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 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * menu.c - the menu idle governor
  *
  * Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
  * Copyright (C) 2009 Intel Corporation
  * Author:
  *        Arjan van de Ven <arjan@linux.intel.com>
  */
 
 #include <linux/kernel.h>
 #include <linux/cpuidle.h>
 #include <linux/time.h>
 #include <linux/ktime.h>
 #include <linux/hrtimer.h>
 #include <linux/tick.h>
 #include <linux/sched.h>
 #include <linux/sched/loadavg.h>
 #include <linux/sched/stat.h>
 #include <linux/math64.h>
 
 #define BUCKETS 12
 #define INTERVAL_SHIFT 3
 #define INTERVALS (1UL << INTERVAL_SHIFT)
 #define RESOLUTION 1024
 #define DECAY 8
 #define MAX_INTERESTING (50000 * NSEC_PER_USEC)
 
 /*
  * 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 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             needs_update;
     int             tick_wakeup;
 
     u64     next_timer_ns;
     unsigned int    bucket;
     unsigned int    correction_factor[BUCKETS];
     unsigned int    intervals[INTERVALS];
     int     interval_ptr;
 };
 
 static inline int which_bucket(u64 duration_ns, unsigned int 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_ns < 10ULL * NSEC_PER_USEC)
         return bucket;
     if (duration_ns < 100ULL * NSEC_PER_USEC)
         return bucket + 1;
     if (duration_ns < 1000ULL * NSEC_PER_USEC)
         return bucket + 2;
     if (duration_ns < 10000ULL * NSEC_PER_USEC)
         return bucket + 3;
     if (duration_ns < 100000ULL * NSEC_PER_USEC)
         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 int nr_iowaiters)
 {
     /* for IO wait tasks (per cpu!) we add 10x each */
     return 1 + 10 * nr_iowaiters;
 }
 
 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,
                      unsigned int predicted_us)
 {
     int i, divisor;
     unsigned int min, max, thresh, avg;
     uint64_t sum, variance;
 
     thresh = INT_MAX; /* Discard outliers above this value */
 
 again:
 
     /* First calculate the average of past intervals */
     min = UINT_MAX;
     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 (value < min)
                 min = value;
         }
     }
 
     /*
      * If the result of the computation is going to be discarded anyway,
      * avoid the computation altogether.
      */
     if (min >= predicted_us)
         return UINT_MAX;
 
     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
  * @stop_tick: indication on whether or not to stop the tick
  */
 static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
                bool *stop_tick)
 {
     struct menu_device *data = this_cpu_ptr(&menu_devices);
     s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
     unsigned int predicted_us;
     u64 predicted_ns;
     u64 interactivity_req;
     unsigned int nr_iowaiters;
     ktime_t delta, delta_tick;
     int i, idx;
 
     if (data->needs_update) {
         menu_update(drv, dev);
         data->needs_update = 0;
     }
 
     /* determine the expected residency time, round up */
     delta = tick_nohz_get_sleep_length(&delta_tick);
     if (unlikely(delta < 0)) {
         delta = 0;
         delta_tick = 0;
     }
     data->next_timer_ns = delta;
 
     nr_iowaiters = nr_iowait_cpu(dev->cpu);
     data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);
 
     if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
         ((data->next_timer_ns < drv->states[1].target_residency_ns ||
           latency_req < drv->states[1].exit_latency_ns) &&
          !dev->states_usage[0].disable)) {
         /*
          * In this case state[0] will be used no matter what, so return
          * it right away and keep the tick running if state[0] is a
          * polling one.
          */
         *stop_tick = !(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
         return 0;
     }
 
     /* Round up the result for half microseconds. */
     predicted_us = div_u64(data->next_timer_ns *
                    data->correction_factor[data->bucket] +
                    (RESOLUTION * DECAY * NSEC_PER_USEC) / 2,
                    RESOLUTION * DECAY * NSEC_PER_USEC);
     /* Use the lowest expected idle interval to pick the idle state. */
     predicted_ns = (u64)min(predicted_us,
                 get_typical_interval(data, predicted_us)) *
                 NSEC_PER_USEC;
 
     if (tick_nohz_tick_stopped()) {
         /*
          * If the tick is already stopped, the cost of possible short
          * idle duration misprediction is much higher, because the CPU
          * may be stuck in a shallow idle state for a long time as a
          * result of it.  In that case say we might mispredict and use
          * the known time till the closest timer event for the idle
          * state selection.
          */
         if (predicted_ns < TICK_NSEC)
             predicted_ns = data->next_timer_ns;
     } else {
         /*
          * Use the performance multiplier and the user-configurable
          * latency_req to determine the maximum exit latency.
          */
         interactivity_req = div64_u64(predicted_ns,
                           performance_multiplier(nr_iowaiters));
         if (latency_req > interactivity_req)
             latency_req = interactivity_req;
     }
 
