| 1 | |
| 2 | |
| 3 | * kernel/sched/loadavg.c
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| 4 | *
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| 5 | * This file contains the magic bits required to compute the global loadavg
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| 6 | * figure. Its a silly number but people think its important. We go through
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| 7 | * great pains to make it work on big machines and tickless kernels.
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| 8 |
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| 9 | #include <linux/sched/nohz.h>
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| 10 | #include "sched.h"
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| 11 |
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| 12 | |
| 13 | * Global load-average calculations
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| 14 | *
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| 15 | * We take a distributed and async approach to calculating the global load-avg
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| 16 | * in order to minimize overhead.
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| 17 | *
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| 18 | * The global load average is an exponentially decaying average of nr_running +
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| 19 | * nr_uninterruptible.
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| 20 | *
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| 21 | * Once every LOAD_FREQ:
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| 22 | *
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| 23 | * nr_active = 0;
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| 24 | * for_each_possible_cpu(cpu)
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| 25 | * nr_active += cpu_of(cpu)->nr_running + cpu_of(cpu)->nr_uninterruptible;
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| 26 | *
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| 27 | * avenrun[n] = avenrun[0] * exp_n + nr_active * (1 - exp_n)
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| 28 | *
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| 29 | * Due to a number of reasons the above turns in the mess below:
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| 30 | *
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| 31 | * - for_each_possible_cpu() is prohibitively expensive on machines with
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| 32 | * serious number of CPUs, therefore we need to take a distributed approach
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| 33 | * to calculating nr_active.
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| 34 | *
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| 35 | * \Sum_i x_i(t) = \Sum_i x_i(t) - x_i(t_0) | x_i(t_0) := 0
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| 36 | * = \Sum_i { \Sum_j=1 x_i(t_j) - x_i(t_j-1) }
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| 37 | *
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| 38 | * So assuming nr_active := 0 when we start out -- true per definition, we
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| 39 | * can simply take per-CPU deltas and fold those into a global accumulate
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| 40 | * to obtain the same result. See calc_load_fold_active().
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| 41 | *
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| 42 | * Furthermore, in order to avoid synchronizing all per-CPU delta folding
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| 43 | * across the machine, we assume 10 ticks is sufficient time for every
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| 44 | * CPU to have completed this task.
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| 45 | *
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| 46 | * This places an upper-bound on the IRQ-off latency of the machine. Then
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| 47 | * again, being late doesn't loose the delta, just wrecks the sample.
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| 48 | *
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| 49 | * - cpu_rq()->nr_uninterruptible isn't accurately tracked per-CPU because
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| 50 | * this would add another cross-CPU cache-line miss and atomic operation
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| 51 | * to the wakeup path. Instead we increment on whatever CPU the task ran
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| 52 | * when it went into uninterruptible state and decrement on whatever CPU
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| 53 | * did the wakeup. This means that only the sum of nr_uninterruptible over
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| 54 | * all CPUs yields the correct result.
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| 55 | *
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| 56 | * This covers the NO_HZ=n code, for extra head-aches, see the comment below.
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| 57 |
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| 58 |
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| 59 |
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| 60 | atomic_long_t calc_load_tasks;
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| 61 | unsigned long calc_load_update;
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| 62 | unsigned long avenrun[3];
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| 63 | EXPORT_SYMBOL(avenrun);
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| 64 |
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| 65 | |
| 66 | * get_avenrun - get the load average array
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| 67 | * @loads: pointer to destination load array
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| 68 | * @offset: offset to add
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| 69 | * @shift: shift count to shift the result left
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| 70 | *
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| 71 | * These values are estimates at best, so no need for locking.
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| 72 |
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| 73 | void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
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| 74 | {
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| 75 | loads[0] = (avenrun[0] + offset) << shift;
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| 76 | loads[1] = (avenrun[1] + offset) << shift;
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| 77 | loads[2] = (avenrun[2] + offset) << shift;
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| 78 | }
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| 79 |
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| 80 | long calc_load_fold_active(struct rq *this_rq, long adjust)
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| 81 | {
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| 82 | long nr_active, delta = 0;
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| 83 |
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| 84 | nr_active = this_rq->nr_running - adjust;
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| 85 | nr_active += (long)this_rq->nr_uninterruptible;
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| 86 |
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| 87 | if (nr_active != this_rq->calc_load_active) {
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| 88 | delta = nr_active - this_rq->calc_load_active;
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| 89 | this_rq->calc_load_active = nr_active;
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| 90 | }
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| 91 |
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| 92 | return delta;
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| 93 | }
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| 94 |
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| 95 | |
| 96 | * fixed_power_int - compute: x^n, in O(log n) time
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| 97 | *
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| 98 | * @x: base of the power
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| 99 | * @frac_bits: fractional bits of @x
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| 100 | * @n: power to raise @x to.
