[PKT_SCHED]: Generic RED layer

Extracts the RED algorithm from sch_red.c and puts it into include/net/red.h
for use by other RED based modules. The statistics are extended to be more
fine grained in order to differ between probability/forced marks/drops.
We now reset the average queue length when setting new parameters, leaving
it might result in an unreasonable qavg for a while depending on the value of W.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 325 insertions, 0 deletions

diff --git a/include/net/red.h b/include/net/red.h
new file mode 100644
index 000000000000..2ed4358e3295
--- /dev/null
+++ b/include/net/red.h
@@ -0,0 +1,325 @@
+#ifndef __NET_SCHED_RED_H
+#define __NET_SCHED_RED_H
+
+#include <linux/config.h>
+#include <linux/types.h>
+#include <net/pkt_sched.h>
+#include <net/inet_ecn.h>
+#include <net/dsfield.h>
+
+/*      Random Early Detection (RED) algorithm.
+        =======================================
+
+        Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
+        for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
+
+        This file codes a "divisionless" version of RED algorithm
+        as written down in Fig.17 of the paper.
+
+        Short description.
+        ------------------
+
+        When a new packet arrives we calculate the average queue length:
+
+        avg = (1-W)*avg + W*current_queue_len,
+
+        W is the filter time constant (chosen as 2^(-Wlog)), it controls
+        the inertia of the algorithm. To allow larger bursts, W should be
+        decreased.
+
+        if (avg > th_max) -> packet marked (dropped).
+        if (avg < th_min) -> packet passes.
+        if (th_min < avg < th_max) we calculate probability:
+
+        Pb = max_P * (avg - th_min)/(th_max-th_min)
+
+        and mark (drop) packet with this probability.
+        Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
+        max_P should be small (not 1), usually 0.01..0.02 is good value.
+
+        max_P is chosen as a number, so that max_P/(th_max-th_min)
+        is a negative power of two in order arithmetics to contain
+        only shifts.
+
+
+        Parameters, settable by user:
+        -----------------------------
+
+        qth_min         - bytes (should be < qth_max/2)
+        qth_max         - bytes (should be at least 2*qth_min and less limit)
+        Wlog            - bits (<32) log(1/W).
+        Plog            - bits (<32)
+
+        Plog is related to max_P by formula:
+
+        max_P = (qth_max-qth_min)/2^Plog;
+
+        F.e. if qth_max=128K and qth_min=32K, then Plog=22
+        corresponds to max_P=0.02
+
+        Scell_log
+        Stab
+
+        Lookup table for log((1-W)^(t/t_ave).
+
+
+        NOTES:
+
+        Upper bound on W.
+        -----------------
+
+        If you want to allow bursts of L packets of size S,
+        you should choose W:
+
+        L + 1 - th_min/S < (1-(1-W)^L)/W
+
+        th_min/S = 32         th_min/S = 4
+
+        log(W)  L
+        -1      33
+        -2      35
+        -3      39
+        -4      46
+        -5      57
+        -6      75
+        -7      101
+        -8      135
+        -9      190
+        etc.
+ */
+
+#define RED_STAB_SIZE   256
+#define RED_STAB_MASK   (RED_STAB_SIZE - 1)
+
+struct red_stats
+{
+        u32             prob_drop;      /* Early probability drops */
+        u32             prob_mark;      /* Early probability marks */
+        u32             forced_drop;    /* Forced drops, qavg > max_thresh */
+        u32             forced_mark;    /* Forced marks, qavg > max_thresh */
+        u32             pdrop;          /* Drops due to queue limits */
+        u32             other;          /* Drops due to drop() calls */
+        u32             backlog;
+};
+
+struct red_parms
+{
+        /* Parameters */
+        u32             qth_min;        /* Min avg length threshold: A scaled */
+        u32             qth_max;        /* Max avg length threshold: A scaled */
+        u32             Scell_max;
+        u32             Rmask;          /* Cached random mask, see red_rmask */
+        u8              Scell_log;
+        u8              Wlog;           /* log(W)               */
+        u8              Plog;           /* random number bits   */
+        u8              Stab[RED_STAB_SIZE];
+
+        /* Variables */
+        int             qcount;         /* Number of packets since last random
+                                           number generation */
+        u32             qR;             /* Cached random number */
+
+        unsigned long   qavg;           /* Average queue length: A scaled */
+        psched_time_t   qidlestart;     /* Start of current idle period */
+};
+
+static inline u32 red_rmask(u8 Plog)
+{
+        return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
+}
+
+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)
+{
+        /* 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
+         */
+        p->qavg         = 0;
+
+        p->qcount       = -1;
+        p->qth_min      = qth_min << Wlog;
+        p->qth_max      = qth_max << Wlog;
+        p->Wlog         = Wlog;
+        p->Plog         = Plog;
+        p->Rmask        = red_rmask(Plog);
+        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)
+{
+        return !PSCHED_IS_PASTPERFECT(p->qidlestart);
+}
+
+static inline void red_start_of_idle_period(struct red_parms *p)
+{
+        PSCHED_GET_TIME(p->qidlestart);
+}
+
+static inline void red_end_of_idle_period(struct red_parms *p)
+{
+        PSCHED_SET_PASTPERFECT(p->qidlestart);
+}
+
+static inline void red_restart(struct red_parms *p)
+{
+        red_end_of_idle_period(p);
+        p->qavg = 0;
+        p->qcount = -1;
+}
+
+static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
+{
+        psched_time_t now;
+        long us_idle;
+        int  shift;
+
+        PSCHED_GET_TIME(now);
+        us_idle = PSCHED_TDIFF_SAFE(now, p->qidlestart, p->Scell_max);
+
+        /*
+         * The problem: ideally, average length queue recalcultion should
+         * be done over constant clock intervals. This is too expensive, so
+         * that the calculation is driven by outgoing packets.
