386 lines
11 KiB
C
386 lines
11 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* kernel/sched/loadavg.c
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*
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* This file contains the magic bits required to compute the global loadavg
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* figure. Its a silly number but people think its important. We go through
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* great pains to make it work on big machines and tickless kernels.
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*/
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#include "sched.h"
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/*
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* Global load-average calculations
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*
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* We take a distributed and async approach to calculating the global load-avg
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* in order to minimize overhead.
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*
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* The global load average is an exponentially decaying average of nr_running +
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* nr_uninterruptible.
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*
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* Once every LOAD_FREQ:
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*
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* nr_active = 0;
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* for_each_possible_cpu(cpu)
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* nr_active += cpu_of(cpu)->nr_running + cpu_of(cpu)->nr_uninterruptible;
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*
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* avenrun[n] = avenrun[0] * exp_n + nr_active * (1 - exp_n)
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*
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* Due to a number of reasons the above turns in the mess below:
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*
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* - for_each_possible_cpu() is prohibitively expensive on machines with
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* serious number of CPUs, therefore we need to take a distributed approach
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* to calculating nr_active.
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*
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* \Sum_i x_i(t) = \Sum_i x_i(t) - x_i(t_0) | x_i(t_0) := 0
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* = \Sum_i { \Sum_j=1 x_i(t_j) - x_i(t_j-1) }
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*
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* So assuming nr_active := 0 when we start out -- true per definition, we
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* can simply take per-CPU deltas and fold those into a global accumulate
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* to obtain the same result. See calc_load_fold_active().
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*
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* Furthermore, in order to avoid synchronizing all per-CPU delta folding
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* across the machine, we assume 10 ticks is sufficient time for every
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* CPU to have completed this task.
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*
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* This places an upper-bound on the IRQ-off latency of the machine. Then
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* again, being late doesn't loose the delta, just wrecks the sample.
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*
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* - cpu_rq()->nr_uninterruptible isn't accurately tracked per-CPU because
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* this would add another cross-CPU cacheline miss and atomic operation
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* to the wakeup path. Instead we increment on whatever CPU the task ran
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* when it went into uninterruptible state and decrement on whatever CPU
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* did the wakeup. This means that only the sum of nr_uninterruptible over
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* all CPUs yields the correct result.
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*
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* This covers the NO_HZ=n code, for extra head-aches, see the comment below.
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*/
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/* Variables and functions for calc_load */
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atomic_long_t calc_load_tasks;
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unsigned long calc_load_update;
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unsigned long avenrun[3];
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EXPORT_SYMBOL(avenrun); /* should be removed */
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/**
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* get_avenrun - get the load average array
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* @loads: pointer to dest load array
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* @offset: offset to add
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* @shift: shift count to shift the result left
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*
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* These values are estimates at best, so no need for locking.
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*/
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void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
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{
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loads[0] = (avenrun[0] + offset) << shift;
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loads[1] = (avenrun[1] + offset) << shift;
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loads[2] = (avenrun[2] + offset) << shift;
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}
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long calc_load_fold_active(struct rq *this_rq, long adjust)
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{
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long nr_active, delta = 0;
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nr_active = this_rq->nr_running - adjust;
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nr_active += (long)this_rq->nr_uninterruptible;
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if (nr_active != this_rq->calc_load_active) {
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delta = nr_active - this_rq->calc_load_active;
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this_rq->calc_load_active = nr_active;
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}
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return delta;
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}
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/**
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* fixed_power_int - compute: x^n, in O(log n) time
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*
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* @x: base of the power
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* @frac_bits: fractional bits of @x
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* @n: power to raise @x to.
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*
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* By exploiting the relation between the definition of the natural power
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* function: x^n := x*x*...*x (x multiplied by itself for n times), and
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* the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
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* (where: n_i \elem {0, 1}, the binary vector representing n),
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* we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
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* of course trivially computable in O(log_2 n), the length of our binary
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* vector.
