xref: /openbmc/linux/drivers/cpuidle/governors/menu.c (revision 4800cd83)
1 /*
2  * menu.c - the menu idle governor
3  *
4  * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
5  * Copyright (C) 2009 Intel Corporation
6  * Author:
7  *        Arjan van de Ven <arjan@linux.intel.com>
8  *
9  * This code is licenced under the GPL version 2 as described
10  * in the COPYING file that acompanies the Linux Kernel.
11  */
12 
13 #include <linux/kernel.h>
14 #include <linux/cpuidle.h>
15 #include <linux/pm_qos_params.h>
16 #include <linux/time.h>
17 #include <linux/ktime.h>
18 #include <linux/hrtimer.h>
19 #include <linux/tick.h>
20 #include <linux/sched.h>
21 #include <linux/math64.h>
22 
23 #define BUCKETS 12
24 #define INTERVALS 8
25 #define RESOLUTION 1024
26 #define DECAY 8
27 #define MAX_INTERESTING 50000
28 #define STDDEV_THRESH 400
29 
30 
31 /*
32  * Concepts and ideas behind the menu governor
33  *
34  * For the menu governor, there are 3 decision factors for picking a C
35  * state:
36  * 1) Energy break even point
37  * 2) Performance impact
38  * 3) Latency tolerance (from pmqos infrastructure)
39  * These these three factors are treated independently.
40  *
41  * Energy break even point
42  * -----------------------
43  * C state entry and exit have an energy cost, and a certain amount of time in
44  * the  C state is required to actually break even on this cost. CPUIDLE
45  * provides us this duration in the "target_residency" field. So all that we
46  * need is a good prediction of how long we'll be idle. Like the traditional
47  * menu governor, we start with the actual known "next timer event" time.
48  *
49  * Since there are other source of wakeups (interrupts for example) than
50  * the next timer event, this estimation is rather optimistic. To get a
51  * more realistic estimate, a correction factor is applied to the estimate,
52  * that is based on historic behavior. For example, if in the past the actual
53  * duration always was 50% of the next timer tick, the correction factor will
54  * be 0.5.
55  *
56  * menu uses a running average for this correction factor, however it uses a
57  * set of factors, not just a single factor. This stems from the realization
58  * that the ratio is dependent on the order of magnitude of the expected
59  * duration; if we expect 500 milliseconds of idle time the likelihood of
60  * getting an interrupt very early is much higher than if we expect 50 micro
61  * seconds of idle time. A second independent factor that has big impact on
62  * the actual factor is if there is (disk) IO outstanding or not.
63  * (as a special twist, we consider every sleep longer than 50 milliseconds
64  * as perfect; there are no power gains for sleeping longer than this)
65  *
66  * For these two reasons we keep an array of 12 independent factors, that gets
67  * indexed based on the magnitude of the expected duration as well as the
68  * "is IO outstanding" property.
69  *
70  * Repeatable-interval-detector
71  * ----------------------------
72  * There are some cases where "next timer" is a completely unusable predictor:
73  * Those cases where the interval is fixed, for example due to hardware
74  * interrupt mitigation, but also due to fixed transfer rate devices such as
75  * mice.
76  * For this, we use a different predictor: We track the duration of the last 8
77  * intervals and if the stand deviation of these 8 intervals is below a
78  * threshold value, we use the average of these intervals as prediction.
79  *
80  * Limiting Performance Impact
81  * ---------------------------
82  * C states, especially those with large exit latencies, can have a real
83  * noticeable impact on workloads, which is not acceptable for most sysadmins,
84  * and in addition, less performance has a power price of its own.
85  *
86  * As a general rule of thumb, menu assumes that the following heuristic
87  * holds:
88  *     The busier the system, the less impact of C states is acceptable
89  *
90  * This rule-of-thumb is implemented using a performance-multiplier:
91  * If the exit latency times the performance multiplier is longer than
92  * the predicted duration, the C state is not considered a candidate
93  * for selection due to a too high performance impact. So the higher
94  * this multiplier is, the longer we need to be idle to pick a deep C
95  * state, and thus the less likely a busy CPU will hit such a deep
96  * C state.
97  *
98  * Two factors are used in determing this multiplier:
99  * a value of 10 is added for each point of "per cpu load average" we have.
100  * a value of 5 points is added for each process that is waiting for
101  * IO on this CPU.
102  * (these values are experimentally determined)
103  *
104  * The load average factor gives a longer term (few seconds) input to the
105  * decision, while the iowait value gives a cpu local instantanious input.
106  * The iowait factor may look low, but realize that this is also already
107  * represented in the system load average.