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| 101 | *
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| 102 | * By exploiting the relation between the definition of the natural power
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| 103 | * function: x^n := x*x*...*x (x multiplied by itself for n times), and
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| 104 | * the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
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| 105 | * (where: n_i \elem {0, 1}, the binary vector representing n),
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| 106 | * we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
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| 107 | * of course trivially computable in O(log_2 n), the length of our binary
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| 108 | * vector.
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| 109 |
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| 110 | static unsigned long
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| 111 | fixed_power_int(unsigned long x, unsigned int frac_bits, unsigned int n)
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| 112 | {
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| 113 | unsigned long result = 1UL << frac_bits;
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| 114 |
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| 115 | if (n) {
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| 116 | for (;;) {
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| 117 | if (n & 1) {
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| 118 | result *= x;
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| 119 | result += 1UL << (frac_bits - 1);
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| 120 | result >>= frac_bits;
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| 121 | }
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| 122 | n >>= 1;
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| 123 | if (!n)
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| 124 | break;
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| 125 | x *= x;
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| 126 | x += 1UL << (frac_bits - 1);
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| 127 | x >>= frac_bits;
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| 128 | }
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| 129 | }
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| 130 |
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| 131 | return result;
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| 132 | }
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| 133 |
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| 134 | |
| 135 | * a1 = a0 * e + a * (1 - e)
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| 136 | *
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| 137 | * a2 = a1 * e + a * (1 - e)
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| 138 | * = (a0 * e + a * (1 - e)) * e + a * (1 - e)
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| 139 | * = a0 * e^2 + a * (1 - e) * (1 + e)
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| 140 | *
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| 141 | * a3 = a2 * e + a * (1 - e)
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| 142 | * = (a0 * e^2 + a * (1 - e) * (1 + e)) * e + a * (1 - e)
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| 143 | * = a0 * e^3 + a * (1 - e) * (1 + e + e^2)
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| 144 | *
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| 145 | * ...
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| 146 | *
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| 147 | * an = a0 * e^n + a * (1 - e) * (1 + e + ... + e^n-1) [1]
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| 148 | * = a0 * e^n + a * (1 - e) * (1 - e^n)/(1 - e)
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| 149 | * = a0 * e^n + a * (1 - e^n)
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| 150 | *
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| 151 | * [1] application of the geometric series:
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| 152 | *
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| 153 | * n 1 - x^(n+1)
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| 154 | * S_n := \Sum x^i = -------------
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| 155 | * i=0 1 - x
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| 156 |
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| 157 | unsigned long
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| 158 | calc_load_n(unsigned long load, unsigned long exp,
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| 159 | unsigned long active, unsigned int n)
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| 160 | {
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| 161 | return calc_load(load, fixed_power_int(exp, FSHIFT, n), active);
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| 162 | }
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| 163 |
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| 164 | #ifdef CONFIG_NO_HZ_COMMON
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| 165 | |
| 166 | * Handle NO_HZ for the global load-average.
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| 167 | *
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| 168 | * Since the above described distributed algorithm to compute the global
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| 169 | * load-average relies on per-CPU sampling from the tick, it is affected by
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| 170 | * NO_HZ.
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| 171 | *
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| 172 | * The basic idea is to fold the nr_active delta into a global NO_HZ-delta upon
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| 173 | * entering NO_HZ state such that we can include this as an 'extra' CPU delta
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| 174 | * when we read the global state.
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| 175 | *
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| 176 | * Obviously reality has to ruin such a delightfully simple scheme:
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| 177 | *
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| 178 | * - When we go NO_HZ idle during the window, we can negate our sample
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| 179 | * contribution, causing under-accounting.
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| 180 | *
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| 181 | * We avoid this by keeping two NO_HZ-delta counters and flipping them
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| 182 | * when the window starts, thus separating old and new NO_HZ load.
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| 183 | *
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| 184 | * The only trick is the slight shift in index flip for read vs write.