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*/
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static unsigned long
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fixed_power_int(unsigned long x, unsigned int frac_bits, unsigned int n)
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{
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unsigned long result = 1UL << frac_bits;
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if (n) {
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for (;;) {
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if (n & 1) {
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result *= x;
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result += 1UL << (frac_bits - 1);
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result >>= frac_bits;
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}
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n >>= 1;
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if (!n)
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break;
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x *= x;
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x += 1UL << (frac_bits - 1);
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x >>= frac_bits;
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}
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}
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return result;
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}
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/*
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* a1 = a0 * e + a * (1 - e)
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*
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* a2 = a1 * e + a * (1 - e)
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* = (a0 * e + a * (1 - e)) * e + a * (1 - e)
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* = a0 * e^2 + a * (1 - e) * (1 + e)
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*
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* a3 = a2 * e + a * (1 - e)
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* = (a0 * e^2 + a * (1 - e) * (1 + e)) * e + a * (1 - e)
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* = a0 * e^3 + a * (1 - e) * (1 + e + e^2)
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*
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* ...
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*
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* an = a0 * e^n + a * (1 - e) * (1 + e + ... + e^n-1) [1]
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* = a0 * e^n + a * (1 - e) * (1 - e^n)/(1 - e)
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* = a0 * e^n + a * (1 - e^n)
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*
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* [1] application of the geometric series:
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*
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* n 1 - x^(n+1)
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* S_n := \Sum x^i = -------------
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* i=0 1 - x
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*/
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unsigned long
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calc_load_n(unsigned long load, unsigned long exp,
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unsigned long active, unsigned int n)
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{
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return calc_load(load, fixed_power_int(exp, FSHIFT, n), active);
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}
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#ifdef CONFIG_NO_HZ_COMMON
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/*
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* Handle NO_HZ for the global load-average.
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*
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* Since the above described distributed algorithm to compute the global
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* load-average relies on per-CPU sampling from the tick, it is affected by
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* NO_HZ.
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*
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* The basic idea is to fold the nr_active delta into a global NO_HZ-delta upon
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* entering NO_HZ state such that we can include this as an 'extra' CPU delta
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* when we read the global state.
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*
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* Obviously reality has to ruin such a delightfully simple scheme:
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*
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* - When we go NO_HZ idle during the window, we can negate our sample
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* contribution, causing under-accounting.
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*
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* We avoid this by keeping two NO_HZ-delta counters and flipping them
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* when the window starts, thus separating old and new NO_HZ load.
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*
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* The only trick is the slight shift in index flip for read vs write.
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*
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* 0s 5s 10s 15s
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* +10 +10 +10 +10
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* |-|-----------|-|-----------|-|-----------|-|
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* r:0 0 1 1 0 0 1 1 0
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* w:0 1 1 0 0 1 1 0 0
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*
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* This ensures we'll fold the old NO_HZ contribution in this window while
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* accumlating the new one.
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*
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* - When we wake up from NO_HZ during the window, we push up our
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* contribution, since we effectively move our sample point to a known
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* busy state.
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*
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* This is solved by pushing the window forward, and thus skipping the
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* sample, for this CPU (effectively using the NO_HZ-delta for this CPU which
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* was in effect at the time the window opened). This also solves the issue
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* of having to deal with a CPU having been in NO_HZ for multiple LOAD_FREQ
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* intervals.
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*
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* When making the ILB scale, we should try to pull this in as well.
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*/
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static atomic_long_t calc_load_nohz[2];
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static int calc_load_idx;
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static inline int calc_load_write_idx(void)
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{
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int idx = calc_load_idx;
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/*
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* See calc_global_nohz(), if we observe the new index, we also
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* need to observe the new update time.
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*/
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smp_rmb();
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/*
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* If the folding window started, make sure we start writing in the
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* next NO_HZ-delta.
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*/
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if (!time_before(jiffies, READ_ONCE(calc_load_update)))
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idx++;
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return idx & 1;
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}
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static inline int calc_load_read_idx(void)
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{
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return calc_load_idx & 1;
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}
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void calc_load_nohz_start(void)
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{
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struct rq *this_rq = this_rq();
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long delta;
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/*
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* We're going into NO_HZ mode, if there's any pending delta, fold it
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* into the pending NO_HZ delta.