108  *
109  */
110 
111 struct menu_device {
112 	int		last_state_idx;
113 	int             needs_update;
114 
115 	unsigned int	expected_us;
116 	u64		predicted_us;
117 	unsigned int	exit_us;
118 	unsigned int	bucket;
119 	u64		correction_factor[BUCKETS];
120 	u32		intervals[INTERVALS];
121 	int		interval_ptr;
122 };
123 
124 
125 #define LOAD_INT(x) ((x) >> FSHIFT)
126 #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
127 
128 static int get_loadavg(void)
129 {
130 	unsigned long this = this_cpu_load();
131 
132 
133 	return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
134 }
135 
136 static inline int which_bucket(unsigned int duration)
137 {
138 	int bucket = 0;
139 
140 	/*
141 	 * We keep two groups of stats; one with no
142 	 * IO pending, one without.
143 	 * This allows us to calculate
144 	 * E(duration)|iowait
145 	 */
146 	if (nr_iowait_cpu(smp_processor_id()))
147 		bucket = BUCKETS/2;
148 
149 	if (duration < 10)
150 		return bucket;
151 	if (duration < 100)
152 		return bucket + 1;
153 	if (duration < 1000)
154 		return bucket + 2;
155 	if (duration < 10000)
156 		return bucket + 3;
157 	if (duration < 100000)
158 		return bucket + 4;
159 	return bucket + 5;
160 }
161 
162 /*
163  * Return a multiplier for the exit latency that is intended
164  * to take performance requirements into account.
165  * The more performance critical we estimate the system
166  * to be, the higher this multiplier, and thus the higher
167  * the barrier to go to an expensive C state.
168  */
169 static inline int performance_multiplier(void)
170 {
171 	int mult = 1;
172 
173 	/* for higher loadavg, we are more reluctant */
174 
175 	mult += 2 * get_loadavg();
176 
177 	/* for IO wait tasks (per cpu!) we add 5x each */
178 	mult += 10 * nr_iowait_cpu(smp_processor_id());
179 
180 	return mult;
181 }
182 
183 static DEFINE_PER_CPU(struct menu_device, menu_devices);
184 
185 static void menu_update(struct cpuidle_device *dev);
186 
187 /* This implements DIV_ROUND_CLOSEST but avoids 64 bit division */
188 static u64 div_round64(u64 dividend, u32 divisor)
189 {
190 	return div_u64(dividend + (divisor / 2), divisor);
191 }
192 
193 /*
194  * Try detecting repeating patterns by keeping track of the last 8
195  * intervals, and checking if the standard deviation of that set
196  * of points is below a threshold. If it is... then use the
197  * average of these 8 points as the estimated value.
198  */
199 static void detect_repeating_patterns(struct menu_device *data)
200 {
201 	int i;
202 	uint64_t avg = 0;
203 	uint64_t stddev = 0; /* contains the square of the std deviation */
204 
205 	/* first calculate average and standard deviation of the past */
206 	for (i = 0; i < INTERVALS; i++)
207 		avg += data->intervals[i];
208 	avg = avg / INTERVALS;
209 
210 	/* if the avg is beyond the known next tick, it's worthless */
211 	if (avg > data->expected_us)
212 		return;
213 
214 	for (i = 0; i < INTERVALS; i++)
215 		stddev += (data->intervals[i] - avg) *
216 			  (data->intervals[i] - avg);
217 
218 	stddev = stddev / INTERVALS;
219 
220 	/*
221 	 * now.. if stddev is small.. then assume we have a
222 	 * repeating pattern and predict we keep doing this.
223 	 */
224 
225 	if (avg && stddev < STDDEV_THRESH)
226 		data->predicted_us = avg;
227 }
228 
229 /**
230  * menu_select - selects the next idle state to enter
231  * @dev: the CPU
232  */
233 static int menu_select(struct cpuidle_device *dev)
234 {
235 	struct menu_device *data = &__get_cpu_var(menu_devices);
236 	int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
237 	unsigned int power_usage = -1;
238 	int i;
239 	int multiplier;
240 
241 	if (data->needs_update) {
242 		menu_update(dev);
243 		data->needs_update = 0;
244 	}
245 
246 	data->last_state_idx = 0;
247 	data->exit_us = 0;
248 
249 	/* Special case when user has set very strict latency requirement */
250 	if (unlikely(latency_req == 0))
251 		return 0;
252 
253 	/* determine the expected residency time, round up */
254 	data->expected_us =
255 	    DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000);
256 
257 
258 	data->bucket = which_bucket(data->expected_us);
259 
260 	multiplier = performance_multiplier();
261 
262 	/*
263 	 * if the correction factor is 0 (eg first time init or cpu hotplug
264 	 * etc), we actually want to start out with a unity factor.