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| 185 | *
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| 186 | * 0s 5s 10s 15s
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| 187 | * +10 +10 +10 +10
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| 188 | * |-|-----------|-|-----------|-|-----------|-|
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| 189 | * r:0 0 1 1 0 0 1 1 0
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| 190 | * w:0 1 1 0 0 1 1 0 0
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| 191 | *
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| 192 | * This ensures we'll fold the old NO_HZ contribution in this window while
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| 193 | * accumulating the new one.
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| 194 | *
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| 195 | * - When we wake up from NO_HZ during the window, we push up our
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| 196 | * contribution, since we effectively move our sample point to a known
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| 197 | * busy state.
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| 198 | *
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| 199 | * This is solved by pushing the window forward, and thus skipping the
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| 200 | * sample, for this CPU (effectively using the NO_HZ-delta for this CPU which
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| 201 | * was in effect at the time the window opened). This also solves the issue
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| 202 | * of having to deal with a CPU having been in NO_HZ for multiple LOAD_FREQ
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| 203 | * intervals.
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| 204 | *
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| 205 | * When making the ILB scale, we should try to pull this in as well.
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| 206 |
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| 207 | static atomic_long_t calc_load_nohz[2];
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| 208 | static int calc_load_idx;
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| 209 |
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| 210 | static inline int calc_load_write_idx(void)
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| 211 | {
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| 212 | int idx = calc_load_idx;
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| 213 |
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| 214 | |
| 215 | * See calc_global_nohz(), if we observe the new index, we also
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| 216 | * need to observe the new update time.
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| 217 |
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| 218 | smp_rmb();
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| 219 |
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| 220 | |
| 221 | * If the folding window started, make sure we start writing in the
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| 222 | * next NO_HZ-delta.
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| 223 |
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| 224 | if (!time_before(jiffies, READ_ONCE(calc_load_update)))
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| 225 | idx++;
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| 226 |
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| 227 | return idx & 1;
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| 228 | }
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| 229 |
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| 230 | static inline int calc_load_read_idx(void)
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| 231 | {
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| 232 | return calc_load_idx & 1;
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| 233 | }
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| 234 |
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| 235 | static void calc_load_nohz_fold(struct rq *rq)
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| 236 | {
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| 237 | long delta;
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| 238 |
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| 239 | delta = calc_load_fold_active(rq, 0);
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| 240 | if (delta) {
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| 241 | int idx = calc_load_write_idx();
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| 242 |
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| 243 | atomic_long_add(delta, &calc_load_nohz[idx]);
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| 244 | }
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| 245 | }
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| 246 |
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| 247 | void calc_load_nohz_start(void)
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| 248 | {
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| 249 | |
| 250 | * We're going into NO_HZ mode, if there's any pending delta, fold it
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| 251 | * into the pending NO_HZ delta.
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| 252 |
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| 253 | calc_load_nohz_fold(this_rq());
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| 254 | }
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| 255 |
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| 256 | |
| 257 | * Keep track of the load for NOHZ_FULL, must be called between
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| 258 | * calc_load_nohz_{start,stop}().
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| 259 |
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| 260 | void calc_load_nohz_remote(struct rq *rq)
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| 261 | {
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| 262 | calc_load_nohz_fold(rq);
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| 263 | }
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| 264 |
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| 265 | void calc_load_nohz_stop(void)
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| 266 | {
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| 267 | struct rq *this_rq = this_rq();
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| 268 |
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| 269 | |
| 270 | * If we're still before the pending sample window, we're done.
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| 271 |
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| 272 | this_rq->calc_load_update = READ_ONCE(calc_load_update);
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| 273 | if (time_before(jiffies, this_rq->calc_load_update))
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| 274 | return;
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| 275 |
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| 276 | |
| 277 | * We woke inside or after the sample window, this means we're already
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| 278 | * accounted through the nohz accounting, so skip the entire deal and
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| 279 | * sync up for the next window.
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| 280 |
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| 281 | if (time_before(jiffies, this_rq->calc_load_update + 10))
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| 282 | this_rq->calc_load_update += LOAD_FREQ;
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| 283 | }
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| 284 |
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| 285 | static long calc_load_nohz_read(void)
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| 286 | {
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| 287 | int idx = calc_load_read_idx();
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| 288 | long delta = 0;
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| 289 |
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| 290 | if (atomic_long_read(&calc_load_nohz[idx]))
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| 291 | delta = atomic_long_xchg(&calc_load_nohz[idx], 0);
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| 292 |
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| 293 | return delta;
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| 294 | }
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| 295 |
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| 296 | |
| 297 | * NO_HZ can leave us missing all per-CPU ticks calling
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| 298 | * calc_load_fold_active(), but since a NO_HZ CPU folds its delta into
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| 299 | * calc_load_nohz per calc_load_nohz_start(), all we need to do is fold
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| 300 | * in the pending NO_HZ delta if our NO_HZ period crossed a load cycle boundary.
