path: root/Documentation/scheduler/sched-hmp.txt

CONTENTS

1. Introduction
   1.1 Heterogeneous Systems
   1.2 CPU Frequency Guidance
2. Window-Based Load Tracking Scheme
   2.1 Synchronized Windows
   2.2 struct ravg
   2.3 Scaling Load Statistics
   2.4 sched_window_stats_policy
   2.5 Task Events
   2.6 update_task_ravg()
   2.7 update_history()
   2.8 Per-task 'initial task load'
3. CPU Capacity
   3.1 Load scale factor
   3.2 CPU Power
4. CPU Power
5. HMP Scheduler
   5.1 Classification of Tasks and CPUs
   5.2 Task Wakeup and select_best_cpu()
   5.3 Scheduler Tick
   5.4 Load Balancer
   5.5 Real Time Tasks
   5.6 Task packing
6. Frequency Guidance
   6.1 Per-CPU Window-Based Stats
   6.1 Per-task Window-Based Stats
   6.3 Effect of various task events
7. Tunables
8. HMP Scheduler Trace Points
   8.1 sched_enq_deq_task
   8.2 sched_task_load
   8.3 sched_cpu_load
   8.4 sched_update_task_ravg
   8.5 sched_update_history
   8.6 sched_reset_all_windows_stats
   8.7 sched_migration_update_sum
   8.8 sched_get_busy
   8.9 sched_freq_alert
   8.10 sched_set_boost

===============
1. INTRODUCTION
===============

Scheduler extensions described in this document serves two goals:

1) handle heterogeneous multi-processor (HMP) systems
2) guide cpufreq governor on proactive changes to cpu frequency

*** 1.1 Heterogeneous systems

Heterogeneous systems have cpus that differ with regard to their performance and
power characteristics. Some cpus could offer peak performance better than
others, although at cost of consuming more power. We shall refer such cpus as
"high performance" or "performance efficient" cpus. Other cpus that offer lesser
peak performance are referred to as "power efficient".

In this situation the scheduler is tasked with the responsibility of assigning
tasks to run on the right cpus where their performance requirements can be met
at the least expense of power.

Achieving that goal is made complicated by the fact that the scheduler has
little clue about performance requirements of tasks and how they may change by
running on power or performance efficient cpus!  One simplifying assumption here
could be that a task's desire for more performance is expressed by its cpu
utilization. A task demanding high cpu utilization on a power-efficient cpu
would likely improve in its performance by running on a performance-efficient
cpu. This idea forms the basis for HMP-related scheduler extensions.

Key inputs required by the HMP scheduler for its task placement decisions are:

a) task load - this reflects cpu utilization or demand of tasks
b) CPU capacity - this reflects peak performance offered by cpus
c) CPU power - this reflects power or energy cost of cpus

Once all 3 pieces of information are available, the HMP scheduler can place
tasks on the lowest power cpus where their demand can be satisfied.

*** 1.2 CPU Frequency guidance

A somewhat separate but related goal of the scheduler extensions described here
is to provide guidance to the cpufreq governor on the need to change cpu
frequency. Most governors that control cpu frequency work on a reactive basis.
CPU utilization is sampled at regular intervals, based on which the need to
change frequency is determined. Higher utilization leads to a frequency increase
and vice-versa.  There are several problems with this approach that scheduler
can help resolve.

a) latency

	Reactive nature introduces latency for cpus to ramp up to desired speed
	which can hurt application performance. This is inevitable as cpufreq
	governors can only track cpu utilization as a whole and not tasks which
	are driving that demand. Scheduler can however keep track of individual
	task demand and can alert the governor on changing task activity. For
	example, request raise in frequency when tasks activity is increasing on
	a cpu because of wakeup or migration or request frequency to be lowered
	when task activity is decreasing because of sleep/exit or migration.

b) part-picture

	Most governors track utilization of each CPU independently. When a task
	migrates from one cpu to another the task's execution time is split
	across the two cpus. The governor can fail to see the full picture of
	task demand in this case and thus the need for increasing frequency,
	affecting the task's performance. Scheduler can keep track of task
	migrations, fix up busy time upon migration and report per-cpu busy time
	to the governor that reflects task demand accurately.

The rest of this document explains key enhancements made to the scheduler to
accomplish both of the aforementioned goals.

====================================
2. WINDOW-BASED LOAD TRACKING SCHEME
====================================

As mentioned in the introduction section, knowledge of the CPU demand exerted by
a task is a prerequisite to knowing where to best place the task in an HMP
system. The per-entity load tracking (PELT) scheme, present in Linux kernel
since v3.7, has some perceived shortcomings when used to place tasks on HMP
systems or provide recommendations on CPU frequency.

