the-tree/lib/win_minmax.c

0001 // SPDX-License-Identifier: GPL-2.0
0002 /**
0003  * lib/minmax.c: windowed min/max tracker
0004  *
0005  * Kathleen Nichols' algorithm for tracking the minimum (or maximum)
0006  * value of a data stream over some fixed time interval.  (E.g.,
0007  * the minimum RTT over the past five minutes.) It uses constant
0008  * space and constant time per update yet almost always delivers
0009  * the same minimum as an implementation that has to keep all the
0010  * data in the window.
0011  *
0012  * The algorithm keeps track of the best, 2nd best & 3rd best min
0013  * values, maintaining an invariant that the measurement time of
0014  * the n'th best >= n-1'th best. It also makes sure that the three
0015  * values are widely separated in the time window since that bounds
0016  * the worse case error when that data is monotonically increasing
0017  * over the window.
0018  *
0019  * Upon getting a new min, we can forget everything earlier because
0020  * it has no value - the new min is <= everything else in the window
0021  * by definition and it's the most recent. So we restart fresh on
0022  * every new min and overwrites 2nd & 3rd choices. The same property
0023  * holds for 2nd & 3rd best.
0024  */
0025 #include <linux/module.h>
0026 #include <linux/win_minmax.h>
0027
0028 /* As time advances, update the 1st, 2nd, and 3rd choices. */
0029 static u32 minmax_subwin_update(struct minmax *m, u32 win,
0030                 const struct minmax_sample *val)
0031 {
0032     u32 dt = val->t - m->s[0].t;
0033
0034     if (unlikely(dt > win)) {
0035         /*
0036          * Passed entire window without a new val so make 2nd
0037          * choice the new val & 3rd choice the new 2nd choice.
0038          * we may have to iterate this since our 2nd choice
0039          * may also be outside the window (we checked on entry
0040          * that the third choice was in the window).
0041          */
0042         m->s[0] = m->s[1];
0043         m->s[1] = m->s[2];
0044         m->s[2] = *val;
0045         if (unlikely(val->t - m->s[0].t > win)) {
0046             m->s[0] = m->s[1];
0047             m->s[1] = m->s[2];
0048             m->s[2] = *val;
0049         }
0050     } else if (unlikely(m->s[1].t == m->s[0].t) && dt > win/4) {
0051         /*
0052          * We've passed a quarter of the window without a new val
0053          * so take a 2nd choice from the 2nd quarter of the window.
0054          */
0055         m->s[2] = m->s[1] = *val;
0056     } else if (unlikely(m->s[2].t == m->s[1].t) && dt > win/2) {
0057         /*
0058          * We've passed half the window without finding a new val
0059          * so take a 3rd choice from the last half of the window
0060          */
0061         m->s[2] = *val;
0062     }
0063     return m->s[0].v;
0064 }
0065
0066 /* Check if new measurement updates the 1st, 2nd or 3rd choice max. */
0067 u32 minmax_running_max(struct minmax *m, u32 win, u32 t, u32 meas)
0068 {
0069     struct minmax_sample val = { .t = t, .v = meas };
0070
0071     if (unlikely(val.v >= m->s[0].v) ||   /* found new max? */
0072         unlikely(val.t - m->s[2].t > win))    /* nothing left in window? */
0073         return minmax_reset(m, t, meas);  /* forget earlier samples */
0074
0075     if (unlikely(val.v >= m->s[1].v))
0076         m->s[2] = m->s[1] = val;
0077     else if (unlikely(val.v >= m->s[2].v))
0078         m->s[2] = val;
0079
0080     return minmax_subwin_update(m, win, &val);
0081 }
0082 EXPORT_SYMBOL(minmax_running_max);
0083
0084 /* Check if new measurement updates the 1st, 2nd or 3rd choice min. */
0085 u32 minmax_running_min(struct minmax *m, u32 win, u32 t, u32 meas)
0086 {
0087     struct minmax_sample val = { .t = t, .v = meas };
0088
0089     if (unlikely(val.v <= m->s[0].v) ||   /* found new min? */
0090         unlikely(val.t - m->s[2].t > win))    /* nothing left in window? */
0091         return minmax_reset(m, t, meas);  /* forget earlier samples */
0092
0093     if (unlikely(val.v <= m->s[1].v))
0094         m->s[2] = m->s[1] = val;
0095     else if (unlikely(val.v <= m->s[2].v))
0096         m->s[2] = val;
0097
0098     return minmax_subwin_update(m, win, &val);
0099 }