// SPDX-License-Identifier: GPL-2.0 /* * Timer events oriented CPU idle governor * * Copyright (C) 2018 - 2021 Intel Corporation * Author: Rafael J. Wysocki */ /** * DOC: teo-description * * The idea of this governor is based on the observation that on many systems * timer events are two or more orders of magnitude more frequent than any * other interrupts, so they are likely to be the most significant cause of CPU * wakeups from idle states. Moreover, information about what happened in the * (relatively recent) past can be used to estimate whether or not the deepest * idle state with target residency within the (known) time till the closest * timer event, referred to as the sleep length, is likely to be suitable for * the upcoming CPU idle period and, if not, then which of the shallower idle * states to choose instead of it. * * Of course, non-timer wakeup sources are more important in some use cases * which can be covered by taking a few most recent idle time intervals of the * CPU into account. However, even in that context it is not necessary to * consider idle duration values greater than the sleep length, because the * closest timer will ultimately wake up the CPU anyway unless it is woken up * earlier. * * Thus this governor estimates whether or not the prospective idle duration of * a CPU is likely to be significantly shorter than the sleep length and selects * an idle state for it accordingly. * * The computations carried out by this governor are based on using bins whose * boundaries are aligned with the target residency parameter values of the CPU * idle states provided by the %CPUIdle driver in the ascending order. That is, * the first bin spans from 0 up to, but not including, the target residency of * the second idle state (idle state 1), the second bin spans from the target * residency of idle state 1 up to, but not including, the target residency of * idle state 2, the third bin spans from the target residency of idle state 2 * up to, but not including, the target residency of idle state 3 and so on. * The last bin spans from the target residency of the deepest idle state * supplied by the driver to infinity. * * Two metrics called "hits" and "intercepts" are associated with each bin. * They are updated every time before selecting an idle state for the given CPU * in accordance with what happened last time. * * The "hits" metric reflects the relative frequency of situations in which the * sleep length and the idle duration measured after CPU wakeup fall into the * same bin (that is, the CPU appears to wake up "on time" relative to the sleep * length). In turn, the "intercepts" metric reflects the relative frequency of * situations in which the measured idle duration is so much shorter than the * sleep length that the bin it falls into corresponds to an idle state * shallower than the one whose bin is fallen into by the sleep length (these * situations are referred to as "intercepts" below). * * In addition to the metrics described above, the governor counts recent * intercepts (that is, intercepts that have occurred during the last * %NR_RECENT invocations of it for the given CPU) for each bin. * * In order to select an idle state for a CPU, the governor takes the following * steps (modulo the possible latency constraint that must be taken into account * too): * * 1. Find the deepest CPU idle state whose target residency does not exceed * the current sleep length (the candidate idle state) and compute 3 sums as * follows: * * - The sum of the "hits" and "intercepts" metrics for the candidate state * and all of the deeper idle states (it represents the cases in which the * CPU was idle long enough to avoid being intercepted if the sleep length * had been equal to the current one). * * - The sum of the "intercepts" metrics for all of the idle states shallower * than the candidate one (it represents the cases in which the CPU was not * idle long enough to avoid being intercepted if the sleep length had been * equal to the current one). * * - The sum of the numbers of recent intercepts for all of the idle states * shallower than the candidate one. * * 2. If the second sum is greater than the first one or the third sum is * greater than %NR_RECENT / 2, the CPU is likely to wake up early, so look * for an alternative idle state to select. * * - Traverse the idle states shallower than the candidate one in the * descending order. * * - For each of them compute the sum of the "intercepts" metrics and the sum * of the numbers of recent intercepts over all of the idle states between * it and the candidate one (including the former and excluding the * latter). * * - If each of these sums that needs to be taken into account (because the * check related to it has indicated that the CPU is likely to wake up * early) is greater than a half of the corresponding sum computed in step * 1 (which means that the target residency of the state in question had * not exceeded the idle duration in over a half of the relevant cases), * select the given idle state instead of the candidate one. * * 3. By default, select the candidate state. */ #include #include #include #include #include /* * The PULSE value is added to metrics when they grow and the DECAY_SHIFT value * is used for decreasing metrics on a regular basis. */ #define PULSE 1024 #define DECAY_SHIFT 3 /* * Number of the most recent idle duration values to take into consideration