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![Rafael J. Wysocki](/assets/img/avatar_default.png)
Make the menu governor check the tick_nohz_get_next_hrtimer() return value so as to avoid dealing with negative "sleep length" values and make it use that value directly when the tick is stopped. While at it, rename local variable delta_next in menu_select() to delta_tick which better reflects its purpose. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
579 lines
18 KiB
C
579 lines
18 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* menu.c - the menu idle governor
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*
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* Copyright (C) 2006-2007 Adam Belay <abelay@novell.com>
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* Copyright (C) 2009 Intel Corporation
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* Author:
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* Arjan van de Ven <arjan@linux.intel.com>
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*/
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#include <linux/kernel.h>
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#include <linux/cpuidle.h>
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#include <linux/time.h>
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#include <linux/ktime.h>
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#include <linux/hrtimer.h>
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#include <linux/tick.h>
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#include <linux/sched.h>
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#include <linux/sched/loadavg.h>
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#include <linux/sched/stat.h>
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#include <linux/math64.h>
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#define BUCKETS 12
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#define INTERVAL_SHIFT 3
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#define INTERVALS (1UL << INTERVAL_SHIFT)
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#define RESOLUTION 1024
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#define DECAY 8
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#define MAX_INTERESTING (50000 * NSEC_PER_USEC)
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/*
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* Concepts and ideas behind the menu governor
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*
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* For the menu governor, there are 3 decision factors for picking a C
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* state:
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* 1) Energy break even point
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* 2) Performance impact
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* 3) Latency tolerance (from pmqos infrastructure)
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* These these three factors are treated independently.
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*
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* Energy break even point
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* -----------------------
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* C state entry and exit have an energy cost, and a certain amount of time in
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* the C state is required to actually break even on this cost. CPUIDLE
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* provides us this duration in the "target_residency" field. So all that we
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* need is a good prediction of how long we'll be idle. Like the traditional
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* menu governor, we start with the actual known "next timer event" time.
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*
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* Since there are other source of wakeups (interrupts for example) than
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* the next timer event, this estimation is rather optimistic. To get a
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* more realistic estimate, a correction factor is applied to the estimate,
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* that is based on historic behavior. For example, if in the past the actual
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* duration always was 50% of the next timer tick, the correction factor will
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* be 0.5.
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*
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* menu uses a running average for this correction factor, however it uses a
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* set of factors, not just a single factor. This stems from the realization
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* that the ratio is dependent on the order of magnitude of the expected
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* duration; if we expect 500 milliseconds of idle time the likelihood of
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* getting an interrupt very early is much higher than if we expect 50 micro
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* seconds of idle time. A second independent factor that has big impact on
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* the actual factor is if there is (disk) IO outstanding or not.
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* (as a special twist, we consider every sleep longer than 50 milliseconds
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* as perfect; there are no power gains for sleeping longer than this)
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*
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* For these two reasons we keep an array of 12 independent factors, that gets
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* indexed based on the magnitude of the expected duration as well as the
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* "is IO outstanding" property.
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*
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* Repeatable-interval-detector
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* ----------------------------
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* There are some cases where "next timer" is a completely unusable predictor:
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* Those cases where the interval is fixed, for example due to hardware
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* interrupt mitigation, but also due to fixed transfer rate devices such as
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* mice.
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* For this, we use a different predictor: We track the duration of the last 8
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* intervals and if the stand deviation of these 8 intervals is below a
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* threshold value, we use the average of these intervals as prediction.
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*
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* Limiting Performance Impact
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* ---------------------------
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* C states, especially those with large exit latencies, can have a real
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* noticeable impact on workloads, which is not acceptable for most sysadmins,
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* and in addition, less performance has a power price of its own.
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*
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* As a general rule of thumb, menu assumes that the following heuristic
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* holds:
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* The busier the system, the less impact of C states is acceptable
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*
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* This rule-of-thumb is implemented using a performance-multiplier:
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* If the exit latency times the performance multiplier is longer than
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* the predicted duration, the C state is not considered a candidate
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* for selection due to a too high performance impact. So the higher
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* this multiplier is, the longer we need to be idle to pick a deep C
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* state, and thus the less likely a busy CPU will hit such a deep
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* C state.
