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-rw-r--r--kernel/sched/loadavg.c398
1 files changed, 398 insertions, 0 deletions
diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
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+// SPDX-License-Identifier: GPL-2.0
+/*
+ * kernel/sched/loadavg.c
+ *
+ * This file contains the magic bits required to compute the global loadavg
+ * figure. Its a silly number but people think its important. We go through
+ * great pains to make it work on big machines and tickless kernels.
+ */
+#include "sched.h"
+
+/*
+ * Global load-average calculations
+ *
+ * We take a distributed and async approach to calculating the global load-avg
+ * in order to minimize overhead.
+ *
+ * The global load average is an exponentially decaying average of nr_running +
+ * nr_uninterruptible.
+ *
+ * Once every LOAD_FREQ:
+ *
+ * nr_active = 0;
+ * for_each_possible_cpu(cpu)
+ * nr_active += cpu_of(cpu)->nr_running + cpu_of(cpu)->nr_uninterruptible;
+ *
+ * avenrun[n] = avenrun[0] * exp_n + nr_active * (1 - exp_n)
+ *
+ * Due to a number of reasons the above turns in the mess below:
+ *
+ * - for_each_possible_cpu() is prohibitively expensive on machines with
+ * serious number of CPUs, therefore we need to take a distributed approach
+ * to calculating nr_active.
+ *
+ * \Sum_i x_i(t) = \Sum_i x_i(t) - x_i(t_0) | x_i(t_0) := 0
+ * = \Sum_i { \Sum_j=1 x_i(t_j) - x_i(t_j-1) }
+ *
+ * So assuming nr_active := 0 when we start out -- true per definition, we
+ * can simply take per-CPU deltas and fold those into a global accumulate
+ * to obtain the same result. See calc_load_fold_active().
+ *
+ * Furthermore, in order to avoid synchronizing all per-CPU delta folding
+ * across the machine, we assume 10 ticks is sufficient time for every
+ * CPU to have completed this task.
+ *
+ * This places an upper-bound on the IRQ-off latency of the machine. Then
+ * again, being late doesn't loose the delta, just wrecks the sample.
+ *
+ * - cpu_rq()->nr_uninterruptible isn't accurately tracked per-CPU because
+ * this would add another cross-CPU cacheline miss and atomic operation
+ * to the wakeup path. Instead we increment on whatever CPU the task ran
+ * when it went into uninterruptible state and decrement on whatever CPU
+ * did the wakeup. This means that only the sum of nr_uninterruptible over
+ * all CPUs yields the correct result.
+ *
+ * This covers the NO_HZ=n code, for extra head-aches, see the comment below.
+ */
+
+/* Variables and functions for calc_load */
+atomic_long_t calc_load_tasks;
+unsigned long calc_load_update;
+unsigned long avenrun[3];
+EXPORT_SYMBOL(avenrun); /* should be removed */
+
+/**
+ * get_avenrun - get the load average array
+ * @loads: pointer to dest load array
+ * @offset: offset to add
+ * @shift: shift count to shift the result left
+ *
+ * These values are estimates at best, so no need for locking.
+ */
+void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
+{
+ loads[0] = (avenrun[0] + offset) << shift;
+ loads[1] = (avenrun[1] + offset) << shift;
+ loads[2] = (avenrun[2] + offset) << shift;
+}
+
+long calc_load_fold_active(struct rq *this_rq, long adjust)
+{
+ long nr_active, delta = 0;
+
+ nr_active = this_rq->nr_running - adjust;
+ nr_active += (long)this_rq->nr_uninterruptible;
+
+ if (nr_active != this_rq->calc_load_active) {
+ delta = nr_active - this_rq->calc_load_active;
+ this_rq->calc_load_active = nr_active;
+ }
+
+ return delta;
+}
+
+/**
+ * fixed_power_int - 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.
+ */
+static unsigned long
+fixed_power_int(unsigned long 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;
+}
+
+/*
+ * a1 = a0 * e + a * (1 - e)
+ *
+ * a2 = a1 * e + a * (1 - e)
+ * = (a0 * e + a * (1 - e)) * e + a * (1 - e)
+ * = a0 * e^2 + a * (1 - e) * (1 + e)
+ *
+ * a3 = a2 * e + a * (1 - e)
+ * = (a0 * e^2 + a * (1 - e) * (1 + e)) * e + a * (1 - e)
+ * = a0 * e^3 + a * (1 - e) * (1 + e + e^2)
+ *
+ * ...
