1 files changed, 139 insertions, 83 deletions

diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index 2601012383fb..ddac2c511e2f 100644
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -18,10 +18,79 @@
 #include "tfrc.h"
 
 #define TFRC_CALC_X_ARRSIZE 500
+#define TFRC_CALC_X_SPLIT   50000       /* 0.05 * 1000000, details below */
+#define TFRC_SMALLEST_P     (TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE)
 
-#define TFRC_CALC_X_SPLIT 50000
-/* equivalent to 0.05 */
-
+/*
+  TFRC TCP Reno Throughput Equation Lookup Table for f(p)
+
+  The following two-column lookup table implements a part of the TCP throughput
+  equation from [RFC 3448, sec. 3.1]:
+
+                                     s
+  X_calc  =  --------------------------------------------------------------
+             R * sqrt(2*b*p/3) + (3 * t_RTO * sqrt(3*b*p/8) * (p + 32*p^3))
+
+  Where:
+        X      is the transmit rate in bytes/second
+        s      is the packet size in bytes
+        R      is the round trip time in seconds
+        p      is the loss event rate, between 0 and 1.0, of the number of loss
+                      events as a fraction of the number of packets transmitted
+        t_RTO  is the TCP retransmission timeout value in seconds
+        b      is the number of packets acknowledged by a single TCP ACK
+
+  We can assume that b = 1 and t_RTO is 4 * R. The equation now becomes:
+
+                                     s
+  X_calc  =  -------------------------------------------------------
+             R * sqrt(p*2/3) + (12 * R * sqrt(p*3/8) * (p + 32*p^3))
+
+  which we can break down into:
+
+                      s
+        X_calc  =  ---------
+                    R * f(p)
+
+  where f(p) is given for 0 < p <= 1 by:
+
+        f(p)  =  sqrt(2*p/3) + 12 * sqrt(3*p/8) *  (p + 32*p^3)
+
+  Since this is kernel code, floating-point arithmetic is avoided in favour of
+  integer arithmetic. This means that nearly all fractional parameters are
+  scaled by 1000000:
+    * the parameters p and R
+    * the return result f(p)
+  The lookup table therefore actually tabulates the following function g(q):
+
+        g(q)  =  1000000 * f(q/1000000)
+
+  Hence, when p <= 1, q must be less than or equal to 1000000. To achieve finer
+  granularity for the practically more relevant case of small values of p (up to
+  5%), the second column is used; the first one ranges up to 100%.  This split
+  corresponds to the value of q = TFRC_CALC_X_SPLIT. At the same time this also
+  determines the smallest resolution possible with this lookup table:
+
+    TFRC_SMALLEST_P   =  TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE
+
+  The entire table is generated by:
+    for(i=0; i < TFRC_CALC_X_ARRSIZE; i++) {
+        lookup[i][0]  =  g((i+1) * 1000000/TFRC_CALC_X_ARRSIZE);
+        lookup[i][1]  =  g((i+1) * TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE);
+    }
+
+  With the given configuration, we have, with M = TFRC_CALC_X_ARRSIZE-1,
+    lookup[0][0]  =  g(1000000/(M+1))            =  1000000 * f(0.2%)
+    lookup[M][0]  =  g(1000000)                  =  1000000 * f(100%)
+    lookup[0][1]  =  g(TFRC_SMALLEST_P)           = 1000000 * f(0.01%)
+    lookup[M][1]  =  g(TFRC_CALC_X_SPLIT)        =  1000000 * f(5%)
+
+  In summary, the two columns represent f(p) for the following ranges:
+    * The first column is for   0.002  <= p <= 1.0
+    * The second column is for  0.0001 <= p <= 0.05
+  Where the columns overlap, the second (finer-grained) is given preference,
+  i.e. the first column is used only for p >= 0.05.
+ */
 static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
         {     37172,   8172 },
         {     53499,  11567 },
@@ -525,85 +594,69 @@ static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
         { 243315981, 271305 }
 };
 
-/* Calculate the send rate as per section 3.1 of RFC3448
- 
-Returns send rate in bytes per second
-
-Integer maths and lookups are used as not allowed floating point in kernel
-
-The function for Xcalc as per section 3.1 of RFC3448 is:
-
-X =                            s
-     -------------------------------------------------------------
-     R*sqrt(2*b*p/3) + (t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p^2)))
-
-where 
-X is the trasmit rate in bytes/second
-s is the packet size in bytes
-R is the round trip time in seconds
-p is the loss event rate, between 0 and 1.0, of the number of loss events 
-  as a fraction of the number of packets transmitted
-t_RTO is the TCP retransmission timeout value in seconds
-b is the number of packets acknowledged by a single TCP acknowledgement
-
-we can assume that b = 1 and t_RTO is 4 * R. With this the equation becomes:
-
-X =                            s
-     -----------------------------------------------------------------------
-     R * sqrt(2 * p / 3) + (12 * R * (sqrt(3 * p / 8) * p * (1 + 32 * p^2)))
-
-
-which we can break down into:
-
-X =     s
-     --------
-     R * f(p)
-
-where f(p) = sqrt(2 * p / 3) + (12 * sqrt(3 * p / 8) * p * (1 + 32 * p * p))
-
-Function parameters:
-s - bytes
-R - RTT in usecs
-p - loss rate (decimal fraction multiplied by 1,000,000)
-
-Returns Xcalc in bytes per second
-
-DON'T alter this code unless you run test cases against it as the code
-has been manipulated to stop underflow/overlow.
+/* return largest index i such that fval <= lookup[i][small] */
+static inline u32 tfrc_binsearch(u32 fval, u8 small)
+{
+        u32 try, low = 0, high = TFRC_CALC_X_ARRSIZE - 1;
+
+        while (low < high) {
+                try = (low + high) / 2;
+                if (fval <= tfrc_calc_x_lookup[try][small])
+                        high = try;
+                else
+                        low  = try + 1;
+        }
+        return high;
+}
 
