// 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 <linux/sched/nohz.h>#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 cache-line 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 destination 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 longfixed_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 longcalc_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 *    accumulating 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 sched_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;}