1 // SPDX-License-Identifier: GPL-2.0 !! 1 /**
2 /* <<
3 * lib/minmax.c: windowed min/max tracker 2 * lib/minmax.c: windowed min/max tracker
4 * 3 *
5 * Kathleen Nichols' algorithm for tracking th 4 * Kathleen Nichols' algorithm for tracking the minimum (or maximum)
6 * value of a data stream over some fixed time 5 * value of a data stream over some fixed time interval. (E.g.,
7 * the minimum RTT over the past five minutes. 6 * the minimum RTT over the past five minutes.) It uses constant
8 * space and constant time per update yet almo 7 * space and constant time per update yet almost always delivers
9 * the same minimum as an implementation that 8 * the same minimum as an implementation that has to keep all the
10 * data in the window. 9 * data in the window.
11 * 10 *
12 * The algorithm keeps track of the best, 2nd 11 * The algorithm keeps track of the best, 2nd best & 3rd best min
13 * values, maintaining an invariant that the m 12 * values, maintaining an invariant that the measurement time of
14 * the n'th best >= n-1'th best. It also makes 13 * the n'th best >= n-1'th best. It also makes sure that the three
15 * values are widely separated in the time win 14 * values are widely separated in the time window since that bounds
16 * the worse case error when that data is mono 15 * the worse case error when that data is monotonically increasing
17 * over the window. 16 * over the window.
18 * 17 *
19 * Upon getting a new min, we can forget every 18 * Upon getting a new min, we can forget everything earlier because
20 * it has no value - the new min is <= everyth 19 * it has no value - the new min is <= everything else in the window
21 * by definition and it's the most recent. So 20 * by definition and it's the most recent. So we restart fresh on
22 * every new min and overwrites 2nd & 3rd choi 21 * every new min and overwrites 2nd & 3rd choices. The same property
23 * holds for 2nd & 3rd best. 22 * holds for 2nd & 3rd best.
24 */ 23 */
25 #include <linux/module.h> 24 #include <linux/module.h>
26 #include <linux/win_minmax.h> 25 #include <linux/win_minmax.h>
27 26
28 /* As time advances, update the 1st, 2nd, and 27 /* As time advances, update the 1st, 2nd, and 3rd choices. */
29 static u32 minmax_subwin_update(struct minmax 28 static u32 minmax_subwin_update(struct minmax *m, u32 win,
30 const struct m 29 const struct minmax_sample *val)
31 { 30 {
32 u32 dt = val->t - m->s[0].t; 31 u32 dt = val->t - m->s[0].t;
33 32
34 if (unlikely(dt > win)) { 33 if (unlikely(dt > win)) {
35 /* 34 /*
36 * Passed entire window withou 35 * Passed entire window without a new val so make 2nd
37 * choice the new val & 3rd ch 36 * choice the new val & 3rd choice the new 2nd choice.
38 * we may have to iterate this 37 * we may have to iterate this since our 2nd choice
39 * may also be outside the win 38 * may also be outside the window (we checked on entry
40 * that the third choice was i 39 * that the third choice was in the window).
41 */ 40 */
42 m->s[0] = m->s[1]; 41 m->s[0] = m->s[1];
43 m->s[1] = m->s[2]; 42 m->s[1] = m->s[2];
44 m->s[2] = *val; 43 m->s[2] = *val;
45 if (unlikely(val->t - m->s[0]. 44 if (unlikely(val->t - m->s[0].t > win)) {
46 m->s[0] = m->s[1]; 45 m->s[0] = m->s[1];
47 m->s[1] = m->s[2]; 46 m->s[1] = m->s[2];
48 m->s[2] = *val; 47 m->s[2] = *val;
49 } 48 }
50 } else if (unlikely(m->s[1].t == m->s[ 49 } else if (unlikely(m->s[1].t == m->s[0].t) && dt > win/4) {
51 /* 50 /*
52 * We've passed a quarter of t 51 * We've passed a quarter of the window without a new val
53 * so take a 2nd choice from t 52 * so take a 2nd choice from the 2nd quarter of the window.
54 */ 53 */
55 m->s[2] = m->s[1] = *val; 54 m->s[2] = m->s[1] = *val;
56 } else if (unlikely(m->s[2].t == m->s[ 55 } else if (unlikely(m->s[2].t == m->s[1].t) && dt > win/2) {
57 /* 56 /*
58 * We've passed half the windo 57 * We've passed half the window without finding a new val
59 * so take a 3rd choice from t 58 * so take a 3rd choice from the last half of the window
60 */ 59 */
61 m->s[2] = *val; 60 m->s[2] = *val;
62 } 61 }
63 return m->s[0].v; 62 return m->s[0].v;
64 } 63 }
65 64
66 /* Check if new measurement updates the 1st, 2 65 /* Check if new measurement updates the 1st, 2nd or 3rd choice max. */
67 u32 minmax_running_max(struct minmax *m, u32 w 66 u32 minmax_running_max(struct minmax *m, u32 win, u32 t, u32 meas)
68 { 67 {
69 struct minmax_sample val = { .t = t, . 68 struct minmax_sample val = { .t = t, .v = meas };
70 69
71 if (unlikely(val.v >= m->s[0].v) || 70 if (unlikely(val.v >= m->s[0].v) || /* found new max? */
72 unlikely(val.t - m->s[2].t > win)) 71 unlikely(val.t - m->s[2].t > win)) /* nothing left in window? */
73 return minmax_reset(m, t, meas 72 return minmax_reset(m, t, meas); /* forget earlier samples */
74 73
75 if (unlikely(val.v >= m->s[1].v)) 74 if (unlikely(val.v >= m->s[1].v))
76 m->s[2] = m->s[1] = val; 75 m->s[2] = m->s[1] = val;
77 else if (unlikely(val.v >= m->s[2].v)) 76 else if (unlikely(val.v >= m->s[2].v))
78 m->s[2] = val; 77 m->s[2] = val;
79 78
80 return minmax_subwin_update(m, win, &v 79 return minmax_subwin_update(m, win, &val);
81 } 80 }
82 EXPORT_SYMBOL(minmax_running_max); 81 EXPORT_SYMBOL(minmax_running_max);
83 82
84 /* Check if new measurement updates the 1st, 2 83 /* Check if new measurement updates the 1st, 2nd or 3rd choice min. */
85 u32 minmax_running_min(struct minmax *m, u32 w 84 u32 minmax_running_min(struct minmax *m, u32 win, u32 t, u32 meas)
86 { 85 {
87 struct minmax_sample val = { .t = t, . 86 struct minmax_sample val = { .t = t, .v = meas };
88 87
89 if (unlikely(val.v <= m->s[0].v) || 88 if (unlikely(val.v <= m->s[0].v) || /* found new min? */
90 unlikely(val.t - m->s[2].t > win)) 89 unlikely(val.t - m->s[2].t > win)) /* nothing left in window? */
91 return minmax_reset(m, t, meas 90 return minmax_reset(m, t, meas); /* forget earlier samples */
92 91
93 if (unlikely(val.v <= m->s[1].v)) 92 if (unlikely(val.v <= m->s[1].v))
94 m->s[2] = m->s[1] = val; 93 m->s[2] = m->s[1] = val;
95 else if (unlikely(val.v <= m->s[2].v)) 94 else if (unlikely(val.v <= m->s[2].v))
96 m->s[2] = val; 95 m->s[2] = val;
97 96
98 return minmax_subwin_update(m, win, &v 97 return minmax_subwin_update(m, win, &val);
99 } 98 }
100 99
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