2 * menu.c - the menu idle governor
4 * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
5 * Copyright (C) 2009 Intel Corporation
7 * Arjan van de Ven <arjan@linux.intel.com>
9 * This code is licenced under the GPL version 2 as described
10 * in the COPYING file that acompanies the Linux Kernel.
13 #include <linux/kernel.h>
14 #include <linux/cpuidle.h>
15 #include <linux/pm_qos.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 #include <linux/module.h>
26 #define RESOLUTION 1024
28 #define MAX_INTERESTING 50000
29 #define STDDEV_THRESH 400
33 * Concepts and ideas behind the menu governor
35 * For the menu governor, there are 3 decision factors for picking a C
37 * 1) Energy break even point
38 * 2) Performance impact
39 * 3) Latency tolerance (from pmqos infrastructure)
40 * These these three factors are treated independently.
42 * Energy break even point
43 * -----------------------
44 * C state entry and exit have an energy cost, and a certain amount of time in
45 * the C state is required to actually break even on this cost. CPUIDLE
46 * provides us this duration in the "target_residency" field. So all that we
47 * need is a good prediction of how long we'll be idle. Like the traditional
48 * menu governor, we start with the actual known "next timer event" time.
50 * Since there are other source of wakeups (interrupts for example) than
51 * the next timer event, this estimation is rather optimistic. To get a
52 * more realistic estimate, a correction factor is applied to the estimate,
53 * that is based on historic behavior. For example, if in the past the actual
54 * duration always was 50% of the next timer tick, the correction factor will
57 * menu uses a running average for this correction factor, however it uses a
58 * set of factors, not just a single factor. This stems from the realization
59 * that the ratio is dependent on the order of magnitude of the expected
60 * duration; if we expect 500 milliseconds of idle time the likelihood of
61 * getting an interrupt very early is much higher than if we expect 50 micro
62 * seconds of idle time. A second independent factor that has big impact on
63 * the actual factor is if there is (disk) IO outstanding or not.
64 * (as a special twist, we consider every sleep longer than 50 milliseconds
65 * as perfect; there are no power gains for sleeping longer than this)
67 * For these two reasons we keep an array of 12 independent factors, that gets
68 * indexed based on the magnitude of the expected duration as well as the
69 * "is IO outstanding" property.
71 * Repeatable-interval-detector
72 * ----------------------------
73 * There are some cases where "next timer" is a completely unusable predictor:
74 * Those cases where the interval is fixed, for example due to hardware
75 * interrupt mitigation, but also due to fixed transfer rate devices such as
77 * For this, we use a different predictor: We track the duration of the last 8
78 * intervals and if the stand deviation of these 8 intervals is below a
79 * threshold value, we use the average of these intervals as prediction.
81 * Limiting Performance Impact
82 * ---------------------------
83 * C states, especially those with large exit latencies, can have a real
84 * noticeable impact on workloads, which is not acceptable for most sysadmins,
85 * and in addition, less performance has a power price of its own.
87 * As a general rule of thumb, menu assumes that the following heuristic
89 * The busier the system, the less impact of C states is acceptable
91 * This rule-of-thumb is implemented using a performance-multiplier:
92 * If the exit latency times the performance multiplier is longer than
93 * the predicted duration, the C state is not considered a candidate
94 * for selection due to a too high performance impact. So the higher
95 * this multiplier is, the longer we need to be idle to pick a deep C
96 * state, and thus the less likely a busy CPU will hit such a deep
99 * Two factors are used in determing this multiplier:
100 * a value of 10 is added for each point of "per cpu load average" we have.
101 * a value of 5 points is added for each process that is waiting for
103 * (these values are experimentally determined)
105 * The load average factor gives a longer term (few seconds) input to the
106 * decision, while the iowait value gives a cpu local instantanious input.
107 * The iowait factor may look low, but realize that this is also already
108 * represented in the system load average.
116 unsigned int expected_us;
118 unsigned int exit_us;
120 u64 correction_factor[BUCKETS];
121 u32 intervals[INTERVALS];
126 #define LOAD_INT(x) ((x) >> FSHIFT)
127 #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
129 static int get_loadavg(void)
131 unsigned long this = this_cpu_load();
134 return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
137 static inline int which_bucket(unsigned int duration)
142 * We keep two groups of stats; one with no
143 * IO pending, one without.
144 * This allows us to calculate
147 if (nr_iowait_cpu(smp_processor_id()))
156 if (duration < 10000)
158 if (duration < 100000)
164 * Return a multiplier for the exit latency that is intended
165 * to take performance requirements into account.
166 * The more performance critical we estimate the system
167 * to be, the higher this multiplier, and thus the higher
168 * the barrier to go to an expensive C state.
170 static inline int performance_multiplier(void)
174 /* for higher loadavg, we are more reluctant */
176 mult += 2 * get_loadavg();
178 /* for IO wait tasks (per cpu!) we add 5x each */
179 mult += 10 * nr_iowait_cpu(smp_processor_id());
184 static DEFINE_PER_CPU(struct menu_device, menu_devices);
186 static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
188 /* This implements DIV_ROUND_CLOSEST but avoids 64 bit division */
189 static u64 div_round64(u64 dividend, u32 divisor)
191 return div_u64(dividend + (divisor / 2), divisor);
195 * Try detecting repeating patterns by keeping track of the last 8
196 * intervals, and checking if the standard deviation of that set
197 * of points is below a threshold. If it is... then use the
198 * average of these 8 points as the estimated value.