Per-entity load tracking does not make a distinction between the ramp up
vs. ramp down time of task load. It also decays task load without exception when
a task sleeps. As an example, a cpu bound task at its peak load (LOAD_AVG_MAX or
47742) can see its load decay to 0 after a sleep of just 213ms! A cpu-bound task
running on a performance-efficient cpu could thus get re-classified as not
requiring such a cpu after a short sleep. In the case of mobile workloads, tasks
could go to sleep due to a lack of user input. When they wakeup it is very
likely their cpu utilization pattern repeats. Resetting their load across sleep
and incurring latency to reclassify them as requiring a high performance cpu can
hurt application performance.

The window-based load tracking scheme described in this document avoids these
drawbacks. It keeps track of N windows of execution for every task. Windows
where a task had no activity are ignored and not recorded. N can be tuned at
compile time (RAVG_HIST_SIZE defined in include/linux/sched.h) or at runtime
(/proc/sys/kernel/sched_ravg_hist_size). The window size, W, is common for all
tasks and currently defaults to 10ms ('sched_ravg_window' defined in
kernel/sched/core.c). The window size can be tuned at boot time via the
sched_ravg_window=W argument to kernel. Alternately it can be tuned after boot
via tunables provided by the interactive governor. More on this later.

Based on the N samples available per-task, a per-task "demand" attribute is
calculated which represents the cpu demand of that task. The demand attribute is
used to classify tasks as to whether or not they need a performance-efficient
CPU and also serves to provide inputs on frequency to the cpufreq governor. More
on this later.  The 'sched_window_stats_policy' tunable (defined in
kernel/sched/core.c) controls how the demand field for a task is derived from
its N past samples.

*** 2.1 Synchronized windows

Windows of observation for task activity are synchronized across cpus. This
greatly aids in the scheduler's frequency guidance feature. Scheduler currently
relies on a synchronized clock (sched_clock()) for this feature to work. It may
be possible to extend this feature to work on systems having an unsynchronized
sched_clock().

struct rq {

	..

	u64	window_start;

	..
};

The 'window_start' attribute represents the time when current window began on a
cpu.  It is updated when key task events such as wakeup or context-switch call
update_task_ravg() to record task activity. The window_start value is expected
to be the same for all cpus, although it could be behind on some cpus where it
has not yet been updated because update_task_ravg() has not been recently
called. For example, when a cpu is idle for a long time its window_start could
be stale.  The window_start value for such cpus is rolled forward upon
occurrence of a task event resulting in a call to update_task_ravg().

*** 2.2 struct ravg

The ravg struct contains information tracked per-task.

struct ravg {
	u64 mark_start;
	u32 sum, demand;
	u32 sum_history[RAVG_HIST_SIZE];
#ifdef CONFIG_SCHED_FREQ_INPUT
	u32 curr_window, prev_window;
#endif
};

struct task_struct {

	..

	struct ravg ravg;

	..
};

sum_history[] 	- stores cpu utilization samples from N previous windows
		  where task had activity

sum		- stores cpu utilization of the task in its most recently
		  tracked window. Once the corresponding window terminates,
		  'sum' will be pushed into the sum_history[] array and is then
		  reset to 0. It is possible that the window corresponding to
		  sum is not the current window being tracked on a cpu.  For
		  example, a task could go to sleep in window X and wakeup in
		  window Y (Y > X).  In this case, sum would correspond to the
		  task's activity seen in window X.  When update_task_ravg() is
		  called during the task's wakeup event it will be seen that
		  window X has elapsed. The sum value will be pushed to
		  'sum_history[]' array before being reset to 0.

demand		- represents task's cpu demand and is derived from the
		  elements in sum_history[]. The section on
		  'sched_window_stats_policy' provides more details on how
		  'demand' is derived from elements in sum_history[] array

mark_start	- records timestamp of the beginning of the most recent task
		  event. See section on 'Task events' for possible events that
		  update 'mark_start'

curr_window	- this is described in the section on 'Frequency guidance'

prev_window	- this is described in the section on 'Frequency guidance'


*** 2.3 Scaling load statistics

Time required for a task to complete its work (and hence its load) depends on,
among various other factors, cpu frequency and its efficiency. In a HMP system,
some cpus are more performance efficient than others. Performance efficiency of
a cpu can be described by its "instructions-per-cycle" (IPC) attribute. History
of task execution could involve task having run at different frequencies and on
cpus with different IPC attributes. To avoid ambiguity of how task load relates
to the frequency and IPC of cpus on which a task has run, task load is captured
in a scaled form, with scaling being done in reference to an "ideal" cpu that
has best possible IPC and frequency. Such an "ideal" cpu, having the best
possible frequency and IPC, may or may not exist in system.

As an example, consider a HMP system, with two types of cpus, A53 and A57. A53
has IPC count of 1024 and can run at maximum frequency of 1 GHz, while A57 has
IPC count of 2048 and can run at maximum frequency of 2 GHz. Ideal cpu in this
case is A57 running at 2 GHz.