for * the detection of recent early wakeup patterns. */ #define NR_RECENT 9 /** * struct teo_bin - Metrics used by the TEO cpuidle governor. * @intercepts: The "intercepts" metric. * @hits: The "hits" metric. * @recent: The number of recent "intercepts". */ struct teo_bin { unsigned int intercepts; unsigned int hits; unsigned int recent; }; /** * struct teo_cpu - CPU data used by the TEO cpuidle governor. * @time_span_ns: Time between idle state selection and post-wakeup update. * @sleep_length_ns: Time till the closest timer event (at the selection time). * @state_bins: Idle state data bins for this CPU. * @total: Grand total of the "intercepts" and "hits" mertics for all bins. * @next_recent_idx: Index of the next @recent_idx entry to update. * @recent_idx: Indices of bins corresponding to recent "intercepts". */ struct teo_cpu { s64 time_span_ns; s64 sleep_length_ns; struct teo_bin state_bins[CPUIDLE_STATE_MAX]; unsigned int total; int next_recent_idx; int recent_idx[NR_RECENT]; }; static DEFINE_PER_CPU(struct teo_cpu, teo_cpus); /** * teo_update - Update CPU metrics after wakeup. * @drv: cpuidle driver containing state data. * @dev: Target CPU. */ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu); int i, idx_timer = 0, idx_duration = 0; u64 measured_ns; if (cpu_data->time_span_ns >= cpu_data->sleep_length_ns) { /* * One of the safety nets has triggered or the wakeup was close * enough to the closest timer event expected at the idle state * selection time to be discarded. */ measured_ns = U64_MAX; } else { u64 lat_ns = drv->states[dev->last_state_idx].exit_latency_ns; /* * The computations below are to determine whether or not the * (saved) time till the next timer event and the measured idle * duration fall into the same "bin", so use last_residency_ns * for that instead of time_span_ns which includes the cpuidle * overhead. */ measured_ns = dev->last_residency_ns; /* * The delay between the wakeup and the first instruction * executed by the CPU is not likely to be worst-case every * time, so take 1/2 of the exit latency as a very rough * approximation of the average of it. */ if (measured_ns >= lat_ns) measured_ns -= lat_ns / 2; else measured_ns /= 2; } cpu_data->total = 0; /* * Decay the "hits" and "intercepts" metrics for all of the bins and * find the bins that the sleep length and the measured idle duration * fall into. */ for (i = 0; i < drv->state_count; i++) { s64 target_residency_ns = drv->states[i].target_residency_ns; struct teo_bin *bin = &cpu_data->state_bins[i]; bin->hits -= bin->hits >> DECAY_SHIFT; bin->intercepts -= bin->intercepts >> DECAY_SHIFT; cpu_data->total += bin->hits + bin->intercepts; if (target_residency_ns <= cpu_data->sleep_length_ns) { idx_timer = i; if (target_residency_ns <= measured_ns) idx_duration = i; } } i = cpu_data->next_recent_idx++; if (cpu_data->next_recent_idx >= NR_RECENT) cpu_data->next_recent_idx = 0; if (cpu_data->recent_idx[i] >= 0) cpu_data->state_bins[cpu_data->recent_idx[i]].recent--; /* * If the measured idle duration falls into the same bin as the sleep * length, this is a "hit", so update the "hits" metric for that bin. * Otherwise, update the "intercepts" metric for the bin fallen into by * the measured idle duration. */ if (idx_timer == idx_duration) { cpu_data->state_bins[idx_timer].hits += PULSE; cpu_data->recent_idx[i] = -1; } else { cpu_data->state_bins[idx_duration].intercepts += PULSE; cpu_data->state_bins[idx_duration].recent++; cpu_data->recent_idx[i] = idx_duration; } cpu_data->total += PULSE; } static bool teo_time_ok(u64 interval_ns) { return !tick_nohz_tick_stopped() || interval_ns >= TICK_NSEC; } static s64 teo_middle_of_bin(int idx, struct cpuidle_driver *drv) { return (drv->states[idx].target_residency_ns + drv->states[idx+1].target_residency_ns) / 2; } /** * teo_find_shallower_state - Find shallower idle state matching given duration. * @drv: cpuidle driver containing state data. * @dev: Target CPU. * @state_idx: Index of the capping idle state. * @duration_ns: Idle duration value to match. */ static int teo_find_shallower_state(struct cpuidle_driver *drv, struct cpuidle_device *dev, int state_idx, s64 duration_ns) { int i; for (i = state_idx - 1; i >= 0; i--) { if (dev->states_usage[i].disable) continue; state_idx = i; if (drv->states[i].target_residency_ns <= duration_ns) break; } return state_idx; } /** * teo_select - Selects the next idle state to enter. * @drv: cpuidle driver containing state data. * @dev: Target CPU. * @stop_tick: Indication on whether or not to stop the scheduler tick. */ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev, bool *stop_tick) { struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu); s64 latency_req = cpuidle_governor_latency_req(dev->cpu); unsigned int idx_intercept_sum = 0; unsigned int intercept_sum = 0; unsigned int idx_recent_sum = 0; unsigned int recent_sum = 0; unsigned int idx_hit_sum = 0; unsigned int hit_sum = 0; int constraint_idx = 0; int idx0 = 0, idx = -1; bool alt_intercepts, alt_recent; ktime_t delta_tick; s64 duration_ns; int i; if (dev->last_state_idx >= 0) { teo_update(drv, dev); dev->last_state_idx = -1; } cpu_data->time_span_ns = local_clock(); duration_ns = tick_nohz_get_sleep_length(&delta_tick); cpu_data->sleep_length_ns = duration_ns; /* Check if there is any choice in the first place. */ if (drv->state_count < 2) { idx = 0; goto end; } if (!dev->states_usage[0].disable) { idx = 0; if (drv->states[1].target_residency_ns > duration_ns) goto end; } /* * Find the deepest idle state whose target residency does not exceed * the current sleep length and the deepest idle state not deeper than * the former whose exit latency does not exceed the current latency * constraint. Compute the sums of metrics for early wakeup pattern * detection. */ for (i = 1; i < drv->state_count; i++) { struct teo_bin *prev_bin = &cpu_data->state_bins[i-1]; struct cpuidle_state *s = &drv->states[i]; /* * Update the sums of idle state mertics for all of the states * shallower than the current one. */ intercept_sum += prev_bin->intercepts; hit_sum += prev_bin->hits; recent_sum += prev_bin->recent; if (dev->states_usage[i].disable) continue; if (idx < 0) { idx = i; /* first enabled state */ idx0 = i; } if (s->target_residency_ns > duration_ns) break; idx = i; if (s->exit_latency_ns <= latency_req) constraint_idx = i; idx_intercept_sum = intercept_sum; idx_hit_sum = hit_sum; idx_recent_sum = recent_sum; } /* Avoid unnecessary overhead. */ if (idx < 0) { idx = 0; /* No states enabled, must use 0. */ goto end; } else if (idx == idx0) { goto end; } /* * If the sum of the intercepts metric for all of the idle states * shallower than the current candidate one (idx) is greater than the * sum of the intercepts and hits metrics for the candidate state and * all of the deeper states, or the sum of the numbers of recent * intercepts over all of the states shallower than the candidate one * is greater than a half of the number of recent events taken into * account, the CPU is likely to wake up early, so find an alternative * idle state to select. */ alt_intercepts = 2 * idx_intercept_sum > cpu_data->total - idx_hit_sum; alt_recent = idx_recent_sum > NR_RECENT / 2; if (alt_recent || alt_intercepts) { s64 first_suitable_span_ns = duration_ns; int first_suitable_idx = idx; /* * Look for the deepest idle state whose target residency had * not exceeded the idle duration in over a half of the relevant * cases (both with respect to intercepts overall and with * respect to the recent intercepts only) in the past. * * Take the possible latency constraint and duration limitation * present if the tick has been stopped already into account. */ intercept_sum = 0; recent_sum = 0; for (i = idx - 1; i >= 0; i--) { struct teo_bin *bin = &cpu_data->state_bins[i]; s64 span_ns; intercept_sum += bin->intercepts; recent_sum += bin->recent; span_ns = teo_middle_of_bin(i, drv); if ((!alt_recent || 2 * recent_sum > idx_recent_sum) && (!alt_intercepts || 2 * intercept_sum > idx_intercept_sum)) { if (teo_time_ok(span_ns) && !dev->states_usage[i].disable) { idx = i; duration_ns = span_ns; } else { /* * The current state is too shallow or * disabled, so take the first enabled * deeper state with suitable time span. */ idx = first_suitable_idx; duration_ns = first_suitable_span_ns; } break; } if (dev->states_usage[i].disable) continue; if (!teo_time_ok(span_ns)) { /* * The current state is too shallow, but if an * alternative candidate state has been found, * it may still turn out to be a better choice. */ if (first_suitable_idx != idx) continue; break; } first_suitable_span_ns = span_ns; first_suitable_idx = i; } } /* * If there is a latency constraint, it may be necessary to select an * idle state shallower than the current candidate one. */ if (idx > constraint_idx) idx = constraint_idx; end: /* * Don't stop the tick if the selected state is a polling one or if the * expected idle duration is shorter than the tick period length. */ if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) || duration_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) { *stop_tick = false; /* * The tick is not going to be stopped, so if the target * residency of the state to be returned is not within the time * till the closest timer including the tick, try to correct * that. */ if (idx > idx0 && drv->states[idx].target_residency_ns > delta_tick) idx = teo_find_shallower_state(drv, dev, idx, delta_tick); } return idx; } /** * teo_reflect - Note that governor data for the CPU need to be updated. * @dev: Target CPU. * @state: Entered state. */ static void teo_reflect(struct cpuidle_device *dev, int state) { struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu); dev->last_state_idx = state; /* * If the wakeup was not "natural", but triggered by one of the safety * nets, assume that the CPU might have been idle for the entire sleep * length time. */ if (dev->poll_time_limit || (tick_nohz_idle_got_tick() && cpu_data->sleep_length_ns > TICK_NSEC)) { dev->poll_time_limit = false; cpu_data->time_span_ns = cpu_data->sleep_length_ns; } else { cpu_data->time_span_ns = local_clock() - cpu_data->time_span_ns; } } /** * teo_enable_device - Initialize the governor's data for the target CPU. * @drv: cpuidle driver (not used). * @dev: Target CPU. */ static int teo_enable_device(struct cpuidle_driver *drv, struct cpuidle_device *dev) { struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu); int i; memset(cpu_data, 0, sizeof(*cpu_data)); for (i = 0; i < NR_RECENT; i++) cpu_data->recent_idx[i] = -1; return 0; } static struct cpuidle_governor teo_governor = { .name = "teo", .rating = 19, .enable = teo_enable_device, .select = teo_select, .reflect = teo_reflect, }; static int __init teo_governor_init(void) { return cpuidle_register_governor(&teo_governor); } postcore_initcall(teo_governor_init);