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*
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* Two factors are used in determing this multiplier:
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* a value of 10 is added for each point of "per cpu load average" we have.
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* a value of 5 points is added for each process that is waiting for
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* IO on this CPU.
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* (these values are experimentally determined)
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*
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* The load average factor gives a longer term (few seconds) input to the
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* decision, while the iowait value gives a cpu local instantanious input.
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* The iowait factor may look low, but realize that this is also already
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* represented in the system load average.
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*
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*/
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struct menu_device {
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int needs_update;
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int tick_wakeup;
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u64 next_timer_ns;
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unsigned int bucket;
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unsigned int correction_factor[BUCKETS];
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unsigned int intervals[INTERVALS];
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int interval_ptr;
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};
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static inline int which_bucket(u64 duration_ns, unsigned long nr_iowaiters)
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{
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int bucket = 0;
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/*
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* We keep two groups of stats; one with no
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* IO pending, one without.
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* This allows us to calculate
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* E(duration)|iowait
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*/
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if (nr_iowaiters)
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bucket = BUCKETS/2;
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if (duration_ns < 10ULL * NSEC_PER_USEC)
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return bucket;
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if (duration_ns < 100ULL * NSEC_PER_USEC)
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return bucket + 1;
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if (duration_ns < 1000ULL * NSEC_PER_USEC)
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return bucket + 2;
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if (duration_ns < 10000ULL * NSEC_PER_USEC)
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return bucket + 3;
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if (duration_ns < 100000ULL * NSEC_PER_USEC)
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return bucket + 4;
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return bucket + 5;
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}
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/*
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* Return a multiplier for the exit latency that is intended
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* to take performance requirements into account.
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* The more performance critical we estimate the system
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* to be, the higher this multiplier, and thus the higher
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* the barrier to go to an expensive C state.
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*/
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static inline int performance_multiplier(unsigned long nr_iowaiters)
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{
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/* for IO wait tasks (per cpu!) we add 10x each */
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return 1 + 10 * nr_iowaiters;
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}
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static DEFINE_PER_CPU(struct menu_device, menu_devices);
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static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
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/*
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* Try detecting repeating patterns by keeping track of the last 8
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* intervals, and checking if the standard deviation of that set
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* of points is below a threshold. If it is... then use the
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* average of these 8 points as the estimated value.
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*/
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static unsigned int get_typical_interval(struct menu_device *data,
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unsigned int predicted_us)
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{
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int i, divisor;
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unsigned int min, max, thresh, avg;
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uint64_t sum, variance;
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thresh = INT_MAX; /* Discard outliers above this value */
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again:
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/* First calculate the average of past intervals */
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min = UINT_MAX;
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max = 0;
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sum = 0;
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divisor = 0;
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for (i = 0; i < INTERVALS; i++) {
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unsigned int value = data->intervals[i];
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if (value <= thresh) {
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sum += value;
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divisor++;
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if (value > max)
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max = value;
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if (value < min)
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min = value;
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}
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}
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/*
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* If the result of the computation is going to be discarded anyway,
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* avoid the computation altogether.
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*/
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if (min >= predicted_us)
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return UINT_MAX;
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if (divisor == INTERVALS)
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avg = sum >> INTERVAL_SHIFT;
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else
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avg = div_u64(sum, divisor);
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/* Then try to determine variance */
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variance = 0;
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for (i = 0; i < INTERVALS; i++) {
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unsigned int value = data->intervals[i];
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if (value <= thresh) {
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int64_t diff = (int64_t)value - avg;
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variance += diff * diff;
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}
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}
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if (divisor == INTERVALS)
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variance >>= INTERVAL_SHIFT;
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else
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do_div(variance, divisor);
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/*
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* The typical interval is obtained when standard deviation is
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* small (stddev <= 20 us, variance <= 400 us^2) or standard
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* deviation is small compared to the average interval (avg >
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* 6*stddev, avg^2 > 36*variance). The average is smaller than
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* UINT_MAX aka U32_MAX, so computing its square does not
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* overflow a u64. We simply reject this candidate average if
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* the standard deviation is greater than 715 s (which is
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* rather unlikely).