A unit of work that takes 100ms to finish on A53 running at 100MHz would get
done in 10ms on A53 running at 1GHz, in 5 ms running on A57 at 1 GHz and 2.5ms
on A57 running at 2 GHz.  Thus a load of 100ms can be expressed as 2.5ms in
reference to ideal cpu of A57 running at 2 GHz.

In order to understand how much load a task will consume on a given cpu, its
scaled load needs to be multiplied by a factor (load scale factor). In above
example, scaled load of 2.5ms needs to be multiplied by a factor of 4 in order
to estimate the load of task on A53 running at 1 GHz.

/proc/sched_debug provides IPC attribute and load scale factor for every cpu.

In summary, task load information stored in a task's sum_history[] array is
scaled for both frequency and efficiency. If a task runs for X ms, then the
value stored in its 'sum' field is derived as:

	X_s = X * (f_cur / max_possible_freq) *
		  (efficiency / max_possible_efficiency)

where:

X   		   	= cpu utilization that needs to be accounted
X_s 		   	= Scaled derivative of X
f_cur		   	= current frequency of the cpu where the task was
			  running
max_possible_freq  	= maximum possible frequency (across all cpus)
efficiency	   	= instructions per cycle (IPC) of cpu where task was
			  running
max_possible_efficiency = maximum IPC offered by any cpu in system


*** 2.4 sched_window_stats_policy

sched_window_stats_policy controls how the 'demand' attribute for a task is
derived from elements in its 'sum_history[]' array.

WINDOW_STATS_RECENT (0)
	demand = recent

WINDOW_STATS_MAX (1)
	demand = max

WINDOW_STATS_MAX_RECENT_AVG (2)
	demand = maximum(average, recent)

WINDOW_STATS_AVG (3)
	demand = average

where:
	M 	= history size specified by
		  /proc/sys/kernel/sched_ravg_hist_size
	average = average of first M samples found in the sum_history[] array
	max	= maximum value of first M samples found in the sum_history[]
		  array
	recent  = most recent sample (sum_history[0])
	demand	= demand attribute found in 'struct ravg'

This policy can be changed at runtime via
/proc/sys/kernel/sched_window_stats_policy. For example, the command
below would select WINDOW_STATS_USE_MAX policy

echo 1 > /proc/sys/kernel/sched_window_stats_policy

*** 2.5 Task events

A number of events results in the window-based stats of a task being
updated. These are:

PICK_NEXT_TASK	- the task is about to start running on a cpu
PUT_PREV_TASK	- the task stopped running on a cpu
TASK_WAKE	- the task is waking from sleep
TASK_MIGRATE	- the task is migrating from one cpu to another
TASK_UPDATE	- this event is invoked on a currently running task to
		  update the task's window-stats and also the cpu's
		  window-stats such as 'window_start'
IRQ_UPDATE	- event to record the busy time spent by an idle cpu
		  processing interrupts

*** 2.6 update_task_ravg()

update_task_ravg() is called to mark the beginning of an event for a task or a
cpu. It serves to accomplish these functions:

a. Update a cpu's window_start value
b. Update a task's window-stats (sum, sum_history[], demand and mark_start)

In addition update_task_ravg() updates the busy time information for the given
cpu, which is used for frequency guidance. This is described further in section
6.

*** 2.7 update_history()

update_history() is called on a task to record its activity in an elapsed
window. 'sum', which represents task's cpu demand in its elapsed window is
pushed onto sum_history[] array and its 'demand' attribute is updated based on
the sched_window_stats_policy in effect.

*** 2.8 Initial task load attribute for a task (init_load_pct)

In some cases, it may be desirable for children of a task to be assigned a
"high" load so that they can start running on best capacity cluster. By default,
newly created tasks are assigned a load defined by tunable sched_init_task_load
(Sec 7.8). Some specialized tasks may need a higher value than the global
default for their child tasks. This will let child tasks run on cpus with best
capacity. This is accomplished by setting the 'initial task load' attribute
(init_load_pct) for a task. Child tasks starting load (ravg.demand and
ravg.sum_history[]) is initialized from their parent's 'initial task load'
attribute. Note that child task's 'initial task load' attribute itself will be 0
by default (i.e it is not inherited from parent).

A task's 'initial task load' attribute can be set in two ways:

**** /proc interface

/proc/[pid]/sched_init_task_load can be written to for setting a task's 'initial
task load' attribute. A numeric value between 0 - 100 (in percent scale) is
accepted for task's 'initial task load' attribute.

Reading /proc/[pid]/sched_init_task_load returns the 'initial task load'
attribute for the given task.

