Root/drivers/cpuidle/governors/menu.c

Source at commit 672917dcc781ead7652a8b11b1fba14e38ac15b8 created 14 years 2 months ago.
By Corrado Zoccolo, cpuidle: menu governor: reduce latency on exit
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
22#define BUCKETS 12
23#define RESOLUTION 1024
24#define DECAY 4
25#define MAX_INTERESTING 50000
26
27/*
28 * Concepts and ideas behind the menu governor
29 *
30 * For the menu governor, there are 3 decision factors for picking a C
31 * state:
32 * 1) Energy break even point
33 * 2) Performance impact
34 * 3) Latency tolerance (from pmqos infrastructure)
35 * These these three factors are treated independently.
36 *
37 * Energy break even point
38 * -----------------------
39 * C state entry and exit have an energy cost, and a certain amount of time in
40 * the C state is required to actually break even on this cost. CPUIDLE
41 * provides us this duration in the "target_residency" field. So all that we
42 * need is a good prediction of how long we'll be idle. Like the traditional
43 * menu governor, we start with the actual known "next timer event" time.
44 *
45 * Since there are other source of wakeups (interrupts for example) than
46 * the next timer event, this estimation is rather optimistic. To get a
47 * more realistic estimate, a correction factor is applied to the estimate,
48 * that is based on historic behavior. For example, if in the past the actual
49 * duration always was 50% of the next timer tick, the correction factor will
50 * be 0.5.
51 *
52 * menu uses a running average for this correction factor, however it uses a
53 * set of factors, not just a single factor. This stems from the realization
54 * that the ratio is dependent on the order of magnitude of the expected
55 * duration; if we expect 500 milliseconds of idle time the likelihood of
56 * getting an interrupt very early is much higher than if we expect 50 micro
57 * seconds of idle time. A second independent factor that has big impact on
58 * the actual factor is if there is (disk) IO outstanding or not.
59 * (as a special twist, we consider every sleep longer than 50 milliseconds
60 * as perfect; there are no power gains for sleeping longer than this)
61 *
62 * For these two reasons we keep an array of 12 independent factors, that gets
63 * indexed based on the magnitude of the expected duration as well as the
64 * "is IO outstanding" property.
65 *
66 * Limiting Performance Impact
67 * ---------------------------
68 * C states, especially those with large exit latencies, can have a real
69 * noticable impact on workloads, which is not acceptable for most sysadmins,
70 * and in addition, less performance has a power price of its own.
71 *
72 * As a general rule of thumb, menu assumes that the following heuristic
73 * holds:
74 * The busier the system, the less impact of C states is acceptable
75 *
76 * This rule-of-thumb is implemented using a performance-multiplier:
77 * If the exit latency times the performance multiplier is longer than
78 * the predicted duration, the C state is not considered a candidate
79 * for selection due to a too high performance impact. So the higher
80 * this multiplier is, the longer we need to be idle to pick a deep C
81 * state, and thus the less likely a busy CPU will hit such a deep
82 * C state.
83 *
84 * Two factors are used in determing this multiplier:
85 * a value of 10 is added for each point of "per cpu load average" we have.
86 * a value of 5 points is added for each process that is waiting for
87 * IO on this CPU.
88 * (these values are experimentally determined)
89 *
90 * The load average factor gives a longer term (few seconds) input to the
91 * decision, while the iowait value gives a cpu local instantanious input.
92 * The iowait factor may look low, but realize that this is also already
93 * represented in the system load average.
94 *
95 */
96
97struct menu_device {
98    int last_state_idx;
99    int needs_update;
100
101    unsigned int expected_us;
102    u64 predicted_us;
103    unsigned int measured_us;
104    unsigned int exit_us;
105    unsigned int bucket;
106    u64 correction_factor[BUCKETS];
107};
108
109
110#define LOAD_INT(x) ((x) >> FSHIFT)
111#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
112
113static int get_loadavg(void)
114{
115    unsigned long this = this_cpu_load();
116
117
118    return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
119}
120
121static inline int which_bucket(unsigned int duration)
122{
123    int bucket = 0;
124
125    /*
126     * We keep two groups of stats; one with no
127     * IO pending, one without.
128     * This allows us to calculate
129     * E(duration)|iowait
130     */
131    if (nr_iowait_cpu())
132        bucket = BUCKETS/2;
133
134    if (duration < 10)
135        return bucket;
136    if (duration < 100)
137        return bucket + 1;
138    if (duration < 1000)
139        return bucket + 2;
140    if (duration < 10000)
141        return bucket + 3;
142    if (duration < 100000)
143        return bucket + 4;
144    return bucket + 5;
145}
146
147/*
148 * Return a multiplier for the exit latency that is intended
149 * to take performance requirements into account.
150 * The more performance critical we estimate the system
151 * to be, the higher this multiplier, and thus the higher
152 * the barrier to go to an expensive C state.
