Line data Source code
1 : // SPDX-License-Identifier: GPL-2.0
2 : /*
3 : * kernel/sched/loadavg.c
4 : *
5 : * This file contains the magic bits required to compute the global loadavg
6 : * figure. Its a silly number but people think its important. We go through
7 : * great pains to make it work on big machines and tickless kernels.
8 : */
9 :
10 : /*
11 : * Global load-average calculations
12 : *
13 : * We take a distributed and async approach to calculating the global load-avg
14 : * in order to minimize overhead.
15 : *
16 : * The global load average is an exponentially decaying average of nr_running +
17 : * nr_uninterruptible.
18 : *
19 : * Once every LOAD_FREQ:
20 : *
21 : * nr_active = 0;
22 : * for_each_possible_cpu(cpu)
23 : * nr_active += cpu_of(cpu)->nr_running + cpu_of(cpu)->nr_uninterruptible;
24 : *
25 : * avenrun[n] = avenrun[0] * exp_n + nr_active * (1 - exp_n)
26 : *
27 : * Due to a number of reasons the above turns in the mess below:
28 : *
29 : * - for_each_possible_cpu() is prohibitively expensive on machines with
30 : * serious number of CPUs, therefore we need to take a distributed approach
31 : * to calculating nr_active.
32 : *
33 : * \Sum_i x_i(t) = \Sum_i x_i(t) - x_i(t_0) | x_i(t_0) := 0
34 : * = \Sum_i { \Sum_j=1 x_i(t_j) - x_i(t_j-1) }
35 : *
36 : * So assuming nr_active := 0 when we start out -- true per definition, we
37 : * can simply take per-CPU deltas and fold those into a global accumulate
38 : * to obtain the same result. See calc_load_fold_active().
39 : *
40 : * Furthermore, in order to avoid synchronizing all per-CPU delta folding
41 : * across the machine, we assume 10 ticks is sufficient time for every
42 : * CPU to have completed this task.
43 : *
44 : * This places an upper-bound on the IRQ-off latency of the machine. Then
45 : * again, being late doesn't loose the delta, just wrecks the sample.
46 : *
47 : * - cpu_rq()->nr_uninterruptible isn't accurately tracked per-CPU because
48 : * this would add another cross-CPU cacheline miss and atomic operation
49 : * to the wakeup path. Instead we increment on whatever CPU the task ran
50 : * when it went into uninterruptible state and decrement on whatever CPU
51 : * did the wakeup. This means that only the sum of nr_uninterruptible over
52 : * all CPUs yields the correct result.
53 : *
54 : * This covers the NO_HZ=n code, for extra head-aches, see the comment below.
55 : */
56 :
57 : /* Variables and functions for calc_load */
58 : atomic_long_t calc_load_tasks;
59 : unsigned long calc_load_update;
60 : unsigned long avenrun[3];
61 : EXPORT_SYMBOL(avenrun); /* should be removed */
62 :
63 : /**
64 : * get_avenrun - get the load average array
65 : * @loads: pointer to dest load array
66 : * @offset: offset to add
67 : * @shift: shift count to shift the result left
68 : *
69 : * These values are estimates at best, so no need for locking.
70 : */
71 0 : void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
72 : {
73 0 : loads[0] = (avenrun[0] + offset) << shift;
74 0 : loads[1] = (avenrun[1] + offset) << shift;
75 0 : loads[2] = (avenrun[2] + offset) << shift;
76 0 : }
77 :
78 0 : long calc_load_fold_active(struct rq *this_rq, long adjust)
79 : {
80 5 : long nr_active, delta = 0;
81 :
82 5 : nr_active = this_rq->nr_running - adjust;
83 5 : nr_active += (int)this_rq->nr_uninterruptible;
84 :
85 5 : if (nr_active != this_rq->calc_load_active) {
86 1 : delta = nr_active - this_rq->calc_load_active;
87 1 : this_rq->calc_load_active = nr_active;
88 : }
89 :
90 0 : return delta;
91 : }
92 :
93 : /**
94 : * fixed_power_int - compute: x^n, in O(log n) time
95 : *
96 : * @x: base of the power
97 : * @frac_bits: fractional bits of @x
98 : * @n: power to raise @x to.
99 : *
100 : * By exploiting the relation between the definition of the natural power
101 : * function: x^n := x*x*...*x (x multiplied by itself for n times), and
102 : * the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
103 : * (where: n_i \elem {0, 1}, the binary vector representing n),
104 : * we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
105 : * of course trivially computable in O(log_2 n), the length of our binary
106 : * vector.