**** kernel API

Following kernel APIs are provided to set or retrieve a given task's 'initial
task load' attribute:

int sched_set_init_task_load(struct task_struct *p, int init_load_pct);
int sched_get_init_task_load(struct task_struct *p);


===============
3. CPU CAPACITY
===============

CPU capacity reflects peak performance offered by a cpu. It is defined both by
maximum frequency at which cpu can run and its efficiency attribute. Capacity of
a cpu is defined in reference to "least" performing cpu such that "least"
performing cpu has capacity of 1024.

	capacity = 1024 * (fmax_cur * / min_max_freq) *
			  (efficiency / min_possible_efficiency)

where:

	fmax_cur    		= maximum frequency at which cpu is currently
				  allowed to run at
	efficiency	    		= IPC of cpu
	min_max_freq 		= max frequency at which "least" performing cpu
				  can run
	min_possible_efficiency	= IPC of "least" performing cpu

'fmax_cur' reflects the fact that a cpu may be constrained at runtime to run at
a maximum frequency less than what is supported. This may be a constraint placed
by user or drivers such as thermal that intends to reduce temperature of a cpu
by restricting its maximum frequency.

'max_possible_capacity' reflects the maximum capacity of a cpu based on the
maximum frequency it supports.

max_possible_capacity = 1024 * (fmax * / min_max_freq) *
			       (efficiency / min_possible_efficiency)

where:
	fmax 	= maximum frequency supported by a cpu

/proc/sched_debug lists capacity and maximum_capacity information for a cpu.

In the example HMP system quoted in Sec 2.3, "least" performing CPU is A53 and
thus min_max_freq = 1GHz and min_possible_efficiency = 1024.

Capacity of A57 = 1024 * (2GHz / 1GHz) * (2048 / 1024) = 4096
Capacity of A53 = 1024 * (1GHz / 1GHz) * (1024 / 1024) = 1024

Capacity of A57 when constrained to run at maximum frequency of 500MHz can be
calculated as:

Capacity of A57 = 1024 * (500MHz / 1GHz) * (2048 / 1024) = 1024

*** 3.1 load_scale_factor

'lsf' or load scale factor attribute of a cpu is used to estimate load of a task
on that cpu when running at its fmax_cur frequency. 'lsf' is defined in
reference to "best" performing cpu such that it's lsf is 1024. 'lsf' for a cpu
is defined as:

	lsf = 1024 * (max_possible_freq / fmax_cur) *
		     (max_possible_efficiency / ipc)

where:
	fmax_cur    		= maximum frequency at which cpu is currently
				  allowed to run at
	ipc	    		= IPC of cpu
	max_possible_freq	= max frequency at which "best" performing cpu
				  can run
	max_possible_efficiency	= IPC of "best" performing cpu

In the example HMP system quoted in Sec 2.3, "best" performing CPU is A57 and
thus max_possible_freq = 2 GHz, max_possible_efficiency = 2048

lsf of A57 = 1024 * (2GHz / 2GHz) * (2048 / 2048) = 1024
lsf of A53 = 1024 * (2GHz / 1 GHz) * (2048 / 1024) = 4096

lsf of A57 constrained to run at maximum frequency of 500MHz can be calculated
as:

lsf of A57 = 1024 * (2GHz / 500Mhz) * (2048 / 2048) = 4096

To estimate load of a task on a given cpu running at its fmax_cur:

	load = scaled_load * lsf / 1024

A task with scaled load of 20% would thus be estimated to consume 80% bandwidth
of A53 running at 1GHz. The same task with scaled load of 20% would be estimated
to consume 160% bandwidth on A53 constrained to run at maximum frequency of
500MHz.

load_scale_factor, thus, is very useful to estimate load of a task on a given
cpu and thus to decide whether it can fit in a cpu or not.

*** 3.2 cpu_power

A metric 'cpu_power' related to 'capacity' is also listed in /proc/sched_debug.
'cpu_power' is ideally same for all cpus (1024) when they are idle and running
at the same frequency. 'cpu_power' of a cpu can be scaled down from its ideal
value to reflect reduced frequency it is operating at and also to reflect the
amount of cpu bandwidth consumed by real-time tasks executing on it.
'cpu_power' metric is used by scheduler to decide task load distribution among
cpus. CPUs with low 'cpu_power' will be assigned less task load compared to cpus
with higher 'cpu_power'

============
4. CPU POWER
============

The HMP scheduler extensions currently depend on an architecture-specific driver
to provide runtime information on cpu power. In the absence of an
architecture-specific driver, the scheduler will resort to using the
max_possible_capacity metric of a cpu as a measure of its power.

================
5. HMP SCHEDULER
================

For normal (SCHED_OTHER/fair class) tasks there are three paths in the
scheduler which these HMP extensions affect. The task wakeup path, the
load balancer, and the scheduler tick are each modified.

Real-time and stop-class tasks are served by different code
paths. These will be discussed separately.

Prior to delving further into the algorithm and implementation however
some definitions are required.

