/* * menu.c - the menu idle governor * * Copyright (C) 2006-2007 Adam Belay * Copyright (C) 2009 Intel Corporation * Author: * Arjan van de Ven * * This code is licenced under the GPL version 2 as described * in the COPYING file that acompanies the Linux Kernel. */ #include #include #include #include #include #include #include #include #include #define BUCKETS 12 #define INTERVALS 8 #define RESOLUTION 1024 #define DECAY 8 #define MAX_INTERESTING 50000 #define STDDEV_THRESH 400 /* * Concepts and ideas behind the menu governor * * For the menu governor, there are 3 decision factors for picking a C * state: * 1) Energy break even point * 2) Performance impact * 3) Latency tolerance (from pmqos infrastructure) * These these three factors are treated independently. * * Energy break even point * ----------------------- * C state entry and exit have an energy cost, and a certain amount of time in * the C state is required to actually break even on this cost. CPUIDLE * provides us this duration in the "target_residency" field. So all that we * need is a good prediction of how long we'll be idle. Like the traditional * menu governor, we start with the actual known "next timer event" time. * * Since there are other source of wakeups (interrupts for example) than * the next timer event, this estimation is rather optimistic. To get a * more realistic estimate, a correction factor is applied to the estimate, * that is based on historic behavior. For example, if in the past the actual * duration always was 50% of the next timer tick, the correction factor will * be 0.5. * * menu uses a running average for this correction factor, however it uses a * set of factors, not just a single factor. This stems from the realization * that the ratio is dependent on the order of magnitude of the expected * duration; if we expect 500 milliseconds of idle time the likelihood of * getting an interrupt very early is much higher than if we expect 50 micro * seconds of idle time. A second independent factor that has big impact on * the actual factor is if there is (disk) IO outstanding or not. * (as a special twist, we consider every sleep longer than 50 milliseconds * as perfect; there are no power gains for sleeping longer than this) * * For these two reasons we keep an array of 12 independent factors, that gets * indexed based on the magnitude of the expected duration as well as the * "is IO outstanding" property. * * Repeatable-interval-detector * ---------------------------- * There are some cases where "next timer" is a completely unusable predictor: * Those cases where the interval is fixed, for example due to hardware * interrupt mitigation, but also due to fixed transfer rate devices such as * mice. * For this, we use a different predictor: We track the duration of the last 8 * intervals and if the stand deviation of these 8 intervals is below a * threshold value, we use the average of these intervals as prediction. * * Limiting Performance Impact * --------------------------- * C states, especially those with large exit latencies, can have a real * noticable impact on workloads, which is not acceptable for most sysadmins, * and in addition, less performance has a power price of its own. * * As a general rule of thumb, menu assumes that the following heuristic * holds: * The busier the system, the less impact of C states is acceptable * * This rule-of-thumb is implemented using a performance-multiplier: * If the exit latency times the performance multiplier is longer than * the predicted duration, the C state is not considered a candidate * for selection due to a too high performance impact. So the higher * this multiplier is, the longer we need to be idle to pick a deep C * state, and thus the less likely a busy CPU will hit such a deep * C state. * * Two factors are used in determing this multiplier: * a value of 10 is added for each point of "per cpu load average" we have. * a value of 5 points is added for each process that is waiting for * IO on this CPU. * (these values are experimentally determined) * * The load average factor gives a longer term (few seconds) input to the * decision, while the iowait value gives a cpu local instantanious input. * The iowait factor may look low, but realize that this is also already * represented in the system load average. * */ struct menu_device { int last_state_idx; int needs_update; unsigned int expected_us; u64 predicted_us; unsigned int exit_us; unsigned int bucket; u64 correction_factor[BUCKETS]; u32 intervals[INTERVALS]; int interval_ptr; }; #define LOAD_INT(x) ((x) >> FSHIFT) #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100) static int get_loadavg(void) { unsigned long this = this_cpu_load(); return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10; } static inline int which_bucket(unsigned int duration) { int bucket = 0; /* * We keep two groups of stats; one with no * IO pending, one without. * This allows us to calculate * E(duration)|iowait */ if (nr_iowait_cpu()) bucket = BUCKETS/2; if (duration < 10) return bucket; if (duration < 100) return bucket + 1; if (duration < 1000) return bucket + 2; if (duration < 10000) return bucket + 3; if (duration < 100000) return bucket + 4; return bucket + 5; } /* * Return a multiplier for the exit latency that is intended * to take performance requirements into account. * The more performance critical we estimate the system * to be, the higher this multiplier, and thus the higher * the barrier to go to an expensive C state. */ static inline int performance_multiplier(void) { int mult = 1; /* for higher loadavg, we are more reluctant */ mult += 2 * get_loadavg(); /* for IO wait tasks (per cpu!) we add 5x each */ mult += 10 * nr_iowait_cpu(); return mult; } static DEFINE_PER_CPU(struct menu_device, menu_devices); static void menu_update(struct cpuidle_device *dev); /* This implements DIV_ROUND_CLOSEST but avoids 64 bit division */ static u64 div_round64(u64 dividend, u32 divisor) { return div_u64(dividend + (divisor / 2), divisor); } /* * Try detecting repeating patterns by keeping track of the last 8 * intervals, and checking if the standard deviation of that set * of points is below a threshold. If it is... then use the * average of these 8 points as the estimated value. */ static void detect_repeating_patterns(struct menu_device *data) { int i; uint64_t avg = 0; uint64_t stddev = 0; /* contains the square of the std deviation */ /* first calculate average and standard deviation of the past */ for (i = 0; i < INTERVALS; i++) avg += data->intervals[i]; avg = avg / INTERVALS; /* if the avg is beyond the known next tick, it's worthless */ if (avg > data->expected_us) return; for (i = 0; i < INTERVALS; i++) stddev += (data->intervals[i] - avg) * (data->intervals[i] - avg); stddev = stddev / INTERVALS; /* * now.. if stddev is small.. then assume we have a * repeating pattern and predict we keep doing this. */ if (avg && stddev < STDDEV_THRESH) data->predicted_us = avg; } /** * menu_select - selects the next idle state to enter * @dev: the CPU */ static int menu_select(struct cpuidle_device *dev) { struct menu_device *data = &__get_cpu_var(menu_devices); int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY); int i; int multiplier; if (data->needs_update) { menu_update(dev); data->needs_update = 0; } data->last_state_idx = 0; data->exit_us = 0; /* Special case when user has set very strict latency requirement */ if (unlikely(latency_req == 0)) return 0; /* determine the expected residency time, round up */ data->expected_us = DIV_ROUND_UP((u32)ktime_to_ns(tick_nohz_get_sleep_length()), 1000); data->bucket = which_bucket(data->expected_us); multiplier = performance_multiplier(); /* * if the correction factor is 0 (eg first time init or cpu hotplug * etc), we actually want to start out with a unity factor. */ if (data->correction_factor[data->bucket] == 0) data->correction_factor[data->bucket] = RESOLUTION * DECAY; /* Make sure to round up for half microseconds */ data->predicted_us = div_round64(data->expected_us * data->correction_factor[data->bucket], RESOLUTION * DECAY); detect_repeating_patterns(data); /* * We want to default to C1 (hlt), not to busy polling * unless the timer is happening really really soon. */ if (data->expected_us > 5) data->last_state_idx = CPUIDLE_DRIVER_STATE_START; /* find the deepest idle state that satisfies our constraints */ for (i = CPUIDLE_DRIVER_STATE_START; i < dev->state_count; i++) { struct cpuidle_state *s = &dev->states[i]; if (s->target_residency > data->predicted_us) break; if (s->exit_latency > latency_req) break; if (s->exit_latency * multiplier > data->predicted_us) break; data->exit_us = s->exit_latency; data->last_state_idx = i; } return data->last_state_idx; } /** * menu_reflect - records that data structures need update * @dev: the CPU * * NOTE: it's important to be fast here because this operation will add to * the overall exit latency. */ static void menu_reflect(struct cpuidle_device *dev) { struct menu_device *data = &__get_cpu_var(menu_devices); data->needs_update = 1; } /** * menu_update - attempts to guess what happened after entry * @dev: the CPU */ static void menu_update(struct cpuidle_device *dev) { struct menu_device *data = &__get_cpu_var(menu_devices); int last_idx = data->last_state_idx; unsigned int last_idle_us = cpuidle_get_last_residency(dev); struct cpuidle_state *target = &dev->states[last_idx]; unsigned int measured_us; u64 new_factor; /* * Ugh, this idle state doesn't support residency measurements, so we * are basically lost in the dark. As a compromise, assume we slept * for the whole expected time. */ if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID))) last_idle_us = data->expected_us; measured_us = last_idle_us; /* * We correct for the exit latency; we are assuming here that the * exit latency happens after the event that we're interested in. */ if (measured_us > data->exit_us) measured_us -= data->exit_us; /* update our correction ratio */ new_factor = data->correction_factor[data->bucket] * (DECAY - 1) / DECAY; if (data->expected_us > 0 && measured_us < MAX_INTERESTING) new_factor += RESOLUTION * measured_us / data->expected_us; else /* * we were idle so long that we count it as a perfect * prediction */ new_factor += RESOLUTION; /* * We don't want 0 as factor; we always want at least * a tiny bit of estimated time. */ if (new_factor == 0) new_factor = 1; data->correction_factor[data->bucket] = new_factor; /* update the repeating-pattern data */ data->intervals[data->interval_ptr++] = last_idle_us; if (data->interval_ptr >= INTERVALS) data->interval_ptr = 0; } /** * menu_enable_device - scans a CPU's states and does setup * @dev: the CPU */ static int menu_enable_device(struct cpuidle_device *dev) { struct menu_device *data = &per_cpu(menu_devices, dev->cpu); memset(data, 0, sizeof(struct menu_device)); return 0; } static struct cpuidle_governor menu_governor = { .name = "menu", .rating = 20, .enable = menu_enable_device, .select = menu_select, .reflect = menu_reflect, .owner = THIS_MODULE, }; /** * init_menu - initializes the governor */ static int __init init_menu(void) { return cpuidle_register_governor(&menu_governor); } /** * exit_menu - exits the governor */ static void __exit exit_menu(void) { cpuidle_unregister_governor(&menu_governor); } MODULE_LICENSE("GPL"); module_init(init_menu); module_exit(exit_menu);