![Tuukka Tikkanen](/assets/img/avatar_default.png)
Field predicted_us value can never exceed expected_us value, but it has a potentially larger type. As there is no need for additional 32 bits of zeroes on 32 bit plaforms, change the type of predicted_us to match the type of expected_us. Field correction_factor is used to store a value that cannot exceed the product of RESOLUTION and DECAY (default 1024*8 = 8192). The constants cannot in practice be incremented to such values, that they'd overflow unsigned int even on 32 bit systems, so the type is changed to avoid unnecessary 64 bit arithmetic on 32 bit systems. One multiplication of (now) 32 bit values needs an added cast to avoid truncation of the result and has been added. In order to avoid another multiplication from 32 bit domain to 64 bit domain, the new correction_factor calculation has been changed from new = old * (DECAY-1) / DECAY to new = old - old / DECAY, which with infinite precision would yeild exactly the same result, but now changes the direction of rounding. The impact is not significant as the maximum accumulated difference cannot exceed the value of DECAY, which is relatively small compared to product of RESOLUTION and DECAY (8 / 8192). Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@linaro.org> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
480 lines
14 KiB
C
480 lines
14 KiB
C
/*
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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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* This code is licenced under the GPL version 2 as described
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* in the COPYING file that acompanies the Linux Kernel.
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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/pm_qos.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/math64.h>
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#include <linux/module.h>
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/*
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* Please note when changing the tuning values:
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* If (MAX_INTERESTING-1) * RESOLUTION > UINT_MAX, the result of
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* a scaling operation multiplication may overflow on 32 bit platforms.
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* In that case, #define RESOLUTION as ULL to get 64 bit result:
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* #define RESOLUTION 1024ULL
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*
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* The default values do not overflow.
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*/
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#define BUCKETS 12
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#define INTERVALS 8
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#define RESOLUTION 1024
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#define DECAY 8
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#define MAX_INTERESTING 50000
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#define STDDEV_THRESH 400
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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 last_state_idx;
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int needs_update;
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unsigned int expected_us;
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unsigned int predicted_us;
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unsigned int exit_us;
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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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#define LOAD_INT(x) ((x) >> FSHIFT)
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#define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
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static int get_loadavg(void)
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{
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unsigned long this = this_cpu_load();
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return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
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}
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static inline int which_bucket(unsigned int duration)
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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_iowait_cpu(smp_processor_id()))
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bucket = BUCKETS/2;
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if (duration < 10)
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return bucket;
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if (duration < 100)
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return bucket + 1;
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if (duration < 1000)
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return bucket + 2;
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if (duration < 10000)
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return bucket + 3;
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if (duration < 100000)
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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(void)
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{
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int mult = 1;
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/* for higher loadavg, we are more reluctant */
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mult += 2 * get_loadavg();
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/* for IO wait tasks (per cpu!) we add 5x each */
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mult += 10 * nr_iowait_cpu(smp_processor_id());
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return mult;
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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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/* This implements DIV_ROUND_CLOSEST but avoids 64 bit division */
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static u64 div_round64(u64 dividend, u32 divisor)
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{
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return div_u64(dividend + (divisor / 2), divisor);
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}
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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 void get_typical_interval(struct menu_device *data)
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{
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int i, divisor;
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unsigned int max, thresh;
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uint64_t avg, stddev;
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thresh = UINT_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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max = 0;
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avg = 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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avg += value;
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divisor++;
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if (value > max)
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max = value;
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}
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}
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do_div(avg, divisor);
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/* Then try to determine standard deviation */
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stddev = 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 = value - avg;
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stddev += diff * diff;
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}
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}
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do_div(stddev, divisor);
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/*
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* The typical interval is obtained when standard deviation is small
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* or standard deviation is small compared to the average interval.
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*
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* int_sqrt() formal parameter type is unsigned long. When the
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* greatest difference to an outlier exceeds ~65 ms * sqrt(divisor)
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* the resulting squared standard deviation exceeds the input domain
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* of int_sqrt on platforms where unsigned long is 32 bits in size.
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* In such case reject the candidate average.
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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(stddev <= ULONG_MAX)) {
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stddev = int_sqrt(stddev);
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if (((avg > stddev * 6) && (divisor * 4 >= INTERVALS * 3))
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|| stddev <= 20) {
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if (data->expected_us > avg)
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data->predicted_us = avg;
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return;
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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;
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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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*/
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static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
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{
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struct menu_device *data = &__get_cpu_var(menu_devices);
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int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
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int i;
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int multiplier;
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struct timespec t;
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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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data->last_state_idx = 0;
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data->exit_us = 0;
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/* Special case when user has set very strict latency requirement */
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if (unlikely(latency_req == 0))
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return 0;
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/* determine the expected residency time, round up */
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t = ktime_to_timespec(tick_nohz_get_sleep_length());
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data->expected_us =
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t.tv_sec * USEC_PER_SEC + t.tv_nsec / NSEC_PER_USEC;
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data->bucket = which_bucket(data->expected_us);
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multiplier = performance_multiplier();
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/*
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* if the correction factor is 0 (eg first time init or cpu hotplug
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* etc), we actually want to start out with a unity factor.