*** 5.1 Classification of Tasks and CPUs

With the extensions described thus far, the following information is
available to the HMP scheduler:

- per-task CPU demand information from either Per-Entity Load Tracking
  (PELT) or the window-based algorithm described above

- a power value for each frequency supported by each CPU via the API
  described in section 4

- current CPU frequency, maximum CPU frequency (may be throttled by at
  runtime due to thermal conditions), maximum possible CPU frequency supported
  by hardware

- data previously maintained within the scheduler such as the number
  of currently runnable tasks on each CPU

Combined with tunable parameters, this information can be used to classify
both tasks and CPUs to aid in the placement of tasks.

- small task

  Small tasks are tasks that have relatively little CPU
  demand. Normally it is desirable to wake a task on an idle CPU to
  minimize the latency for it to execute - this may mean waking the
  idle CPU up out of a deep power-saving state. For small tasks
  however this may not be the case.  Because a small task is expected
  to run for very little time, it may be better to put it on a CPU
  which is not idle, but lightly loaded.

  The small task threshold is set by the value
  /proc/sys/kenrel/sched_small_task. This value is a percentage. If the
  task consumes this much or less of the minimum CPU in the system, the
  task is considered "small."

- big task

  A big task is one that exerts a CPU demand too high for a particular
  CPU to satisfy. The scheduler will attempt to find a CPU with more
  capacity for such a task.

  The definition of "big" is specific to a task *and* a CPU. A task
  may be considered big on one CPU in the system and not big on
  another if the first CPU has less capacity than the second.

  What task demand is "too high" for a particular CPU? One obvious
  answer would be a task demand which, as measured by PELT or
  window-based load tracking, matches or exceeds the capacity of that
  CPU. A task which runs on a CPU for a long time, for example, might
  meet this criteria as it would report 100% demand of that CPU. It
  may be desirable however to classify tasks which use less than 100%
  of a particular CPU as big so that the task has some "headroom" to grow
  without its CPU bandwidth getting capped and its performance requirements
  not being met. This task demand is therefore a tunable parameter:

  /proc/sys/kernel/sched_upmigrate

  This value is a percentage. If a task consumes more than this much of
  a particular CPU, that CPU will be considered too small for the task.

- mostly_idle

  The "mostly_idle" classification applies to CPUs. This
  classification attempts to answer the following question: if a task
  is put on this CPU, is it likely to be able to run with low contention for
  bandwidth? One possible way to answer this question would be to just check
  whether the CPU is idle or not. That may be too conservative however. The CPU
  may be currently executing a very small task and could become idle soon. Since
  the scheduler is tracking the demand of each task it can make an educated
  guess as to whether a CPU will become idle in the near future.

  There are three tunable parameters which are used to determine whether
  a CPU is mostly idle:

  /sys/devices/system/cpu/cpuX/sched_mostly_idle_nr_run
  /sys/devices/system/cpu/cpuX/sched_mostly_idle_load
  /sys/devices/system/cpu/cpuX/sched_mostly_idle_freq

  Note that these tunables are per-cpu. If a CPU does not have more than
  sched_mostly_idle_nr_run runnable tasks and is not more than
  sched_mostly_idle_load percent busy, it is considered mostly idle.
  Additionally if a cpu's sched_mostly_idle_freq is non-zero and its current
  frequency is less than threshold, then scheduler will attempt to pack
  tasks on the most power-efficient cpu in the cluster.

- spill threshold

  The spill threshold determines how much task load the scheduler
  should put on a CPU before considering that CPU busy and putting the
  load elsewhere. This allows a configurable level of task packing within
  one or more CPUs in the system. How aggressively should the scheduler
  attempt to fill CPUs with task demand before utilizing other CPUs?

  These two tunable parameters together define the spill threshold.

  /proc/sys/kernel/sched_spill_nr_run
  /proc/sys/kernel/sched_spill_load

  If placing a task on a CPU would cause it to have more than
  sched_spill_nr_run runnable tasks, or would cause the CPU to be more
  than sched_spill_load percent busy, the scheduler will interpret that as
  causing the CPU to cross its spill threshold. Spill threshold is only
  considered when having to consider whether a task, which can fit in
  a power-efficient cpu, should spill over to a high-performance CPU because
  the aggregate load of power-efficient cpus exceed their spill threshold.

- power band

  The scheduler may be faced with a tradeoff between power and performance when
  placing a task. If the scheduler sees two CPUs which can accommodate a task:

  CPU 1, power cost of 20, load of 10
  CPU 2, power cost of 10, load of 15

  It is not clear what the right choice of CPU is. The HMP scheduler
  offers the sched_powerband_limit tunable to determine how this
  situation should be handled. When the power delta between two CPUs
  is less than sched_powerband_limit_pct, load will be prioritized as
  the deciding factor as to which CPU is selected. If the power delta
  between two CPUs exceeds that, the lower power CPU is considered to
  be in a different "band" and it is selected, despite perhaps having
  a higher current task load.

