kernel / pub / scm / linux / kernel / git / keithp / linux / ef0a59924a795ccb4ced0ae1722a337745a1b045 / . / kernel / sched / proc.c

/* | |

* kernel/sched/proc.c | |

* | |

* Kernel load calculations, forked from sched/core.c | |

*/ | |

#include <linux/export.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 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 nr_active, delta = 0; | |

nr_active = this_rq->nr_running; | |

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; | |

} | |

/* | |

* a1 = a0 * e + a * (1 - e) | |

*/ | |

static unsigned long | |

calc_load(unsigned long load, unsigned long exp, unsigned long active) | |

{ | |

load *= exp; | |

load += active * (FIXED_1 - exp); | |

load += 1UL << (FSHIFT - 1); | |

return load >> FSHIFT; | |

} | |

#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 idle-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 idle-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 idle contribution in this window while | |

* accumlating the new one. | |

* | |

* - When we wake up from NO_HZ idle 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 idle-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 NOHZ idle 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_idle[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 idle-delta. | |

*/ | |

if (!time_before(jiffies, calc_load_update)) | |

idx++; | |

return idx & 1; | |

} | |

static inline int calc_load_read_idx(void) | |

{ | |

return calc_load_idx & 1; | |

} | |

void calc_load_enter_idle(void) | |

{ | |

struct rq *this_rq = this_rq(); | |

long delta; | |

/* | |

* We're going into NOHZ mode, if there's any pending delta, fold it | |

* into the pending idle delta. | |

*/ | |

delta = calc_load_fold_active(this_rq); | |

if (delta) { | |

int idx = calc_load_write_idx(); | |

atomic_long_add(delta, &calc_load_idle[idx]); | |

} | |

} | |

void calc_load_exit_idle(void) | |

{ | |

struct rq *this_rq = this_rq(); | |

/* | |

* If we're still before the sample window, we're done. | |

*/ | |

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. | |

*/ | |

this_rq->calc_load_update = calc_load_update; | |

if (time_before(jiffies, this_rq->calc_load_update + 10)) | |

this_rq->calc_load_update += LOAD_FREQ; | |

} | |

static long calc_load_fold_idle(void) | |

{ | |

int idx = calc_load_read_idx(); | |

long delta = 0; | |

if (atomic_long_read(&calc_load_idle[idx])) | |

delta = atomic_long_xchg(&calc_load_idle[idx], 0); | |

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 | |

*/ | |

static 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); | |

} | |

/* | |

* NO_HZ can leave us missing all per-cpu ticks calling | |

* calc_load_account_active(), but since an idle CPU folds its delta into | |

* calc_load_tasks_idle per calc_load_account_idle(), all we need to do is fold | |

* in the pending idle delta if our idle 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) | |

{ | |

long delta, active, n; | |

if (!time_before(jiffies, calc_load_update + 10)) { | |

/* | |

* Catch-up, fold however many we are behind still | |

*/ | |

delta = jiffies - calc_load_update - 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); | |

calc_load_update += n * LOAD_FREQ; | |

} | |

/* | |

* Flip the idle 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_fold_idle(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. | |

*/ | |

void calc_global_load(unsigned long ticks) | |

{ | |

long active, delta; | |

if (time_before(jiffies, calc_load_update + 10)) | |

return; | |

/* | |

* Fold the 'old' idle-delta to include all NO_HZ cpus. | |

*/ | |

delta = calc_load_fold_idle(); | |

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); | |

calc_load_update += LOAD_FREQ; | |

/* | |

* In case we idled for multiple LOAD_FREQ intervals, catch up in bulk. | |

*/ | |

calc_global_nohz(); | |

} | |

/* | |

* Called from update_cpu_load() to periodically update this CPU's | |

* active count. | |

*/ | |

static void calc_load_account_active(struct rq *this_rq) | |

{ | |

long delta; | |

if (time_before(jiffies, this_rq->calc_load_update)) | |

return; | |

delta = calc_load_fold_active(this_rq); | |

if (delta) | |

atomic_long_add(delta, &calc_load_tasks); | |

this_rq->calc_load_update += LOAD_FREQ; | |

} | |

/* | |

* End of global load-average stuff | |

*/ | |

/* | |

* The exact cpuload at various idx values, calculated at every tick would be | |

* load = (2^idx - 1) / 2^idx * load + 1 / 2^idx * cur_load | |

* | |

* If a cpu misses updates for n-1 ticks (as it was idle) and update gets called | |