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*
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* Use this result only if there is no timer to wake us up sooner.
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*/
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if (likely(variance <= U64_MAX/36)) {
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if ((((u64)avg*avg > variance*36) && (divisor * 4 >= INTERVALS * 3))
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|| variance <= 400) {
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return avg;
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}
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}
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/*
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* If we have outliers to the upside in our distribution, discard
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* those by setting the threshold to exclude these outliers, then
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* calculate the average and standard deviation again. Once we get
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* down to the bottom 3/4 of our samples, stop excluding samples.
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*
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* This can deal with workloads that have long pauses interspersed
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* with sporadic activity with a bunch of short pauses.
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*/
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if ((divisor * 4) <= INTERVALS * 3)
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return UINT_MAX;
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thresh = max - 1;
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goto again;
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}
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/**
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* menu_select - selects the next idle state to enter
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* @drv: cpuidle driver containing state data
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* @dev: the CPU
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* @stop_tick: indication on whether or not to stop the tick
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*/
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static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
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bool *stop_tick)
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{
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struct menu_device *data = this_cpu_ptr(&menu_devices);
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s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
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unsigned int predicted_us;
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u64 predicted_ns;
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u64 interactivity_req;
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unsigned long nr_iowaiters;
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ktime_t delta, delta_tick;
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int i, idx;
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if (data->needs_update) {
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menu_update(drv, dev);
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data->needs_update = 0;
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}
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/* determine the expected residency time, round up */
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delta = tick_nohz_get_sleep_length(&delta_tick);
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if (unlikely(delta < 0)) {
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delta = 0;
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delta_tick = 0;
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}
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data->next_timer_ns = delta;
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nr_iowaiters = nr_iowait_cpu(dev->cpu);
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data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);
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if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
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((data->next_timer_ns < drv->states[1].target_residency_ns ||
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latency_req < drv->states[1].exit_latency_ns) &&
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!dev->states_usage[0].disable)) {
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/*
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* In this case state[0] will be used no matter what, so return
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* it right away and keep the tick running if state[0] is a
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* polling one.
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*/
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*stop_tick = !(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
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return 0;
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}
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/* Round up the result for half microseconds. */
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predicted_us = div_u64(data->next_timer_ns *
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data->correction_factor[data->bucket] +
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(RESOLUTION * DECAY * NSEC_PER_USEC) / 2,
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RESOLUTION * DECAY * NSEC_PER_USEC);
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/* Use the lowest expected idle interval to pick the idle state. */
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predicted_ns = (u64)min(predicted_us,
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get_typical_interval(data, predicted_us)) *
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NSEC_PER_USEC;
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if (tick_nohz_tick_stopped()) {
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/*
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* If the tick is already stopped, the cost of possible short
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* idle duration misprediction is much higher, because the CPU
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* may be stuck in a shallow idle state for a long time as a
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* result of it. In that case say we might mispredict and use
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* the known time till the closest timer event for the idle
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* state selection.
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*/
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if (predicted_ns < TICK_NSEC)
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predicted_ns = data->next_timer_ns;
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} else {
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/*
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* Use the performance multiplier and the user-configurable
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* latency_req to determine the maximum exit latency.
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*/
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interactivity_req = div64_u64(predicted_ns,
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performance_multiplier(nr_iowaiters));
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if (latency_req > interactivity_req)
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latency_req = interactivity_req;
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}
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/*
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* Find the idle state with the lowest power while satisfying
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* our constraints.
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*/
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idx = -1;
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for (i = 0; i < drv->state_count; i++) {
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struct cpuidle_state *s = &drv->states[i];
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if (dev->states_usage[i].disable)
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continue;
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if (idx == -1)
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idx = i; /* first enabled state */
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if (s->target_residency_ns > predicted_ns) {
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/*
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* Use a physical idle state, not busy polling, unless
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* a timer is going to trigger soon enough.
