/* * 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 #include /* * Please note when changing the tuning values: * If (MAX_INTERESTING-1) * RESOLUTION > UINT_MAX, the result of * a scaling operation multiplication may overflow on 32 bit platforms. * In that case, #define RESOLUTION as ULL to get 64 bit result: * #define RESOLUTION 1024ULL * * The default values do not overflow. */ #define BUCKETS 12 #define INTERVAL_SHIFT 3 #define INTERVALS (1UL << INTERVAL_SHIFT) #define RESOLUTION 1024 #define DECAY 8 #define MAX_INTERESTING 50000 /* * 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 * noticeable 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 next_timer_us; unsigned int predicted_us; unsigned int bucket; unsigned int correction_factor[BUCKETS]; unsigned int intervals[INTERVALS]; int interval_ptr; }; #define LOAD_INT(x) ((x) >> FSHIFT) #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100) static inline int get_loadavg(unsigned long load) { return LOAD_INT(load) * 10 + LOAD_FRAC(load) / 10; } static inline int which_bucket(unsigned int duration, unsigned long nr_iowaiters) { 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_iowaiters) 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(unsigned long nr_iowaiters, unsigned long load) { int mult = 1; /* for higher loadavg, we are more reluctant */ /* * this doesn't work as intended - it is almost always 0, but can * sometimes, depending on workload, spike very high into the hundreds * even when the average cpu load is under 10%. */ /* mult += 2 * get_loadavg(); */ /* for IO wait tasks (per cpu!) we add 5x each */ mult += 10 * nr_iowaiters; return mult; } static DEFINE_PER_CPU(struct menu_device, menu_devices); static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev); /* * 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 unsigned int get_typical_interval(struct menu_device *data) { int i, divisor; unsigned int max, thresh, avg; uint64_t sum, variance; thresh = UINT_MAX; /* Discard outliers above this value */ again: /* First calculate the average of past intervals */ max = 0; sum = 0; divisor = 0; for (i = 0; i < INTERVALS; i++) { unsigned int value = data->intervals[i]; if (value <= thresh) { sum += value; divisor++; if (value > max) max = value; } } if (divisor == INTERVALS) avg = sum >> INTERVAL_SHIFT; else avg = div_u64(sum, divisor); /* Then try to determine variance */ variance = 0; for (i = 0; i < INTERVALS; i++) { unsigned int value = data->intervals[i]; if (value <= thresh) { int64_t diff = (int64_t)value - avg; variance += diff * diff; } } if (divisor == INTERVALS) variance >>= INTERVAL_SHIFT; else do_div(variance, divisor); /* * The typical interval is obtained when standard deviation is * small (stddev <= 20 us, variance <= 400 us^2) or standard * deviation is small compared to the average interval (avg > * 6*stddev, avg^2 > 36*variance). The average is smaller than * UINT_MAX aka U32_MAX, so computing its square does not * overflow a u64. We simply reject this candidate average if * the standard deviation is greater than 715 s (which is * rather unlikely). * * Use this result only if there is no timer to wake us up sooner. */ if (likely(variance <= U64_MAX/36)) { if ((((u64)avg*avg > variance*36) && (divisor * 4 >= INTERVALS * 3)) || variance <= 400) { return avg; } } /* * If we have outliers to the upside in our distribution, discard * those by setting the threshold to exclude these outliers, then * calculate the average and standard deviation again. Once we get * down to the bottom 3/4 of our samples, stop excluding samples. * * This can deal with workloads that have long pauses interspersed * with sporadic activity with a bunch of short pauses. */ if ((divisor * 4) <= INTERVALS * 3) return UINT_MAX; thresh = max - 1; goto again; } /** * menu_select - selects the next idle state to enter * @drv: cpuidle driver containing state data * @dev: the CPU */ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct menu_device *data = this_cpu_ptr(&menu_devices); int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY); int i; unsigned int interactivity_req; unsigned int expected_interval; unsigned long nr_iowaiters, cpu_load; if (data->needs_update) { menu_update(drv, dev); data->needs_update = 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->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length()); get_iowait_load(&nr_iowaiters, &cpu_load); data->bucket = which_bucket(data->next_timer_us, nr_iowaiters); /* * Force the result of multiplication to be 64 bits even if both * operands are 32 bits. * Make sure to round up for half microseconds. */ data->predicted_us = DIV_ROUND_CLOSEST_ULL((uint64_t)data->next_timer_us * data->correction_factor[data->bucket], RESOLUTION * DECAY); expected_interval = get_typical_interval(data); expected_interval = min(expected_interval, data->next_timer_us); if (CPUIDLE_DRIVER_STATE_START > 0) { struct cpuidle_state *s = &drv->states[CPUIDLE_DRIVER_STATE_START]; unsigned int polling_threshold; /* * We want to default to C1 (hlt), not to busy polling * unless the timer is happening really really soon, or * C1's exit latency exceeds the user configured limit. */ polling_threshold = max_t(unsigned int, 20, s->target_residency); if (data->next_timer_us > polling_threshold && latency_req > s->exit_latency && !s->disabled && !dev->states_usage[CPUIDLE_DRIVER_STATE_START].disable) data->last_state_idx = CPUIDLE_DRIVER_STATE_START; else data->last_state_idx = CPUIDLE_DRIVER_STATE_START - 1; } else { data->last_state_idx = CPUIDLE_DRIVER_STATE_START; } /* * Use the lowest expected idle interval to pick the idle state. */ data->predicted_us = min(data->predicted_us, expected_interval); /* * Use the performance multiplier and the user-configurable * latency_req to determine the maximum exit latency. */ interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load); if (latency_req > interactivity_req) latency_req = interactivity_req; /* * Find the idle state with the lowest power while satisfying * our constraints. */ for (i = data->last_state_idx + 1; i < drv->state_count; i++) { struct cpuidle_state *s = &drv->states[i]; struct cpuidle_state_usage *su = &dev->states_usage[i]; if (s->disabled || su->disable) continue; if (s->target_residency > data->predicted_us) continue; if (s->exit_latency > latency_req) continue; data->last_state_idx = i; } return data->last_state_idx; } /** * menu_reflect - records that data structures need update * @dev: the CPU * @index: the index of actual entered state * * 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, int index) { struct menu_device *data = this_cpu_ptr(&menu_devices); data->last_state_idx = index; data->needs_update = 1; } /** * menu_update - attempts to guess what happened after entry * @drv: cpuidle driver containing state data * @dev: the CPU */ static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct menu_device *data = this_cpu_ptr(&menu_devices); int last_idx = data->last_state_idx; struct cpuidle_state *target = &drv->states[last_idx]; unsigned int measured_us; unsigned int new_factor; /* * Try to figure out how much time passed between entry to low * power state and occurrence of the wakeup event. * * If the entered idle state didn't support residency measurements, * we use them anyway if they are short, and if long, * truncate to the whole expected time. * * Any measured amount of time will include the exit latency. * Since we are interested in when the wakeup begun, not when it * was completed, we must subtract the exit latency. However, if * the measured amount of time is less than the exit latency, * assume the state was never reached and the exit latency is 0. */ /* measured value */ measured_us = cpuidle_get_last_residency(dev); /* Deduct exit latency */ if (measured_us > 2 * target->exit_latency) measured_us -= target->exit_latency; else measured_us /= 2; /* Make sure our coefficients do not exceed unity */ if (measured_us > data->next_timer_us) measured_us = data->next_timer_us; /* Update our correction ratio */ new_factor = data->correction_factor[data->bucket]; new_factor -= new_factor / DECAY; if (data->next_timer_us > 0 && measured_us < MAX_INTERESTING) new_factor += RESOLUTION * measured_us / data->next_timer_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. Fortunately, due to rounding, * new_factor will stay nonzero regardless of measured_us values * and the compiler can eliminate this test as long as DECAY > 1. */ if (DECAY == 1 && unlikely(new_factor == 0)) new_factor = 1; data->correction_factor[data->bucket] = new_factor; /* update the repeating-pattern data */ data->intervals[data->interval_ptr++] = measured_us; if (data->interval_ptr >= INTERVALS) data->interval_ptr = 0; } /** * menu_enable_device - scans a CPU's states and does setup * @drv: cpuidle driver * @dev: the CPU */ static int menu_enable_device(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct menu_device *data = &per_cpu(menu_devices, dev->cpu); int i; memset(data, 0, sizeof(struct menu_device)); /* * if the correction factor is 0 (eg first time init or cpu hotplug * etc), we actually want to start out with a unity factor. */ for(i = 0; i < BUCKETS; i++) data->correction_factor[i] = RESOLUTION * DECAY; 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); } postcore_initcall(init_menu);