107 : */
108 : static unsigned long
109 : fixed_power_int(unsigned long x, unsigned int frac_bits, unsigned int n)
110 : {
111 0 : unsigned long result = 1UL << frac_bits;
112 :
113 0 : if (n) {
114 : for (;;) {
115 0 : if (n & 1) {
116 0 : result *= x;
117 0 : result += 1UL << (frac_bits - 1);
118 0 : result >>= frac_bits;
119 : }
120 0 : n >>= 1;
121 0 : if (!n)
122 : break;
123 0 : x *= x;
124 0 : x += 1UL << (frac_bits - 1);
125 0 : x >>= frac_bits;
126 : }
127 : }
128 :
129 : return result;
130 : }
131 :
132 : /*
133 : * a1 = a0 * e + a * (1 - e)
134 : *
135 : * a2 = a1 * e + a * (1 - e)
136 : * = (a0 * e + a * (1 - e)) * e + a * (1 - e)
137 : * = a0 * e^2 + a * (1 - e) * (1 + e)
138 : *
139 : * a3 = a2 * e + a * (1 - e)
140 : * = (a0 * e^2 + a * (1 - e) * (1 + e)) * e + a * (1 - e)
141 : * = a0 * e^3 + a * (1 - e) * (1 + e + e^2)
142 : *
143 : * ...
144 : *
145 : * an = a0 * e^n + a * (1 - e) * (1 + e + ... + e^n-1) [1]
146 : * = a0 * e^n + a * (1 - e) * (1 - e^n)/(1 - e)
147 : * = a0 * e^n + a * (1 - e^n)
148 : *
149 : * [1] application of the geometric series:
150 : *
151 : * n 1 - x^(n+1)
152 : * S_n := \Sum x^i = -------------
153 : * i=0 1 - x
154 : */
155 : unsigned long
156 0 : calc_load_n(unsigned long load, unsigned long exp,
157 : unsigned long active, unsigned int n)
158 : {
159 0 : return calc_load(load, fixed_power_int(exp, FSHIFT, n), active);
160 : }
161 :
162 : #ifdef CONFIG_NO_HZ_COMMON
163 : /*
164 : * Handle NO_HZ for the global load-average.
165 : *
166 : * Since the above described distributed algorithm to compute the global
167 : * load-average relies on per-CPU sampling from the tick, it is affected by
168 : * NO_HZ.
169 : *
170 : * The basic idea is to fold the nr_active delta into a global NO_HZ-delta upon
171 : * entering NO_HZ state such that we can include this as an 'extra' CPU delta
172 : * when we read the global state.
173 : *
174 : * Obviously reality has to ruin such a delightfully simple scheme:
175 : *
176 : * - When we go NO_HZ idle during the window, we can negate our sample
177 : * contribution, causing under-accounting.
178 : *
179 : * We avoid this by keeping two NO_HZ-delta counters and flipping them
180 : * when the window starts, thus separating old and new NO_HZ load.
181 : *
182 : * The only trick is the slight shift in index flip for read vs write.
183 : *
184 : * 0s 5s 10s 15s
185 : * +10 +10 +10 +10
186 : * |-|-----------|-|-----------|-|-----------|-|
187 : * r:0 0 1 1 0 0 1 1 0
188 : * w:0 1 1 0 0 1 1 0 0
189 : *
190 : * This ensures we'll fold the old NO_HZ contribution in this window while
191 : * accumulating the new one.
192 : *
193 : * - When we wake up from NO_HZ during the window, we push up our
194 : * contribution, since we effectively move our sample point to a known
195 : * busy state.
196 : *
197 : * This is solved by pushing the window forward, and thus skipping the
198 : * sample, for this CPU (effectively using the NO_HZ-delta for this CPU which
199 : * was in effect at the time the window opened). This also solves the issue
200 : * of having to deal with a CPU having been in NO_HZ for multiple LOAD_FREQ
201 : * intervals.
202 : *
203 : * When making the ILB scale, we should try to pull this in as well.
204 : */
205 : static atomic_long_t calc_load_nohz[2];
206 : static int calc_load_idx;
207 :
208 : static inline int calc_load_write_idx(void)
209 : {
210 : int idx = calc_load_idx;
211 :
212 : /*
213 : * See calc_global_nohz(), if we observe the new index, we also
214 : * need to observe the new update time.
215 : */
216 : smp_rmb();
217 :
218 : /*
219 : * If the folding window started, make sure we start writing in the
220 : * next NO_HZ-delta.
221 : */
222 : if (!time_before(jiffies, READ_ONCE(calc_load_update)))
223 : idx++;
224 :
225 : return idx & 1;
226 : }
227 :
228 : static inline int calc_load_read_idx(void)
229 : {
230 : return calc_load_idx & 1;
231 : }
232 :
233 : static void calc_load_nohz_fold(struct rq *rq)
234 : {
235 : long delta;
236 :
237 : delta = calc_load_fold_active(rq, 0);
238 : if (delta) {
239 : int idx = calc_load_write_idx();
240 :
241 : atomic_long_add(delta, &calc_load_nohz[idx]);
242 : }
243 : }
244 :
245 : void calc_load_nohz_start(void)
246 : {
247 : /*
248 : * We're going into NO_HZ mode, if there's any pending delta, fold it
249 : * into the pending NO_HZ delta.
250 : */
251 : calc_load_nohz_fold(this_rq());
252 : }
253 :
254 : /*
255 : * Keep track of the load for NOHZ_FULL, must be called between
256 : * calc_load_nohz_{start,stop}().
