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dm-mpath: streamline setup work in the HST path selector

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fixed_power() is convoluted and does unnecessary work here, since
hst_set_weights() calculates x^n for n = [1,64]. x^1 is x, and x^(n) is
just x * x^(n-1), so all these values can be trivially computed from the
previous value.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
This commit is contained in:
Benjamin Marzinski 2026-10-08 16:28:07 -04:00 committed by Mikulas Patocka
parent c0df022cb0
commit fe9d8c26ab
1 changed files with 6 additions and 43 deletions
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49
drivers/md/dm-ps-historical-service-time.c
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@ -36,6 +36,7 @@
#define HST_FIXED_SHIFT 10 /* 10 bits of decimal precision */
#define HST_FIXED_MAX (ULLONG_MAX >> HST_FIXED_SHIFT)
#define HST_FIXED_1 (1 << HST_FIXED_SHIFT)
#define HST_FIXED_POINT_5 (1 << (HST_FIXED_SHIFT - 1))
#define HST_FIXED_95 972
#define HST_MAX_INFLIGHT HST_FIXED_1
#define HST_BUCKET_SHIFT 24 /* Buckets are ~ 16ms */
@ -66,46 +67,6 @@ struct path_info {
u64 outstanding;
};
/**
* fixed_power - compute: x^n, in O(log n) time
*
* @x: base of the power
* @frac_bits: fractional bits of @x
* @n: power to raise @x to.
*
* By exploiting the relation between the definition of the natural power
* function: x^n := x*x*...*x (x multiplied by itself for n times), and
* the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
* (where: n_i \elem {0, 1}, the binary vector representing n),
* we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
* of course trivially computable in O(log_2 n), the length of our binary
* vector.
*
* (see: kernel/sched/loadavg.c)
*/
static u64 fixed_power(u64 x, unsigned int frac_bits, unsigned int n)
{
unsigned long result = 1UL << frac_bits;
if (n) {
for (;;) {
if (n & 1) {
result *= x;
result += 1UL << (frac_bits - 1);
result >>= frac_bits;
}
n >>= 1;
if (!n)
break;
x *= x;
x += 1UL << (frac_bits - 1);
x >>= frac_bits;
}
}
return result;
}
/*
* Calculate the next value of an exponential moving average
* a_1 = a_0 * e + a * (1 - e)
@ -123,7 +84,7 @@ static u64 fixed_ema(u64 last, u64 next, u64 weight)
{
last *= weight;
last += next * (HST_FIXED_1 - weight);
last += 1ULL << (HST_FIXED_SHIFT - 1);
last += HST_FIXED_POINT_5;
return last >> HST_FIXED_SHIFT;
}
@ -167,8 +128,10 @@ static void hst_set_weights(struct path_selector *ps, unsigned int base)
if (base >= HST_FIXED_1)
return;
for (i = 0; i < HST_WEIGHT_COUNT - 1; i++)
s->weights[i] = fixed_power(base, HST_FIXED_SHIFT, i + 1);
s->weights[0] = base;
for (i = 1; i < HST_WEIGHT_COUNT - 1; i++)
s->weights[i] = (base * s->weights[i - 1] +
HST_FIXED_POINT_5) >> HST_FIXED_SHIFT;
s->weights[HST_WEIGHT_COUNT - 1] = 0;
}