*** 5.2 Task Wakeup and select_best_cpu()

CPU placement of a waking task is the single most important decision
made by the HMP scheduler. This section will describe the call flow
and algorithm used in detail.

The primary entry point for a task wakeup operation is
try_to_wake_up(), located in kernel/sched/core.c. This function relies
on select_task_rq() to determine the target CPU for the waking
task. For fair-class (SCHED_OTHER) tasks, that request will be routed
to select_task_rq_fair() in kernel/sched/fair.c. As part of these
scheduler extensions a hook has been inserted into the top of that
function. If HMP scheduling is enabled the normal scheduling behavior
will be replaced by a call to select_best_cpu(). This function,
select_best_cpu(), represents the heart of the HMP scheduling
algorithm described in this document.

The behavior of select_best_cpu() differs depending on whether the
task being placed is a small task or not and the value of the sched_prefer_idle
tunable.

--- Wakeup Logic a Non-Small Task "p"

The order of CPU preference for a non-small task when sched_prefer_idle = 1 is
the following:

  1. The shallowest-cstate idle CPU in the lowest-power cluster which can fit
     the task. Where there is a tie of two CPUs with the same load, the CPU with
     the lowest power cost is chosen.

  2. The least-loaded CPU the task is allowed to run on in the lowest power band
     where the task will fit and where the placement will not result in cpu
     exceeding spill level. When there is a tie of two CPUs at same load, the
     CPU with the lowest power cost is chosen.

  3. The least-loaded mostly idle CPU that the task is allowed to run on where
     the task won't fit (since there was no CPU where the task would fit).

  4. The CPU which the task last ran on.

The order of CPU preference for a non-small task when sched_prefer_idle = 0
is the following:

  1. The least-loaded non-idle mostly idle CPU the task is allowed to run on in
     the lowest power band where the task will fit. When there is a tie of two
     CPUs at same load, the CPU with the lowest power cost is chosen.

  2. The shallowest-cstate idle CPU in the lowest-power cluster which can fit
     the task. Where there is a tie of two CPUs with the same load, the CPU with
     the lowest power cost is chosen.

  3. The least-loaded CPU the task is allowed to run on in the lowest power band
     where the task will fit and where the placement will not result in the CPU
     exceeding spill level. When there is a tie of two CPUs at the same load,
     the CPU with the lowest power cost is chosen.

  4. The least-loaded mostly idle CPU that the task is allowed to run on where
     the task won't fit (since there was no CPU where the task would fit).

  5. The CPU which the task last ran on.

--- Wakeup Logic a Small Task "p"

The order of CPU preference for a small task is the following:

  1. The lowest-power CPU, if it is not idle but is mostly idle.

  2. A non-idle CPU in the lowest power band which is mostly idle. The first
     such CPU found is selected.

  3. An idle CPU in the lowest power band that is in the least shallow C-state.

  4. The least busy CPU in the lowest power band where adding the task will not
     result in exceeding the spill threshold.

  5. The most power-efficient CPU outside of the lowest power band.

*** 5.3 Scheduler Tick

Every CPU is interrupted periodically to let kernel update various statistics
and possibly preempt the currently running task in favor of a waiting task. This
periodicity, determined by CONFIG_HZ value, is set at 10ms. There are various
optimizations by which a CPU however can skip taking these interrupts (ticks).
A cpu going idle for considerable time in one such case.

HMP scheduler extensions brings in a change in processing of tick
(scheduler_tick()) that can result in task migration. In case the currently
running task on a cpu belongs to fair_sched class, a check is made if it needs
to be migrated. Possible reasons for migrating task could be:

a) A big task is running on a power-efficient cpu and a high-performance cpu is
available (idle) to service it

b) The task is not a small task and a more power-efficient cpu is available to
service the task

In case the test for migration turns out positive (which is expected to be rare
event), a candidate cpu is identified for task migration. To avoid multiple task
migrations to the same candidate cpu(s), identification of candidate cpu is
serialized via global spinlock (migration_lock).

*** 5.4 Load Balancer

Load balance is a key functionality of scheduler that strives to distribute task
across available cpus in a "fair" manner. Most of the complexity associated with
this feature involves balancing fair_sched class tasks. Changes made to load
balance code serve these goals:

1. Restrict flow of tasks from power-efficient cpus to high-performance cpu.
   Provide a spill-over threshold, defined in terms of number of tasks
   (sched_spill_nr_run) and cpu demand (sched_spill_load), beyond which tasks
   can spill over from power-efficient cpu to high-performance cpus.

2. Allow idle power-efficient cpus to pick up extra load from over-loaded
   performance-efficient cpu

3. Allow idle high-performance cpu to pick up big tasks from power-efficient cpu

4. Allow a cpu with lower power rating to pick up load from another cpu with
   higher power rating. Power rating of cpus is provided by an
   architecture-specific driver, described in Sec 4

5. Allow small-task packing. Normally a cpu with more than one task would kick
   an idle cpu in tickless state and have it pull task from it. That is
   undesirable when, say, a cpu has couple of small tasks.