257 : */
258 : void calc_load_nohz_remote(struct rq *rq)
259 : {
260 : calc_load_nohz_fold(rq);
261 : }
262 :
263 : void calc_load_nohz_stop(void)
264 : {
265 : struct rq *this_rq = this_rq();
266 :
267 : /*
268 : * If we're still before the pending sample window, we're done.
269 : */
270 : this_rq->calc_load_update = READ_ONCE(calc_load_update);
271 : if (time_before(jiffies, this_rq->calc_load_update))
272 : return;
273 :
274 : /*
275 : * We woke inside or after the sample window, this means we're already
276 : * accounted through the nohz accounting, so skip the entire deal and
277 : * sync up for the next window.
278 : */
279 : if (time_before(jiffies, this_rq->calc_load_update + 10))
280 : this_rq->calc_load_update += LOAD_FREQ;
281 : }
282 :
283 : static long calc_load_nohz_read(void)
284 : {
285 : int idx = calc_load_read_idx();
286 : long delta = 0;
287 :
288 : if (atomic_long_read(&calc_load_nohz[idx]))
289 : delta = atomic_long_xchg(&calc_load_nohz[idx], 0);
290 :
291 : return delta;
292 : }
293 :
294 : /*
295 : * NO_HZ can leave us missing all per-CPU ticks calling
296 : * calc_load_fold_active(), but since a NO_HZ CPU folds its delta into
297 : * calc_load_nohz per calc_load_nohz_start(), all we need to do is fold
298 : * in the pending NO_HZ delta if our NO_HZ period crossed a load cycle boundary.
299 : *
300 : * Once we've updated the global active value, we need to apply the exponential
301 : * weights adjusted to the number of cycles missed.
302 : */
303 : static void calc_global_nohz(void)
304 : {
305 : unsigned long sample_window;
306 : long delta, active, n;
307 :
308 : sample_window = READ_ONCE(calc_load_update);
309 : if (!time_before(jiffies, sample_window + 10)) {
310 : /*
311 : * Catch-up, fold however many we are behind still
312 : */
313 : delta = jiffies - sample_window - 10;
314 : n = 1 + (delta / LOAD_FREQ);
315 :
316 : active = atomic_long_read(&calc_load_tasks);
317 : active = active > 0 ? active * FIXED_1 : 0;
318 :
319 : avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
320 : avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
321 : avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
322 :
323 : WRITE_ONCE(calc_load_update, sample_window + n * LOAD_FREQ);
324 : }
325 :
326 : /*
327 : * Flip the NO_HZ index...
328 : *
329 : * Make sure we first write the new time then flip the index, so that
330 : * calc_load_write_idx() will see the new time when it reads the new
331 : * index, this avoids a double flip messing things up.
332 : */
333 : smp_wmb();
334 : calc_load_idx++;
335 : }
336 : #else /* !CONFIG_NO_HZ_COMMON */
337 :
338 : static inline long calc_load_nohz_read(void) { return 0; }
339 : static inline void calc_global_nohz(void) { }
340 :
341 : #endif /* CONFIG_NO_HZ_COMMON */
342 :
343 : /*
344 : * calc_load - update the avenrun load estimates 10 ticks after the
345 : * CPUs have updated calc_load_tasks.
346 : *
347 : * Called from the global timer code.
348 : */
349 2943 : void calc_global_load(void)
350 : {
351 : unsigned long sample_window;
352 : long active, delta;
353 :
354 2943 : sample_window = READ_ONCE(calc_load_update);
355 2943 : if (time_before(jiffies, sample_window + 10))
356 : return;
357 :
358 : /*
359 : * Fold the 'old' NO_HZ-delta to include all NO_HZ CPUs.
360 : */
361 5 : delta = calc_load_nohz_read();
362 : if (delta)
363 : atomic_long_add(delta, &calc_load_tasks);
364 :
365 5 : active = atomic_long_read(&calc_load_tasks);
366 5 : active = active > 0 ? active * FIXED_1 : 0;
367 :
368 10 : avenrun[0] = calc_load(avenrun[0], EXP_1, active);
369 10 : avenrun[1] = calc_load(avenrun[1], EXP_5, active);
370 10 : avenrun[2] = calc_load(avenrun[2], EXP_15, active);
371 :
372 5 : WRITE_ONCE(calc_load_update, sample_window + LOAD_FREQ);
373 :
374 : /*
375 : * In case we went to NO_HZ for multiple LOAD_FREQ intervals
376 : * catch up in bulk.
377 : */
378 : calc_global_nohz();
379 : }
380 :
381 : /*
382 : * Called from scheduler_tick() to periodically update this CPU's
383 : * active count.
384 : */
385 2943 : void calc_global_load_tick(struct rq *this_rq)
386 : {
387 : long delta;
388 :
389 2943 : if (time_before(jiffies, this_rq->calc_load_update))
390 : return;
391 :
392 5 : delta = calc_load_fold_active(this_rq, 0);
393 5 : if (delta)
394 : atomic_long_add(delta, &calc_load_tasks);
395 :
396 5 : this_rq->calc_load_update += LOAD_FREQ;
397 : }
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