mirror of https://github.com/Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
398 lines
11 KiB
398 lines
11 KiB
// 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 += (int)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 |
|
* 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 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; |
|
}
|
|
|