* on nth tick when cpu may be busy, then we have: | |

* load = ((2^idx - 1) / 2^idx)^(n-1) * load | |

* load = (2^idx - 1) / 2^idx) * load + 1 / 2^idx * cur_load | |

* | |

* decay_load_missed() below does efficient calculation of | |

* load = ((2^idx - 1) / 2^idx)^(n-1) * load | |

* avoiding 0..n-1 loop doing load = ((2^idx - 1) / 2^idx) * load | |

* | |

* The calculation is approximated on a 128 point scale. | |

* degrade_zero_ticks is the number of ticks after which load at any | |

* particular idx is approximated to be zero. | |

* degrade_factor is a precomputed table, a row for each load idx. | |

* Each column corresponds to degradation factor for a power of two ticks, | |

* based on 128 point scale. | |

* Example: | |

* row 2, col 3 (=12) says that the degradation at load idx 2 after | |

* 8 ticks is 12/128 (which is an approximation of exact factor 3^8/4^8). | |

* | |

* With this power of 2 load factors, we can degrade the load n times | |

* by looking at 1 bits in n and doing as many mult/shift instead of | |

* n mult/shifts needed by the exact degradation. | |

*/ | |

#define DEGRADE_SHIFT 7 | |

static const unsigned char | |

degrade_zero_ticks[CPU_LOAD_IDX_MAX] = {0, 8, 32, 64, 128}; | |

static const unsigned char | |

degrade_factor[CPU_LOAD_IDX_MAX][DEGRADE_SHIFT + 1] = { | |

{0, 0, 0, 0, 0, 0, 0, 0}, | |

{64, 32, 8, 0, 0, 0, 0, 0}, | |

{96, 72, 40, 12, 1, 0, 0}, | |

{112, 98, 75, 43, 15, 1, 0}, | |

{120, 112, 98, 76, 45, 16, 2} }; | |

/* | |

* Update cpu_load for any missed ticks, due to tickless idle. The backlog | |

* would be when CPU is idle and so we just decay the old load without | |

* adding any new load. | |

*/ | |

static unsigned long | |

decay_load_missed(unsigned long load, unsigned long missed_updates, int idx) | |

{ | |

int j = 0; | |

if (!missed_updates) | |

return load; | |

if (missed_updates >= degrade_zero_ticks[idx]) | |

return 0; | |

if (idx == 1) | |

return load >> missed_updates; | |

while (missed_updates) { | |

if (missed_updates % 2) | |

load = (load * degrade_factor[idx][j]) >> DEGRADE_SHIFT; | |

missed_updates >>= 1; | |

j++; | |

} | |

return load; | |

} | |

/* | |

* Update rq->cpu_load[] statistics. This function is usually called every | |

* scheduler tick (TICK_NSEC). With tickless idle this will not be called | |

* every tick. We fix it up based on jiffies. | |

*/ | |

static void __update_cpu_load(struct rq *this_rq, unsigned long this_load, | |

unsigned long pending_updates) | |

{ | |

int i, scale; | |

this_rq->nr_load_updates++; | |

/* Update our load: */ | |

this_rq->cpu_load[0] = this_load; /* Fasttrack for idx 0 */ | |

for (i = 1, scale = 2; i < CPU_LOAD_IDX_MAX; i++, scale += scale) { | |

unsigned long old_load, new_load; | |

/* scale is effectively 1 << i now, and >> i divides by scale */ | |

old_load = this_rq->cpu_load[i]; | |

old_load = decay_load_missed(old_load, pending_updates - 1, i); | |

new_load = this_load; | |

/* | |

* Round up the averaging division if load is increasing. This | |

* prevents us from getting stuck on 9 if the load is 10, for | |

* example. | |

*/ | |

if (new_load > old_load) | |

new_load += scale - 1; | |

this_rq->cpu_load[i] = (old_load * (scale - 1) + new_load) >> i; | |

} | |

sched_avg_update(this_rq); | |

} | |

#ifdef CONFIG_SMP | |

static inline unsigned long get_rq_runnable_load(struct rq *rq) | |

{ | |

return rq->cfs.runnable_load_avg; | |

} | |

#else | |

static inline unsigned long get_rq_runnable_load(struct rq *rq) | |

{ | |

return rq->load.weight; | |

} | |

#endif | |

#ifdef CONFIG_NO_HZ_COMMON | |

/* | |

* There is no sane way to deal with nohz on smp when using jiffies because the | |

* cpu doing the jiffies update might drift wrt the cpu doing the jiffy reading | |

* causing off-by-one errors in observed deltas; {0,2} instead of {1,1}. | |

* | |

* Therefore we cannot use the delta approach from the regular tick since that | |

* would seriously skew the load calculation. However we'll make do for those | |

* updates happening while idle (nohz_idle_balance) or coming out of idle | |

* (tick_nohz_idle_exit). | |

* | |

* This means we might still be one tick off for nohz periods. | |

*/ | |

/* | |

* Called from nohz_idle_balance() to update the load ratings before doing the | |

* idle balance. | |

*/ | |

void update_idle_cpu_load(struct rq *this_rq) | |

{ | |

unsigned long curr_jiffies = ACCESS_ONCE(jiffies); | |

unsigned long load = get_rq_runnable_load(this_rq); | |

unsigned long pending_updates; | |

/* | |

* bail if there's load or we're actually up-to-date. | |

*/ | |

if (load || curr_jiffies == this_rq->last_load_update_tick) | |

return; | |

pending_updates = curr_jiffies - this_rq->last_load_update_tick; | |

this_rq->last_load_update_tick = curr_jiffies; | |

__update_cpu_load(this_rq, load, pending_updates); | |

} | |

/* | |

* Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed. | |

*/ | |

void update_cpu_load_nohz(void) | |

{ | |

struct rq *this_rq = this_rq(); | |

unsigned long curr_jiffies = ACCESS_ONCE(jiffies); | |

unsigned long pending_updates; | |

if (curr_jiffies == this_rq->last_load_update_tick) | |

return; | |

raw_spin_lock(&this_rq->lock); | |

pending_updates = curr_jiffies - this_rq->last_load_update_tick; | |

if (pending_updates) { | |

this_rq->last_load_update_tick = curr_jiffies; | |

/* | |

* We were idle, this means load 0, the current load might be | |

* !0 due to remote wakeups and the sort. | |

*/ | |

__update_cpu_load(this_rq, 0, pending_updates); | |

} | |

raw_spin_unlock(&this_rq->lock); | |

} | |

#endif /* CONFIG_NO_HZ */ | |

/* | |

* Called from scheduler_tick() | |

*/ | |

void update_cpu_load_active(struct rq *this_rq) | |

{ | |

unsigned long load = get_rq_runnable_load(this_rq); | |

/* | |

* See the mess around update_idle_cpu_load() / update_cpu_load_nohz(). | |

*/ | |

this_rq->last_load_update_tick = jiffies; | |

__update_cpu_load(this_rq, load, 1); | |

calc_load_account_active(this_rq); | |

} |