| // 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; |
| } |
| EXPORT_SYMBOL_GPL(get_avenrun); |
| |
| 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(unsigned long ticks) |
| { |
| 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; |
| } |