*** 5.5 Real Time Tasks

Minimal changes introduced in treatment of real-time tasks by HMP scheduler
aims at preferring scheduling of real-time tasks on cpus with low
power-rating.

Prior to HMP scheduler, the fast-path cpu selection for placing a real-time task
(at wakeup) is its previous cpu, provided the currently running task on its
previous cpu is not a real-time task or a real-time task with lower priority.
Failing this, cpu selection in slow-path involves building a list of candidate
cpus where the waking real-time task will be of highest priority and thus can be
run immediately. The first cpu from this candidate list is chosen for the waking
real-time task. Much of the premise for this simple approach is the assumption
that real-time tasks often execute for very short intervals and thus the focus
is to place them on a cpu where they can be run immediately.

HMP scheduler brings in a change which avoids fast-path and always resorts to
slow-path. Further cpu with lowest power-rating from candidate list of cpus is
chosen as cpu for placing waking real-time task.

*** 5.6 Task packing

Task packing is letting one cpu take up more than one task in an attempt to
improve power (and in some cases performance). Power benefit is derived by
avoiding wakeup cost for idle cpus from their deep sleep states. For example,
consider a system with one cpu busy while other cpus are idle and in deep
sleep state. A small task in this situation needs to be placed on a suitable
cpu. Placing the small task on the busy cpu will likely not hurt its
performance (it is after all a low-demand task) while helping gain on power
because we avoid the cost associated with waking idle cpu from deep sleep
state.

Task packing can have good or bad implications for power and performance.

a. Power implications

As described in the small task wakeup example, task packing can be beneficial
for power. However, the adverse impact on power can arise when packing on one
cpu can increase its busy time and hence result in frequency raise.

b. Performance implications

The most obvious negative impact on performance because of packing is
increased scheduling latencies for tasks that can occur. Positive impact on
performance from packing has also been seen. This arises from the fact
that a waking task, when woken to busy cpu because of packing, will incur very
low latency to run immediately, when compared to being woken to a idle cpu in
deep sleep state. In later case, task has to wait for cpu to exit sleep state,
considerable enough in some cases to hurt performance.

Packing thus is a delicate matter to play with! The following parameters control
packing behavior.

- sched_small_task
	This parameter specifies demand threshold below which a task will be
classified as "small". As described in Sec 5.2 ("Task Wakeup and
select_best_cpu()"), for small tasks wakeups, a busy cpu is prefered as target
rather than idle cpu.

- mostly_idle_load and mostly_idle_nr_run

These are per-cpu parameters that define mostly_idle thresholds for a cpu. A cpu
whose load < mostly_idle_load AND whose nr_running is < mostly_idle_nr_run is
classified as mostly_idle. See further description of "mostly_idle" thresholds
in Sec 5.

- mostly_idle_freq

This is a per-cpu parameter. If non-zero for a cpu which is part of a cluster
and cluster current frequency is less than this threshold, then scheduler will
poack all tasks on a single cpu in cluster. The cpu chosen is the first most
power-efficient cpu found while scanning cluster's online cpus.

For some low band of frequency, spread of task on all available cpus can be
groslly power-inefficient. As an example, consider two tasks that each need
500MHz. Packing them on one cpu could lead to 1GHz. In spread case, we incur
cost of two cpus running at 500MHz, while in packed case, we incur the cost of
one cpu running at 1GHz. Based on the silicon characteristics, where leakage
power can be dominant factor, former can be worse on power rather than latter.
Running at slow frequency (in spread case) can actually makes it worse on
leakage power (especially if 500MHz and 1GHz share the same voltage point).
sched_mostly_idle_freq is set based on silicon characteristics and can provide
a winning argument for both power and performance.


=====================
6. FREQUENCY GUIDANCE
=====================

As mentioned in the introduction section the scheduler is in a unique
position to assist with the determination of CPU frequency. Because
the scheduler now maintains an estimate of per-task CPU demand, task
activity can be tracked, aggregated and provided to the CPUfreq
governor as a replacement for simple CPU busy time. CONFIG_SCHED_FREQ_INPUT
kernel configuration variable needs to be enabled for this feature to be active.

Two of the most popular CPUfreq governors, interactive and ondemand,
utilize a window-based approach for measuring CPU busy time. This
works well with the window-based load tracking scheme previously
described. The following APIs are provided to allow the CPUfreq
governor to query busy time from the scheduler instead of using the
basic CPU busy time value derived via get_cpu_idle_time_us() and
get_cpu_iowait_time_us() APIs.

  int sched_set_window(u64 window_start, unsigned int window_size)

    This API is invoked by governor at initialization time or whenever
    window size is changed. 'window_size' argument (in jiffy units)
    indicates the size of window to be used. The first window of size
    'window_size' is set to beging at jiffy 'window_start'

    -EINVAL is returned if per-entity load tracking is in use rather
    than window-based load tracking, otherwise a success value of 0
    is returned.

  int sched_get_busy(int cpu)

    Returns the busy time for the given CPU in the most recent
    complete window. The value returned is microseconds of busy
    time at fmax of given CPU.