153 */
154static inline int performance_multiplier(void)
155{
156    int mult = 1;
157
158    /* for higher loadavg, we are more reluctant */
159
160    mult += 2 * get_loadavg();
161
162    /* for IO wait tasks (per cpu!) we add 5x each */
163    mult += 10 * nr_iowait_cpu();
164
165    return mult;
166}
167
168static DEFINE_PER_CPU(struct menu_device, menu_devices);
169
170static void menu_update(struct cpuidle_device *dev);
171
172/**
173 * menu_select - selects the next idle state to enter
174 * @dev: the CPU
175 */
176static int menu_select(struct cpuidle_device *dev)
177{
178    struct menu_device *data = &__get_cpu_var(menu_devices);
179    int latency_req = pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY);
180    int i;
181    int multiplier;
182
183    data->last_state_idx = 0;
184    data->exit_us = 0;
185
186    if (data->needs_update) {
187        menu_update(dev);
188        data->needs_update = 0;
189    }
190
191    /* Special case when user has set very strict latency requirement */
192    if (unlikely(latency_req == 0))
193        return 0;
194
195    /* determine the expected residency time, round up */
196    data->expected_us =
197        DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000);
198
199
200    data->bucket = which_bucket(data->expected_us);
201
202    multiplier = performance_multiplier();
203
204    /*
205     * if the correction factor is 0 (eg first time init or cpu hotplug
206     * etc), we actually want to start out with a unity factor.
207     */
208    if (data->correction_factor[data->bucket] == 0)
209        data->correction_factor[data->bucket] = RESOLUTION * DECAY;
210
211    /* Make sure to round up for half microseconds */
212    data->predicted_us = DIV_ROUND_CLOSEST(
213        data->expected_us * data->correction_factor[data->bucket],
214        RESOLUTION * DECAY);
215
216    /*
217     * We want to default to C1 (hlt), not to busy polling
218     * unless the timer is happening really really soon.
219     */
220    if (data->expected_us > 5)
221        data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
222
223
224    /* find the deepest idle state that satisfies our constraints */
225    for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) {
226        struct cpuidle_state *s = &dev->states[i];
227
228        if (s->target_residency > data->predicted_us)
229            break;
230        if (s->exit_latency > latency_req)
231            break;
232        if (s->exit_latency * multiplier > data->predicted_us)
233            break;
234        data->exit_us = s->exit_latency;
235        data->last_state_idx = i;
236    }
237
238    return data->last_state_idx;
239}
240
241/**
242 * menu_reflect - records that data structures need update
243 * @dev: the CPU
244 *
245 * NOTE: it's important to be fast here because this operation will add to
246 * the overall exit latency.
247 */
248static void menu_reflect(struct cpuidle_device *dev)
249{
250    struct menu_device *data = &__get_cpu_var(menu_devices);
251    data->needs_update = 1;
252}
253
254/**
255 * menu_update - attempts to guess what happened after entry
256 * @dev: the CPU
257 */
258static void menu_update(struct cpuidle_device *dev)
259{
260    struct menu_device *data = &__get_cpu_var(menu_devices);
261    int last_idx = data->last_state_idx;
262    unsigned int last_idle_us = cpuidle_get_last_residency(dev);
263    struct cpuidle_state *target = &dev->states[last_idx];
264    unsigned int measured_us;
265    u64 new_factor;
266
267    /*
268     * Ugh, this idle state doesn't support residency measurements, so we
269     * are basically lost in the dark. As a compromise, assume we slept
270     * for the whole expected time.
271     */
272    if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
273        last_idle_us = data->expected_us;
274
275
276    measured_us = last_idle_us;
277
278    /*
279     * We correct for the exit latency; we are assuming here that the
280     * exit latency happens after the event that we're interested in.
281     */
282    if (measured_us > data->exit_us)
283        measured_us -= data->exit_us;
284
285
286    /* update our correction ratio */
287
288    new_factor = data->correction_factor[data->bucket]
289            * (DECAY - 1) / DECAY;
290
291    if (data->expected_us > 0 && data->measured_us < MAX_INTERESTING)
292        new_factor += RESOLUTION * measured_us / data->expected_us;
293    else
294        /*
295         * we were idle so long that we count it as a perfect
296         * prediction
297         */
298        new_factor += RESOLUTION;
299
300    /*
301     * We don't want 0 as factor; we always want at least
302     * a tiny bit of estimated time.
303     */
304    if (new_factor == 0)
305        new_factor = 1;
306
307    data->correction_factor[data->bucket] = new_factor;
308}
309
310/**
311 * menu_enable_device - scans a CPU's states and does setup
312 * @dev: the CPU
313 */
314static int menu_enable_device(struct cpuidle_device *dev)
315{
316    struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
317
318    memset(data, 0, sizeof(struct menu_device));
319
320    return 0;
321}
322
323static struct cpuidle_governor menu_governor = {
324    .name = "menu",
325    .rating = 20,
326    .enable = menu_enable_device,
327    .select = menu_select,
328    .reflect = menu_reflect,
329    .owner = THIS_MODULE,
330};
331
332/**
333 * init_menu - initializes the governor
334 */
335static int __init init_menu(void)
336{
337    return cpuidle_register_governor(&menu_governor);
338}
339
340/**
341 * exit_menu - exits the governor
342 */
343static void __exit exit_menu(void)
344{
345    cpuidle_unregister_governor(&menu_governor);
346}
347
348MODULE_LICENSE("GPL");
349module_init(init_menu);
350module_exit(exit_menu);
351

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