target/linux/generic/patches-3.3/045-codel-use-u16-field-instead-of-31bits-for-rec_inv_sq.patch

   1 From 03333931c17d9c62ba4063d4e4fec1578c0729a7 Mon Sep 17 00:00:00 2001
   2 From: Eric Dumazet <eric.dumazet@gmail.com>
   3 Date: Sat, 12 May 2012 21:23:23 +0000
   4 Subject: [PATCH] codel: use u16 field instead of 31bits for rec_inv_sqrt
   5
   6 commit 6ff272c9ad65eda219cd975b9da2dbc31cc812ee upstream.
   7
   8 David pointed out gcc might generate poor code with 31bit fields.
   9
  10 Using u16 is more than enough and permits a better code output.
  11
  12 Also make the code intent more readable using constants, fixed point arithmetic
  13 not being trivial for everybody.
  14
  15 Suggested-by: David Miller <davem@davemloft.net>
  16 Signed-off-by: Eric Dumazet <edumazet@google.com>
  17 Signed-off-by: David S. Miller <davem@davemloft.net>
  18 ---
  19  include/net/codel.h |   25 +++++++++++++++----------
  20  1 file changed, 15 insertions(+), 10 deletions(-)
  21
  22 --- a/include/net/codel.h
  23 +++ b/include/net/codel.h
  24 @@ -133,13 +133,17 @@ struct codel_params {
  25  struct codel_vars {
  26         u32             count;
  27         u32             lastcount;
  28 -       bool            dropping:1;
  29 -       u32             rec_inv_sqrt:31;
  30 +       bool            dropping;
  31 +       u16             rec_inv_sqrt;
  32         codel_time_t    first_above_time;
  33         codel_time_t    drop_next;
  34         codel_time_t    ldelay;
  35  };
  36
  37 +#define REC_INV_SQRT_BITS (8 * sizeof(u16)) /* or sizeof_in_bits(rec_inv_sqrt) */
  38 +/* needed shift to get a Q0.32 number from rec_inv_sqrt */
  39 +#define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS)
  40 +
  41  /**
  42   * struct codel_stats - contains codel shared variables and stats
  43   * @maxpacket: largest packet we've seen so far
  44 @@ -173,17 +177,18 @@ static void codel_stats_init(struct code
  45   * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
  46   * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
  47   *
  48 - * Here, invsqrt is a fixed point number (< 1.0), 31bit mantissa)
  49 + * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
  50   */
  51  static void codel_Newton_step(struct codel_vars *vars)
  52  {
  53 -       u32 invsqrt = vars->rec_inv_sqrt;
  54 -       u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
  55 -       u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
  56 +       u32 invsqrt = ((u32)vars->rec_inv_sqrt) << REC_INV_SQRT_SHIFT;
  57 +       u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
  58 +       u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);
  59
  60 -       val = (val * invsqrt) >> 32;
  61 +       val >>= 2; /* avoid overflow in following multiply */
  62 +       val = (val * invsqrt) >> (32 - 2 + 1);
  63
  64 -       vars->rec_inv_sqrt = val;
  65 +       vars->rec_inv_sqrt = val >> REC_INV_SQRT_SHIFT;
  66  }
  67
  68  /*
  69 @@ -195,7 +200,7 @@ static codel_time_t codel_control_law(co
  70                                       codel_time_t interval,
  71                                       u32 rec_inv_sqrt)
  72  {
  73 -       return t + reciprocal_divide(interval, rec_inv_sqrt << 1);
  74 +       return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
  75  }
  76
  77
  78 @@ -326,7 +331,7 @@ static struct sk_buff *codel_dequeue(str
  79                         codel_Newton_step(vars);
  80                 } else {
  81                         vars->count = 1;
  82 -                       vars->rec_inv_sqrt = 0x7fffffff;
  83 +                       vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
  84                 }
  85                 vars->lastcount = vars->count;
  86                 vars->drop_next = codel_control_law(now, params->interval,