-*/
+/**
+ * tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448
+ *
+ *  @s: packet size          in bytes
+ *  @R: RTT                  scaled by 1000000   (i.e., microseconds)
+ *  @p: loss ratio estimate  scaled by 1000000
+ *  Returns X_calc           in bytes per second (not scaled).
+ *
+ * Note: DO NOT alter this code unless you run test cases against it,
+ *       as the code has been optimized to stop underflow/overflow.
+ */
 u32 tfrc_calc_x(u16 s, u32 R, u32 p)
 {
         int index;
         u32 f;
         u64 tmp1, tmp2;
 
-        if (p < TFRC_CALC_X_SPLIT)
-                index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;
-        else
-                index = (p / (1000000 / TFRC_CALC_X_ARRSIZE)) - 1;
+        /* check against invalid parameters and divide-by-zero   */
+        BUG_ON(p >  1000000);           /* p must not exceed 100%   */
+        BUG_ON(p == 0);                 /* f(0) = 0, divide by zero */
+        if (R == 0) {                   /* possible  divide by zero */
+                DCCP_CRIT("WARNING: RTT is 0, returning maximum X_calc.");
+                return ~0U;
+        }
 
-        if (index < 0)
-                /* p should be 0 unless there is a bug in my code */
-                index = 0;
+        if (p <= TFRC_CALC_X_SPLIT)             {     /* 0.0000 < p <= 0.05   */
+                if (p < TFRC_SMALLEST_P) {            /* 0.0000 < p <  0.0001 */
+                        DCCP_WARN("Value of p (%d) below resolution. "
+                                  "Substituting %d\n", p, TFRC_SMALLEST_P);
+                        index = 0;
+                } else                                /* 0.0001 <= p <= 0.05  */
+                        index =  p/TFRC_SMALLEST_P - 1;
 
-        if (R == 0) {
-                DCCP_WARN("RTT==0, setting to 1\n");
-                R = 1; /* RTT can't be zero or else divide by zero */
-        }
+                f = tfrc_calc_x_lookup[index][1];
 
-        BUG_ON(index >= TFRC_CALC_X_ARRSIZE);
+        } else {                                      /* 0.05   <  p <= 1.00  */
+                index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1;
 
-        if (p >= TFRC_CALC_X_SPLIT)
                 f = tfrc_calc_x_lookup[index][0];
-        else
-                f = tfrc_calc_x_lookup[index][1];
+        }
 
+        /* The following computes X = s/(R*f(p)) in bytes per second. Since f(p)
+         * and R are both scaled by 1000000, we need to multiply by 1000000^2.
+         * ==> DO NOT alter this unless you test against overflow on 32 bit   */
         tmp1 = ((u64)s * 100000000);
         tmp2 = ((u64)R * (u64)f);
         do_div(tmp2, 10000);
         do_div(tmp1, tmp2); 
-        /* Don't alter above math unless you test due to overflow on 32 bit */
 
         return (u32)tmp1; 
 }
@@ -611,33 +664,36 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
 EXPORT_SYMBOL_GPL(tfrc_calc_x);
 
 /*
- * args: fvalue - function value to match
- * returns: p closest to that value
+ *  tfrc_calc_x_reverse_lookup  -  try to find p given f(p)
  *
- * both fvalue and p are multiplied by 1,000,000 to use ints
+ *  @fvalue: function value to match, scaled by 1000000
+ *  Returns closest match for p, also scaled by 1000000
  */
 u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
 {
-        int ctr = 0;
-        int small;
+        int index;
 
-        if (fvalue < tfrc_calc_x_lookup[0][1])
+        if (fvalue == 0)        /* f(p) = 0  whenever  p = 0 */
                 return 0;
 
-        if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1])
-                small = 1;
-        else if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0])
+        /* Error cases. */
+        if (fvalue < tfrc_calc_x_lookup[0][1]) {
+                DCCP_WARN("fvalue %d smaller than resolution\n", fvalue);
+                return tfrc_calc_x_lookup[0][1];
+        }
+        if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0]) {
+                DCCP_WARN("fvalue %d exceeds bounds!\n", fvalue);
                 return 1000000;
-        else
-                small = 0;
-
-        while (fvalue > tfrc_calc_x_lookup[ctr][small])
-                ctr++;
+        }
 
-        if (small)
-                return TFRC_CALC_X_SPLIT * ctr / TFRC_CALC_X_ARRSIZE;
-        else
-                return 1000000 * ctr / TFRC_CALC_X_ARRSIZE;
+        if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) {
+                index = tfrc_binsearch(fvalue, 1);
+                return (index + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE;
+        }
+ 
+        /* else ... it must be in the coarse-grained column */
+        index = tfrc_binsearch(fvalue, 0);
+        return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
 }
 
 EXPORT_SYMBOL_GPL(tfrc_calc_x_reverse_lookup);