+         * When the queue is idle we have to model this clock by hand.
+         *
+         * SF+VJ proposed to "generate":
+         *
+         *      m = idletime / (average_pkt_size / bandwidth)
+         *
+         * dummy packets as a burst after idle time, i.e.
+         *
+         *      p->qavg *= (1-W)^m
+         *
+         * This is an apparently overcomplicated solution (f.e. we have to
+         * precompute a table to make this calculation in reasonable time)
+         * I believe that a simpler model may be used here,
+         * but it is field for experiments.
+         */
+
+        shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
+
+        if (shift)
+                return p->qavg >> shift;
+        else {
+                /* Approximate initial part of exponent with linear function:
+                 *
+                 *      (1-W)^m ~= 1-mW + ...
+                 *
+                 * Seems, it is the best solution to
+                 * problem of too coarse exponent tabulation.
+                 */
+                us_idle = (p->qavg * us_idle) >> p->Scell_log;
+
+                if (us_idle < (p->qavg >> 1))
+                        return p->qavg - us_idle;
+                else
+                        return p->qavg >> 1;
+        }
+}
+
+static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
+                                                       unsigned int backlog)
+{
+        /*
+         * NOTE: p->qavg is fixed point number with point at Wlog.
+         * The formula below is equvalent to floating point
+         * version:
+         *
+         *      qavg = qavg*(1-W) + backlog*W;
+         *
+         * --ANK (980924)
+         */
+        return p->qavg + (backlog - (p->qavg >> p->Wlog));
+}
+
+static inline unsigned long red_calc_qavg(struct red_parms *p,
+                                          unsigned int backlog)
+{
+        if (!red_is_idling(p))
+                return red_calc_qavg_no_idle_time(p, backlog);
+        else
+                return red_calc_qavg_from_idle_time(p);
+}
+
+static inline u32 red_random(struct red_parms *p)
+{
+        return net_random() & p->Rmask;
+}
+
+static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
+{
+        /* The formula used below causes questions.
+
+           OK. qR is random number in the interval 0..Rmask
+           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
+           below have the following floating point equivalent:
+
+           max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
+
+           Any questions? --ANK (980924)
+         */
+        return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
+}
+
+enum {
+        RED_BELOW_MIN_THRESH,
+        RED_BETWEEN_TRESH,
+        RED_ABOVE_MAX_TRESH,
+};
+
+static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
+{
+        if (qavg < p->qth_min)
+                return RED_BELOW_MIN_THRESH;
+        else if (qavg >= p->qth_max)
+                return RED_ABOVE_MAX_TRESH;
+        else
+                return RED_BETWEEN_TRESH;
+}
+
+enum {
+        RED_DONT_MARK,
+        RED_PROB_MARK,
+        RED_HARD_MARK,
+};
+
+static inline int red_action(struct red_parms *p, unsigned long qavg)
+{
+        switch (red_cmp_thresh(p, qavg)) {
+                case RED_BELOW_MIN_THRESH:
+                        p->qcount = -1;
+                        return RED_DONT_MARK;
+
+                case RED_BETWEEN_TRESH:
+                        if (++p->qcount) {
+                                if (red_mark_probability(p, qavg)) {
+                                        p->qcount = 0;
+                                        p->qR = red_random(p);
+                                        return RED_PROB_MARK;
+                                }
+                        } else
+                                p->qR = red_random(p);
+
+                        return RED_DONT_MARK;
+
+                case RED_ABOVE_MAX_TRESH:
+                        p->qcount = -1;
+                        return RED_HARD_MARK;
+        }
+
+        BUG();
+        return RED_DONT_MARK;
+}
+
+#endif