200 static void detect_repeating_patterns(struct menu_device *data)
204 uint64_t stddev = 0; /* contains the square of the std deviation */
206 /* first calculate average and standard deviation of the past */
207 for (i = 0; i < INTERVALS; i++)
208 avg += data->intervals[i];
209 avg = avg / INTERVALS;
211 /* if the avg is beyond the known next tick, it's worthless */
212 if (avg > data->expected_us)
215 for (i = 0; i < INTERVALS; i++)
216 stddev += (data->intervals[i] - avg) *
217 (data->intervals[i] - avg);
219 stddev = stddev / INTERVALS;
222 * now.. if stddev is small.. then assume we have a
223 * repeating pattern and predict we keep doing this.
226 if (avg && stddev < STDDEV_THRESH)
227 data->predicted_us = avg;
231 * menu_select - selects the next idle state to enter
232 * @drv: cpuidle driver containing state data
235 static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
237 struct menu_device *data = &__get_cpu_var(menu_devices);
238 int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
239 int power_usage = -1;
244 if (data->needs_update) {
245 menu_update(drv, dev);
246 data->needs_update = 0;
249 data->last_state_idx = 0;
252 /* Special case when user has set very strict latency requirement */
253 if (unlikely(latency_req == 0))
256 /* determine the expected residency time, round up */
257 t = ktime_to_timespec(tick_nohz_get_sleep_length());
259 t.tv_sec * USEC_PER_SEC + t.tv_nsec / NSEC_PER_USEC;
262 data->bucket = which_bucket(data->expected_us);
264 multiplier = performance_multiplier();
267 * if the correction factor is 0 (eg first time init or cpu hotplug
268 * etc), we actually want to start out with a unity factor.
270 if (data->correction_factor[data->bucket] == 0)
271 data->correction_factor[data->bucket] = RESOLUTION * DECAY;
273 /* Make sure to round up for half microseconds */
274 data->predicted_us = div_round64(data->expected_us * data->correction_factor[data->bucket],
277 detect_repeating_patterns(data);
280 * We want to default to C1 (hlt), not to busy polling
281 * unless the timer is happening really really soon.
283 if (data->expected_us > 5 &&
284 !drv->states[CPUIDLE_DRIVER_STATE_START].disabled &&
285 dev->states_usage[CPUIDLE_DRIVER_STATE_START].disable == 0)
286 data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
289 * Find the idle state with the lowest power while satisfying
292 for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
293 struct cpuidle_state *s = &drv->states[i];
294 struct cpuidle_state_usage *su = &dev->states_usage[i];
296 if (s->disabled || su->disable)
298 if (s->target_residency > data->predicted_us)
300 if (s->exit_latency > latency_req)
302 if (s->exit_latency * multiplier > data->predicted_us)
305 if (s->power_usage < power_usage) {
306 power_usage = s->power_usage;
307 data->last_state_idx = i;
308 data->exit_us = s->exit_latency;
312 return data->last_state_idx;
316 * menu_reflect - records that data structures need update
318 * @index: the index of actual entered state
320 * NOTE: it's important to be fast here because this operation will add to
321 * the overall exit latency.
323 static void menu_reflect(struct cpuidle_device *dev, int index)
325 struct menu_device *data = &__get_cpu_var(menu_devices);
326 data->last_state_idx = index;
328 data->needs_update = 1;
332 * menu_update - attempts to guess what happened after entry
333 * @drv: cpuidle driver containing state data
336 static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
338 struct menu_device *data = &__get_cpu_var(menu_devices);
339 int last_idx = data->last_state_idx;
340 unsigned int last_idle_us = cpuidle_get_last_residency(dev);
341 struct cpuidle_state *target = &drv->states[last_idx];
342 unsigned int measured_us;
346 * Ugh, this idle state doesn't support residency measurements, so we
347 * are basically lost in the dark. As a compromise, assume we slept
348 * for the whole expected time.
350 if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
351 last_idle_us = data->expected_us;
354 measured_us = last_idle_us;
357 * We correct for the exit latency; we are assuming here that the
358 * exit latency happens after the event that we're interested in.
360 if (measured_us > data->exit_us)
361 measured_us -= data->exit_us;
364 /* update our correction ratio */
366 new_factor = data->correction_factor[data->bucket]
367 * (DECAY - 1) / DECAY;
369 if (data->expected_us > 0 && measured_us < MAX_INTERESTING)
370 new_factor += RESOLUTION * measured_us / data->expected_us;
373 * we were idle so long that we count it as a perfect
376 new_factor += RESOLUTION;
379 * We don't want 0 as factor; we always want at least
380 * a tiny bit of estimated time.
385 data->correction_factor[data->bucket] = new_factor;
387 /* update the repeating-pattern data */
388 data->intervals[data->interval_ptr++] = last_idle_us;
389 if (data->interval_ptr >= INTERVALS)
390 data->interval_ptr = 0;
394 * menu_enable_device - scans a CPU's states and does setup
395 * @drv: cpuidle driver
398 static int menu_enable_device(struct cpuidle_driver *drv,
399 struct cpuidle_device *dev)
401 struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
403 memset(data, 0, sizeof(struct menu_device));
408 static struct cpuidle_governor menu_governor = {
411 .enable = menu_enable_device,
412 .select = menu_select,
413 .reflect = menu_reflect,
414 .owner = THIS_MODULE,
418 * init_menu - initializes the governor
420 static int __init init_menu(void)
422 return cpuidle_register_governor(&menu_governor);
426 * exit_menu - exits the governor
428 static void __exit exit_menu(void)
430 cpuidle_unregister_governor(&menu_governor);
433 MODULE_LICENSE("GPL");
434 module_init(init_menu);
435 module_exit(exit_menu);