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*/
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delta = calc_load_fold_active(this_rq, 0);
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if (delta) {
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int idx = calc_load_write_idx();
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atomic_long_add(delta, &calc_load_nohz[idx]);
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}
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}
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void calc_load_nohz_stop(void)
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{
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struct rq *this_rq = this_rq();
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/*
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* If we're still before the pending sample window, we're done.
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*/
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this_rq->calc_load_update = READ_ONCE(calc_load_update);
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if (time_before(jiffies, this_rq->calc_load_update))
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return;
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/*
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* We woke inside or after the sample window, this means we're already
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* accounted through the nohz accounting, so skip the entire deal and
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* sync up for the next window.
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*/
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if (time_before(jiffies, this_rq->calc_load_update + 10))
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this_rq->calc_load_update += LOAD_FREQ;
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}
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static long calc_load_nohz_fold(void)
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{
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int idx = calc_load_read_idx();
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long delta = 0;
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if (atomic_long_read(&calc_load_nohz[idx]))
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delta = atomic_long_xchg(&calc_load_nohz[idx], 0);
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return delta;
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}
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/*
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* NO_HZ can leave us missing all per-CPU ticks calling
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* calc_load_fold_active(), but since a NO_HZ CPU folds its delta into
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* calc_load_nohz per calc_load_nohz_start(), all we need to do is fold
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* in the pending NO_HZ delta if our NO_HZ period crossed a load cycle boundary.
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*
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* Once we've updated the global active value, we need to apply the exponential
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* weights adjusted to the number of cycles missed.
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*/
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static void calc_global_nohz(void)
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{
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unsigned long sample_window;
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long delta, active, n;
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sample_window = READ_ONCE(calc_load_update);
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if (!time_before(jiffies, sample_window + 10)) {
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/*
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* Catch-up, fold however many we are behind still
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*/
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delta = jiffies - sample_window - 10;
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n = 1 + (delta / LOAD_FREQ);
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active = atomic_long_read(&calc_load_tasks);
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active = active > 0 ? active * FIXED_1 : 0;
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avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
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avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
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avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
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WRITE_ONCE(calc_load_update, sample_window + n * LOAD_FREQ);
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}
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/*
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* Flip the NO_HZ index...
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*
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* Make sure we first write the new time then flip the index, so that
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* calc_load_write_idx() will see the new time when it reads the new
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* index, this avoids a double flip messing things up.
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*/
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smp_wmb();
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calc_load_idx++;
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}
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#else /* !CONFIG_NO_HZ_COMMON */
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static inline long calc_load_nohz_fold(void) { return 0; }
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static inline void calc_global_nohz(void) { }
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#endif /* CONFIG_NO_HZ_COMMON */
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/*
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* calc_load - update the avenrun load estimates 10 ticks after the
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* CPUs have updated calc_load_tasks.
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*
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* Called from the global timer code.
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*/
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void calc_global_load(unsigned long ticks)
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{
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unsigned long sample_window;
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long active, delta;
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sample_window = READ_ONCE(calc_load_update);
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if (time_before(jiffies, sample_window + 10))
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return;
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/*
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* Fold the 'old' NO_HZ-delta to include all NO_HZ CPUs.
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*/
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delta = calc_load_nohz_fold();
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if (delta)
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atomic_long_add(delta, &calc_load_tasks);
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active = atomic_long_read(&calc_load_tasks);
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active = active > 0 ? active * FIXED_1 : 0;
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avenrun[0] = calc_load(avenrun[0], EXP_1, active);
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avenrun[1] = calc_load(avenrun[1], EXP_5, active);
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avenrun[2] = calc_load(avenrun[2], EXP_15, active);
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WRITE_ONCE(calc_load_update, sample_window + LOAD_FREQ);
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/*
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* In case we went to NO_HZ for multiple LOAD_FREQ intervals
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* catch up in bulk.
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*/
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calc_global_nohz();
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}
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/*
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* Called from scheduler_tick() to periodically update this CPU's
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* active count.
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*/
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void calc_global_load_tick(struct rq *this_rq)
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{
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long delta;
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if (time_before(jiffies, this_rq->calc_load_update))
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return;
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delta = calc_load_fold_active(this_rq, 0);
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if (delta)
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atomic_long_add(delta, &calc_load_tasks);
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this_rq->calc_load_update += LOAD_FREQ;
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}
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