     /*
      * Find the idle state with the lowest power while satisfying
      * our constraints.
      */
     idx = -1;
     for (i = 0; i < drv->state_count; i++) {
         struct cpuidle_state *s = &drv->states[i];
 
         if (dev->states_usage[i].disable)
             continue;
 
         if (idx == -1)
             idx = i; /* first enabled state */
 
         if (s->target_residency_ns > predicted_ns) {
             /*
              * Use a physical idle state, not busy polling, unless
              * a timer is going to trigger soon enough.
              */
             if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
                 s->exit_latency_ns <= latency_req &&
                 s->target_residency_ns <= data->next_timer_ns) {
                 predicted_ns = s->target_residency_ns;
                 idx = i;
                 break;
             }
             if (predicted_ns < TICK_NSEC)
                 break;
 
             if (!tick_nohz_tick_stopped()) {
                 /*
                  * If the state selected so far is shallow,
                  * waking up early won't hurt, so retain the
                  * tick in that case and let the governor run
                  * again in the next iteration of the loop.
                  */
                 predicted_ns = drv->states[idx].target_residency_ns;
                 break;
             }
 
             /*
              * If the state selected so far is shallow and this
              * state's target residency matches the time till the
              * closest timer event, select this one to avoid getting
              * stuck in the shallow one for too long.
              */
             if (drv->states[idx].target_residency_ns < TICK_NSEC &&
                 s->target_residency_ns <= delta_tick)
                 idx = i;
 
             return idx;
         }
         if (s->exit_latency_ns > latency_req)
             break;
 
         idx = i;
     }
 
     if (idx == -1)
         idx = 0; /* No states enabled. Must use 0. */
 
     /*
      * 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) ||
          predicted_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
         *stop_tick = false;
 
         if (idx > 0 && drv->states[idx].target_residency_ns > delta_tick) {
             /*
              * The tick is not going to be stopped and the target
              * residency of the state to be returned is not within
              * the time until the next timer event including the
              * tick, so try to correct that.
              */
             for (i = idx - 1; i >= 0; i--) {
                 if (dev->states_usage[i].disable)
                     continue;
 
                 idx = i;
                 if (drv->states[i].target_residency_ns <= delta_tick)
                     break;
             }
         }
     }
 
     return 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);
 
     dev->last_state_idx = index;
     data->needs_update = 1;
     data->tick_wakeup = tick_nohz_idle_got_tick();
 }
 
 /**
  * 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 = dev->last_state_idx;
     struct cpuidle_state *target = &drv->states[last_idx];
     u64 measured_ns;
     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.
      */
 
     if (data->tick_wakeup && data->next_timer_ns > TICK_NSEC) {
         /*
          * The nohz code said that there wouldn't be any events within
          * the tick boundary (if the tick was stopped), but the idle
          * duration predictor had a differing opinion.  Since the CPU
          * was woken up by a tick (that wasn't stopped after all), the
          * predictor was not quite right, so assume that the CPU could
          * have been idle long (but not forever) to help the idle
          * duration predictor do a better job next time.
          */
         measured_ns = 9 * MAX_INTERESTING / 10;
     } else if ((drv->states[last_idx].flags & CPUIDLE_FLAG_POLLING) &&
            dev->poll_time_limit) {
         /*
          * The CPU exited the "polling" state due to a time limit, so
          * the idle duration prediction leading to the selection of that
          * state was inaccurate.  If a better prediction had been made,
          * the CPU might have been woken up from idle by the next timer.
          * Assume that to be the case.
          */
         measured_ns = data->next_timer_ns;
     } else {
         /* measured value */
         measured_ns = dev->last_residency_ns;
 
         /* Deduct exit latency */
         if (measured_ns > 2 * target->exit_latency_ns)
             measured_ns -= target->exit_latency_ns;
         else
             measured_ns /= 2;
     }
 
     /* Make sure our coefficients do not exceed unity */
     if (measured_ns > data->next_timer_ns)
         measured_ns = data->next_timer_ns;
 
     /* Update our correction ratio */
     new_factor = data->correction_factor[data->bucket];
     new_factor -= new_factor / DECAY;
 
     if (data->next_timer_ns > 0 && measured_ns < MAX_INTERESTING)
         new_factor += div64_u64(RESOLUTION * measured_ns,
                     data->next_timer_ns);
     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++] = ktime_to_us(measured_ns);
     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,
 };
 
 /**
  * init_menu - initializes the governor
  */
 static int __init init_menu(void)
 {
     return cpuidle_register_governor(&menu_governor);
 }
 
 postcore_initcall(init_menu);

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