265 	 */
266 	if (data->correction_factor[data->bucket] == 0)
267 		data->correction_factor[data->bucket] = RESOLUTION * DECAY;
268 
269 	/* Make sure to round up for half microseconds */
270 	data->predicted_us = div_round64(data->expected_us * data->correction_factor[data->bucket],
271 					 RESOLUTION * DECAY);
272 
273 	detect_repeating_patterns(data);
274 
275 	/*
276 	 * We want to default to C1 (hlt), not to busy polling
277 	 * unless the timer is happening really really soon.
278 	 */
279 	if (data->expected_us > 5)
280 		data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
281 
282 	/*
283 	 * Find the idle state with the lowest power while satisfying
284 	 * our constraints.
285 	 */
286 	for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {
287 		struct cpuidle_state *s = &dev->states[i];
288 
289 		if (s->flags & CPUIDLE_FLAG_IGNORE)
290 			continue;
291 		if (s->target_residency > data->predicted_us)
292 			continue;
293 		if (s->exit_latency > latency_req)
294 			continue;
295 		if (s->exit_latency * multiplier > data->predicted_us)
296 			continue;
297 
298 		if (s->power_usage < power_usage) {
299 			power_usage = s->power_usage;
300 			data->last_state_idx = i;
301 			data->exit_us = s->exit_latency;
302 		}
303 	}
304 
305 	return data->last_state_idx;
306 }
307 
308 /**
309  * menu_reflect - records that data structures need update
310  * @dev: the CPU
311  *
312  * NOTE: it's important to be fast here because this operation will add to
313  *       the overall exit latency.
314  */
315 static void menu_reflect(struct cpuidle_device *dev)
316 {
317 	struct menu_device *data = &__get_cpu_var(menu_devices);
318 	data->needs_update = 1;
319 }
320 
321 /**
322  * menu_update - attempts to guess what happened after entry
323  * @dev: the CPU
324  */
325 static void menu_update(struct cpuidle_device *dev)
326 {
327 	struct menu_device *data = &__get_cpu_var(menu_devices);
328 	int last_idx = data->last_state_idx;
329 	unsigned int last_idle_us = cpuidle_get_last_residency(dev);
330 	struct cpuidle_state *target = &dev->states[last_idx];
331 	unsigned int measured_us;
332 	u64 new_factor;
333 
334 	/*
335 	 * Ugh, this idle state doesn't support residency measurements, so we
336 	 * are basically lost in the dark.  As a compromise, assume we slept
337 	 * for the whole expected time.
338 	 */
339 	if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
340 		last_idle_us = data->expected_us;
341 
342 
343 	measured_us = last_idle_us;
344 
345 	/*
346 	 * We correct for the exit latency; we are assuming here that the
347 	 * exit latency happens after the event that we're interested in.
348 	 */
349 	if (measured_us > data->exit_us)
350 		measured_us -= data->exit_us;
351 
352 
353 	/* update our correction ratio */
354 
355 	new_factor = data->correction_factor[data->bucket]
356 			* (DECAY - 1) / DECAY;
357 
358 	if (data->expected_us > 0 && measured_us < MAX_INTERESTING)
359 		new_factor += RESOLUTION * measured_us / data->expected_us;
360 	else
361 		/*
362 		 * we were idle so long that we count it as a perfect
363 		 * prediction
364 		 */
365 		new_factor += RESOLUTION;
366 
367 	/*
368 	 * We don't want 0 as factor; we always want at least
369 	 * a tiny bit of estimated time.
370 	 */
371 	if (new_factor == 0)
372 		new_factor = 1;
373 
374 	data->correction_factor[data->bucket] = new_factor;
375 
376 	/* update the repeating-pattern data */
377 	data->intervals[data->interval_ptr++] = last_idle_us;
378 	if (data->interval_ptr >= INTERVALS)
379 		data->interval_ptr = 0;
380 }
381 
382 /**
383  * menu_enable_device - scans a CPU's states and does setup
384  * @dev: the CPU
385  */
386 static int menu_enable_device(struct cpuidle_device *dev)
387 {
388 	struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
389 
390 	memset(data, 0, sizeof(struct menu_device));
391 
392 	return 0;
393 }
394 
395 static struct cpuidle_governor menu_governor = {
396 	.name =		"menu",
397 	.rating =	20,
398 	.enable =	menu_enable_device,
399 	.select =	menu_select,
400 	.reflect =	menu_reflect,
401 	.owner =	THIS_MODULE,
402 };
403 
404 /**
405  * init_menu - initializes the governor
406  */
407 static int __init init_menu(void)
408 {
409 	return cpuidle_register_governor(&menu_governor);
410 }
411 
412 /**
413  * exit_menu - exits the governor
414  */
415 static void __exit exit_menu(void)
416 {
417 	cpuidle_unregister_governor(&menu_governor);
418 }
419 
420 MODULE_LICENSE("GPL");
421 module_init(init_menu);
422 module_exit(exit_menu);
423