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| 301 | *
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| 302 | * Once we've updated the global active value, we need to apply the exponential
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| 303 | * weights adjusted to the number of cycles missed.
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| 304 |
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| 305 | static void calc_global_nohz(void)
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| 306 | {
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| 307 | unsigned long sample_window;
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| 308 | long delta, active, n;
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| 309 |
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| 310 | sample_window = READ_ONCE(calc_load_update);
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| 311 | if (!time_before(jiffies, sample_window + 10)) {
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| 312 | |
| 313 | * Catch-up, fold however many we are behind still
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| 314 |
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| 315 | delta = jiffies - sample_window - 10;
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| 316 | n = 1 + (delta / LOAD_FREQ);
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| 317 |
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| 318 | active = atomic_long_read(&calc_load_tasks);
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| 319 | active = active > 0 ? active * FIXED_1 : 0;
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| 320 |
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| 321 | avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
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| 322 | avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
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| 323 | avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
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| 324 |
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| 325 | WRITE_ONCE(calc_load_update, sample_window + n * LOAD_FREQ);
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| 326 | }
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| 327 |
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| 328 | |
| 329 | * Flip the NO_HZ index...
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| 330 | *
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| 331 | * Make sure we first write the new time then flip the index, so that
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| 332 | * calc_load_write_idx() will see the new time when it reads the new
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| 333 | * index, this avoids a double flip messing things up.
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| 334 |
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| 335 | smp_wmb();
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| 336 | calc_load_idx++;
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| 337 | }
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| 338 | #else
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| 339 |
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| 340 | static inline long calc_load_nohz_read(void) { return 0; }
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| 341 | static inline void calc_global_nohz(void) { }
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| 342 |
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| 343 | #endif
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| 344 |
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| 345 | |
| 346 | * calc_load - update the avenrun load estimates 10 ticks after the
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| 347 | * CPUs have updated calc_load_tasks.
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| 348 | *
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| 349 | * Called from the global timer code.
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| 350 |
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| 351 | void calc_global_load(void)
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| 352 | {
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| 353 | unsigned long sample_window;
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| 354 | long active, delta;
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| 355 |
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| 356 | sample_window = READ_ONCE(calc_load_update);
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| 357 | if (time_before(jiffies, sample_window + 10))
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| 358 | return;
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| 359 |
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| 360 | |
| 361 | * Fold the 'old' NO_HZ-delta to include all NO_HZ CPUs.
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| 362 |
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| 363 | delta = calc_load_nohz_read();
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| 364 | if (delta)
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| 365 | atomic_long_add(delta, &calc_load_tasks);
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| 366 |
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| 367 | active = atomic_long_read(&calc_load_tasks);
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| 368 | active = active > 0 ? active * FIXED_1 : 0;
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| 369 |
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| 370 | avenrun[0] = calc_load(avenrun[0], EXP_1, active);
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| 371 | avenrun[1] = calc_load(avenrun[1], EXP_5, active);
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| 372 | avenrun[2] = calc_load(avenrun[2], EXP_15, active);
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| 373 |
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| 374 | WRITE_ONCE(calc_load_update, sample_window + LOAD_FREQ);
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| 375 |
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| 376 | |
| 377 | * In case we went to NO_HZ for multiple LOAD_FREQ intervals
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| 378 | * catch up in bulk.
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| 379 |
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| 380 | calc_global_nohz();
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| 381 | }
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| 382 |
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| 383 | |
| 384 | * Called from sched_tick() to periodically update this CPU's
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| 385 | * active count.
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| 386 |
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| 387 | void calc_global_load_tick(struct rq *this_rq)
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| 388 | {
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| 389 | long delta;
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| 390 |
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| 391 | if (time_before(jiffies, this_rq->calc_load_update))
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| 392 | return;
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| 393 |
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| 394 | delta = calc_load_fold_active(this_rq, 0);
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| 395 | if (delta)
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| 396 | atomic_long_add(delta, &calc_load_tasks);
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| 397 |
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| 398 | this_rq->calc_load_update += LOAD_FREQ;
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| 399 | }
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