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*/
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if (data->correction_factor[data->bucket] == 0)
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data->correction_factor[data->bucket] = RESOLUTION * DECAY;
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/*
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* Force the result of multiplication to be 64 bits even if both
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* operands are 32 bits.
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* Make sure to round up for half microseconds.
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*/
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data->predicted_us = div_round64((uint64_t)data->expected_us *
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data->correction_factor[data->bucket],
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RESOLUTION * DECAY);
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get_typical_interval(data);
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/*
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* We want to default to C1 (hlt), not to busy polling
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* unless the timer is happening really really soon.
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*/
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if (data->expected_us > 5 &&
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!drv->states[CPUIDLE_DRIVER_STATE_START].disabled &&
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dev->states_usage[CPUIDLE_DRIVER_STATE_START].disable == 0)
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data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
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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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for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
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struct cpuidle_state *s = &drv->states[i];
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struct cpuidle_state_usage *su = &dev->states_usage[i];
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if (s->disabled || su->disable)
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continue;
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if (s->target_residency > data->predicted_us)
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continue;
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if (s->exit_latency > latency_req)
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continue;
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if (s->exit_latency * multiplier > data->predicted_us)
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continue;
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data->last_state_idx = i;
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data->exit_us = s->exit_latency;
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}
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return data->last_state_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 = &__get_cpu_var(menu_devices);
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data->last_state_idx = index;
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if (index >= 0)
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data->needs_update = 1;
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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 = &__get_cpu_var(menu_devices);
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int last_idx = data->last_state_idx;
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unsigned int last_idle_us = cpuidle_get_last_residency(dev);
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struct cpuidle_state *target = &drv->states[last_idx];
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unsigned int measured_us;
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unsigned int new_factor;
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/*
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* Ugh, this idle state doesn't support residency measurements, so we
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* are basically lost in the dark. As a compromise, assume we slept
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* for the whole expected time.
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*/
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if (unlikely(!(target->flags & CPUIDLE_FLAG_TIME_VALID)))
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last_idle_us = data->expected_us;
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measured_us = last_idle_us;
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/*
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* We correct for the exit latency; we are assuming here that the
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* exit latency happens after the event that we're interested in.
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*/
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if (measured_us > data->exit_us)
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measured_us -= data->exit_us;
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/* Update our correction ratio */
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new_factor = data->correction_factor[data->bucket];
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new_factor -= new_factor / DECAY;
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if (data->expected_us > 0 && measured_us < MAX_INTERESTING)
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new_factor += RESOLUTION * measured_us / data->expected_us;
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else
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/*
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* we were idle so long that we count it as a perfect
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* prediction
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*/
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new_factor += RESOLUTION;
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/*
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* We don't want 0 as factor; we always want at least
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* a tiny bit of estimated time. Fortunately, due to rounding,
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* new_factor will stay nonzero regardless of measured_us values
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* and the compiler can eliminate this test as long as DECAY > 1.
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*/
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if (DECAY == 1 && unlikely(new_factor == 0))
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new_factor = 1;
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data->correction_factor[data->bucket] = new_factor;
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/* update the repeating-pattern data */
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data->intervals[data->interval_ptr++] = last_idle_us;
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if (data->interval_ptr >= INTERVALS)
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data->interval_ptr = 0;
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}
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/**
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* menu_enable_device - scans a CPU's states and does setup
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* @drv: cpuidle driver
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* @dev: the CPU
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*/
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static int menu_enable_device(struct cpuidle_driver *drv,
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struct cpuidle_device *dev)
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{
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struct menu_device *data = &per_cpu(menu_devices, dev->cpu);
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memset(data, 0, sizeof(struct menu_device));
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return 0;
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}
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static struct cpuidle_governor menu_governor = {
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.name = "menu",
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.rating = 20,
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.enable = menu_enable_device,
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.select = menu_select,
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.reflect = menu_reflect,
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.owner = THIS_MODULE,
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};
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/**
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* init_menu - initializes the governor
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*/
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static int __init init_menu(void)
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{
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return cpuidle_register_governor(&menu_governor);
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}
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postcore_initcall(init_menu);
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