+ *
+ * an = a0 * e^n + a * (1 - e) * (1 + e + ... + e^n-1) [1]
+ * = a0 * e^n + a * (1 - e) * (1 - e^n)/(1 - e)
+ * = a0 * e^n + a * (1 - e^n)
+ *
+ * [1] application of the geometric series:
+ *
+ * n 1 - x^(n+1)
+ * S_n := \Sum x^i = -------------
+ * i=0 1 - x
+ */
+unsigned long
+calc_load_n(unsigned long load, unsigned long exp,
+ unsigned long active, unsigned int n)
+{
+ return calc_load(load, fixed_power_int(exp, FSHIFT, n), active);
+}
+
+#ifdef CONFIG_NO_HZ_COMMON
+/*
+ * Handle NO_HZ for the global load-average.
+ *
+ * Since the above described distributed algorithm to compute the global
+ * load-average relies on per-CPU sampling from the tick, it is affected by
+ * NO_HZ.
+ *
+ * The basic idea is to fold the nr_active delta into a global NO_HZ-delta upon
+ * entering NO_HZ state such that we can include this as an 'extra' CPU delta
+ * when we read the global state.
+ *
+ * Obviously reality has to ruin such a delightfully simple scheme:
+ *
+ * - When we go NO_HZ idle during the window, we can negate our sample
+ * contribution, causing under-accounting.
+ *
+ * We avoid this by keeping two NO_HZ-delta counters and flipping them
+ * when the window starts, thus separating old and new NO_HZ load.
+ *
+ * The only trick is the slight shift in index flip for read vs write.
+ *
+ * 0s 5s 10s 15s
+ * +10 +10 +10 +10
+ * |-|-----------|-|-----------|-|-----------|-|
+ * r:0 0 1 1 0 0 1 1 0
+ * w:0 1 1 0 0 1 1 0 0
+ *
+ * This ensures we'll fold the old NO_HZ contribution in this window while
+ * accumlating the new one.
+ *
+ * - When we wake up from NO_HZ during the window, we push up our
+ * contribution, since we effectively move our sample point to a known
+ * busy state.
+ *
+ * This is solved by pushing the window forward, and thus skipping the
+ * sample, for this CPU (effectively using the NO_HZ-delta for this CPU which
+ * was in effect at the time the window opened). This also solves the issue
+ * of having to deal with a CPU having been in NO_HZ for multiple LOAD_FREQ
+ * intervals.
+ *
+ * When making the ILB scale, we should try to pull this in as well.
+ */
+static atomic_long_t calc_load_nohz[2];
+static int calc_load_idx;
+
+static inline int calc_load_write_idx(void)
+{
+ int idx = calc_load_idx;
+
+ /*
+ * See calc_global_nohz(), if we observe the new index, we also
+ * need to observe the new update time.
+ */
+ smp_rmb();
+
+ /*
+ * If the folding window started, make sure we start writing in the
+ * next NO_HZ-delta.
+ */
+ if (!time_before(jiffies, READ_ONCE(calc_load_update)))
+ idx++;
+
+ return idx & 1;
+}
+
+static inline int calc_load_read_idx(void)
+{
+ return calc_load_idx & 1;
+}
+
+static void calc_load_nohz_fold(struct rq *rq)
+{
+ long delta;
+
+ delta = calc_load_fold_active(rq, 0);
+ if (delta) {
+ int idx = calc_load_write_idx();
+
+ atomic_long_add(delta, &calc_load_nohz[idx]);
+ }
+}
+
+void calc_load_nohz_start(void)
+{
+ /*
+ * We're going into NO_HZ mode, if there's any pending delta, fold it
+ * into the pending NO_HZ delta.
+ */
+ calc_load_nohz_fold(this_rq());
+}
+
+/*
+ * Keep track of the load for NOHZ_FULL, must be called between
+ * calc_load_nohz_{start,stop}().
+ */
+void calc_load_nohz_remote(struct rq *rq)
+{
+ calc_load_nohz_fold(rq);
+}
+
+void calc_load_nohz_stop(void)
+{
+ struct rq *this_rq = this_rq();
+
+ /*
+ * If we're still before the pending sample window, we're done.