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*/
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if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
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s->exit_latency_ns <= latency_req &&
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s->target_residency_ns <= data->next_timer_ns) {
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predicted_ns = s->target_residency_ns;
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idx = i;
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break;
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}
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if (predicted_ns < TICK_NSEC)
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break;
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if (!tick_nohz_tick_stopped()) {
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/*
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* If the state selected so far is shallow,
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* waking up early won't hurt, so retain the
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* tick in that case and let the governor run
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* again in the next iteration of the loop.
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*/
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predicted_ns = drv->states[idx].target_residency_ns;
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break;
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}
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/*
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* If the state selected so far is shallow and this
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* state's target residency matches the time till the
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* closest timer event, select this one to avoid getting
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* stuck in the shallow one for too long.
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*/
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if (drv->states[idx].target_residency_ns < TICK_NSEC &&
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s->target_residency_ns <= delta_tick)
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idx = i;
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return idx;
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}
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if (s->exit_latency_ns > latency_req)
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break;
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idx = i;
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}
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if (idx == -1)
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idx = 0; /* No states enabled. Must use 0. */
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/*
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* Don't stop the tick if the selected state is a polling one or if the
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* expected idle duration is shorter than the tick period length.
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*/
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if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
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predicted_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
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*stop_tick = false;
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if (idx > 0 && drv->states[idx].target_residency_ns > delta_tick) {
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/*
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* The tick is not going to be stopped and the target
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* residency of the state to be returned is not within
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* the time until the next timer event including the
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* tick, so try to correct that.
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*/
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for (i = idx - 1; i >= 0; i--) {
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if (dev->states_usage[i].disable)
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continue;
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idx = i;
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if (drv->states[i].target_residency_ns <= delta_tick)
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break;
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}
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}
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}
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return idx;
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}
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/**
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* menu_reflect - records that data structures need update
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* @dev: the CPU
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* @index: the index of actual entered state
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*
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* NOTE: it's important to be fast here because this operation will add to
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* the overall exit latency.
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*/
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static void menu_reflect(struct cpuidle_device *dev, int index)
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{
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struct menu_device *data = this_cpu_ptr(&menu_devices);
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dev->last_state_idx = index;
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data->needs_update = 1;
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data->tick_wakeup = tick_nohz_idle_got_tick();
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}
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/**
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* menu_update - attempts to guess what happened after entry
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* @drv: cpuidle driver containing state data
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* @dev: the CPU
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*/
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static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
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{
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struct menu_device *data = this_cpu_ptr(&menu_devices);
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int last_idx = dev->last_state_idx;
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struct cpuidle_state *target = &drv->states[last_idx];
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u64 measured_ns;
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unsigned int new_factor;
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/*
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* Try to figure out how much time passed between entry to low
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* power state and occurrence of the wakeup event.
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*
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* If the entered idle state didn't support residency measurements,
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* we use them anyway if they are short, and if long,
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* truncate to the whole expected time.
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*
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* Any measured amount of time will include the exit latency.
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* Since we are interested in when the wakeup begun, not when it
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* was completed, we must subtract the exit latency. However, if
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* the measured amount of time is less than the exit latency,
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* assume the state was never reached and the exit latency is 0.
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*/
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if (data->tick_wakeup && data->next_timer_ns > TICK_NSEC) {
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/*
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* The nohz code said that there wouldn't be any events within
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* the tick boundary (if the tick was stopped), but the idle
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* duration predictor had a differing opinion. Since the CPU
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* was woken up by a tick (that wasn't stopped after all), the
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* predictor was not quite right, so assume that the CPU could
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* have been idle long (but not forever) to help the idle
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* duration predictor do a better job next time.
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*/
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measured_ns = 9 * MAX_INTERESTING / 10;
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} else if ((drv->states[last_idx].flags & CPUIDLE_FLAG_POLLING) &&
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dev->poll_time_limit) {
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/*
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* The CPU exited the "polling" state due to a time limit, so
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* the idle duration prediction leading to the selection of that
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* state was inaccurate. If a better prediction had been made,
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* the CPU might have been woken up from idle by the next timer.
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* Assume that to be the case.
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*/
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measured_ns = data->next_timer_ns;
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|
} 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);
|