static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
+static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p)
 {
         s64 delta = ktime_us_delta(ktime_get(), p->qidlestart);
         long us_idle = min_t(s64, delta, p->Scell_max);
@@ -215,7 +261,7 @@ static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
         }
 }
 
-static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
+static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p,
                                                        unsigned int backlog)
 {
         /*
@@ -230,7 +276,7 @@ static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
         return p->qavg + (backlog - (p->qavg >> p->Wlog));
 }
 
-static inline unsigned long red_calc_qavg(struct red_parms *p,
+static inline unsigned long red_calc_qavg(const struct red_parms *p,
                                           unsigned int backlog)
 {
         if (!red_is_idling(p))
@@ -239,23 +285,24 @@ static inline unsigned long red_calc_qavg(struct red_parms *p,
                 return red_calc_qavg_from_idle_time(p);
 }
 
-static inline u32 red_random(struct red_parms *p)
+
+static inline u32 red_random(const struct red_parms *p)
 {
-        return net_random() & p->Rmask;
+        return reciprocal_divide(net_random(), p->max_P_reciprocal);
 }
 
-static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
+static inline int red_mark_probability(const struct red_parms *p, unsigned long qavg)
 {
         /* The formula used below causes questions.
 
-           OK. qR is random number in the interval 0..Rmask
+           OK. qR is random number in the interval
+                (0..1/max_P)*(qth_max-qth_min)
            i.e. 0..(2^Plog). If we used floating point
            arithmetics, it would be: (2^Plog)*rnd_num,
            where rnd_num is less 1.
 
            Taking into account, that qavg have fixed
-           point at Wlog, and Plog is related to max_P by
-           max_P = (qth_max-qth_min)/2^Plog; two lines
+           point at Wlog, two lines
            below have the following floating point equivalent:
 
            max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
@@ -315,4 +362,24 @@ static inline int red_action(struct red_parms *p, unsigned long qavg)
         return RED_DONT_MARK;
 }
 
+static inline void red_adaptative_algo(struct red_parms *p)
+{
+        unsigned long qavg;
+        u32 max_p_delta;
+
+        qavg = p->qavg;
+        if (red_is_idling(p))
+                qavg = red_calc_qavg_from_idle_time(p);
+
+        /* p->qavg is fixed point number with point at Wlog */
+        qavg >>= p->Wlog;
+
+        if (qavg > p->target_max && p->max_P <= MAX_P_MAX)
+                p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */
+        else if (qavg < p->target_min && p->max_P >= MAX_P_MIN)
+                p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */
+
+        max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
+        p->max_P_reciprocal = reciprocal_value(max_p_delta);
+}
 #endif