+ */
+ this_rq->calc_load_update = READ_ONCE(calc_load_update);
+ if (time_before(jiffies, this_rq->calc_load_update))
+ return;
+
+ /*
+ * We woke inside or after the sample window, this means we're already
+ * accounted through the nohz accounting, so skip the entire deal and
+ * sync up for the next window.
+ */
+ if (time_before(jiffies, this_rq->calc_load_update + 10))
+ this_rq->calc_load_update += LOAD_FREQ;
+}
+
+static long calc_load_nohz_read(void)
+{
+ int idx = calc_load_read_idx();
+ long delta = 0;
+
+ if (atomic_long_read(&calc_load_nohz[idx]))
+ delta = atomic_long_xchg(&calc_load_nohz[idx], 0);
+
+ return delta;
+}
+
+/*
+ * NO_HZ can leave us missing all per-CPU ticks calling
+ * calc_load_fold_active(), but since a NO_HZ CPU folds its delta into
+ * calc_load_nohz per calc_load_nohz_start(), all we need to do is fold
+ * in the pending NO_HZ delta if our NO_HZ period crossed a load cycle boundary.
+ *
+ * Once we've updated the global active value, we need to apply the exponential
+ * weights adjusted to the number of cycles missed.
+ */
+static void calc_global_nohz(void)
+{
+ unsigned long sample_window;
+ long delta, active, n;
+
+ sample_window = READ_ONCE(calc_load_update);
+ if (!time_before(jiffies, sample_window + 10)) {
+ /*
+ * Catch-up, fold however many we are behind still
+ */
+ delta = jiffies - sample_window - 10;
+ n = 1 + (delta / LOAD_FREQ);
+
+ active = atomic_long_read(&calc_load_tasks);
+ active = active > 0 ? active * FIXED_1 : 0;
+
+ avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
+ avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
+ avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
+
+ WRITE_ONCE(calc_load_update, sample_window + n * LOAD_FREQ);
+ }
+
+ /*
+ * Flip the NO_HZ index...
+ *
+ * Make sure we first write the new time then flip the index, so that
+ * calc_load_write_idx() will see the new time when it reads the new
+ * index, this avoids a double flip messing things up.
+ */
+ smp_wmb();
+ calc_load_idx++;
+}
+#else /* !CONFIG_NO_HZ_COMMON */
+
+static inline long calc_load_nohz_read(void) { return 0; }
+static inline void calc_global_nohz(void) { }
+
+#endif /* CONFIG_NO_HZ_COMMON */
+
+/*
+ * calc_load - update the avenrun load estimates 10 ticks after the
+ * CPUs have updated calc_load_tasks.
+ *
+ * Called from the global timer code.
+ */
+void calc_global_load(void)
+{
+ unsigned long sample_window;
+ long active, delta;
+
+ sample_window = READ_ONCE(calc_load_update);
+ if (time_before(jiffies, sample_window + 10))
+ return;
+
+ /*
+ * Fold the 'old' NO_HZ-delta to include all NO_HZ CPUs.
+ */
+ delta = calc_load_nohz_read();
+ if (delta)
+ atomic_long_add(delta, &calc_load_tasks);
+
+ active = atomic_long_read(&calc_load_tasks);
+ active = active > 0 ? active * FIXED_1 : 0;
+
+ avenrun[0] = calc_load(avenrun[0], EXP_1, active);
+ avenrun[1] = calc_load(avenrun[1], EXP_5, active);
+ avenrun[2] = calc_load(avenrun[2], EXP_15, active);
+
+ WRITE_ONCE(calc_load_update, sample_window + LOAD_FREQ);
+
+ /*
+ * In case we went to NO_HZ for multiple LOAD_FREQ intervals
+ * catch up in bulk.
+ */
+ calc_global_nohz();
+}
+
+/*
+ * Called from scheduler_tick() to periodically update this CPU's
+ * active count.
+ */
+void calc_global_load_tick(struct rq *this_rq)
+{
+ long delta;
+
+ if (time_before(jiffies, this_rq->calc_load_update))
+ return;
+
+ delta = calc_load_fold_active(this_rq, 0);
+ if (delta)
+ atomic_long_add(delta, &calc_load_tasks);
+
+ this_rq->calc_load_update += LOAD_FREQ;
+}
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