The values returned by sched_get_busy() take a bit of explanation,
both in what they mean and also how they are derived.

*** 6.1 Per-CPU Window-Based Stats

In addition to the per-task window-based demand, the HMP scheduler
extensions also track the aggregate demand seen on each CPU. This is
done using the same windows that the task demand is tracked with
(which is in turn set by the governor when frequency guidance is in
use). There are two quantities maintained for each CPU by the HMP scheduler:

  curr_runnable_sum: aggregate demand from all tasks which executed during
  the current (not yet completed) window

  prev_runnable_sum: aggregate demand from all tasks which executed during
  the most recent completed window

When the scheduler is updating a task's window-based stats it also
updates these values. Like per-task window-based demand these
quantities are normalized against the max possible frequency and max
efficiency (instructions per cycle) in the system. If an update occurs
and a window rollover is observed, curr_runnable_sum is copied into
prev_runnable_sum before being reset to 0. The sched_get_busy() API
returns prev_runnable_sum, scaled to the efficiency and fmax of given
CPU.

*** 6.2 Per-task window-based stats

Corresponding to curr_runnable_sum and prev_runnable_sum, two counters are
maintained per-task

curr_window - represents cpu demand of task in its most recently tracked
	      window
prev_window - represents cpu demand of task in the window prior to the one
	      being tracked by curr_window

"cpu demand" of a task includes its execution time and can also include its
wait time. 'sched_freq_account_wait_time' tunable controls whether task's wait
time is included in its 'curr_window' and 'prev_window' counters or not.

Needless to say, curr_runnable_sum counter of a cpu is derived from curr_window
counter of various tasks that ran on it in its most recent window.

*** 6.3 Effect of various task events

We now consider various events and how they affect above mentioned counters.

PICK_NEXT_TASK
	This represents beginning of execution for a task. Provided the task
	refers to a non-idle task, a portion of task's wait time that
	corresponds to the current window being tracked on a cpu is added to
	task's curr_window counter, provided sched_freq_account_wait_time is
	set. The same quantum is also added to cpu's curr_runnable_sum counter.
	The remaining portion, which corresponds to task's wait time in previous
	window is added to task's prev_window and cpu's prev_runnable_sum
	counters.

PUT_PREV_TASK
	This represents end of execution of a time-slice for a task, where the
	task could refer to a cpu's idle task also. In case the task is non-idle
	or (in case of task being idle with cpu having non-zero rq->nr_iowait
	count and sched_io_is_busy =1), a portion of task's execution time, that
	corresponds to current window being tracked on a cpu is added to task's
	curr_window_counter and also to cpu's curr_runnable_sum counter. Portion
	of task's execution that corresponds to the previous window is added to
	task's prev_window and cpu's prev_runnable_sum counters.

TASK_UPDATE
	This event is called on a cpu's currently running task and hence
	behaves effectively as PUT_PREV_TASK. Task continues executing after
	this event, until PUT_PREV_TASK event occurs on the task (during
	context switch).

TASK_WAKE
	This event signifies a task waking from sleep. Since many windows
	could have elapsed since the task went to sleep, its curr_window
	and prev_window are updated to reflect task's demand in the most
	recent and its previous window that is being tracked on a cpu.

TASK_MIGRATE
	This event signifies task migration across cpus. It is invoked on the
	task prior to being moved. Thus at the time of this event, the task
	can be considered to be in "waiting" state on src_cpu. In that way
	this event reflects actions taken under PICK_NEXT_TASK (i.e its
	wait time is added to task's curr/prev_window counters as well
	as src_cpu's curr/prev_runnable_sum counters, provided
	sched_freq_account_wait_time tunable is non-zero). After that update,
	src_cpu's curr_runnable_sum is reduced by task's curr_window value
	and dst_cpu's curr_runnable_sum is increased by task's curr_window
	value, provided sched_migration_fixup = 1. Similarly, src_cpu's
	prev_runnable_sum is reduced by task's prev_window value and dst_cpu's
	prev_runnable_sum is increased by task's prev_window value,
	provided sched_migration_fixup = 1

IRQ_UPDATE
	This event signifies end of execution of an interrupt handler. This
	event results in update of cpu's busy time counters, curr_runnable_sum
	and prev_runnable_sum, provided cpu was idle.
	When sched_io_is_busy = 0, only the interrupt handling time is added
	to cpu's curr_runnable_sum and prev_runnable_sum counters. When
	sched_io_is_busy = 1, the event mirrors actions taken under
	TASK_UPDATED event i.e  time since last accounting of idle task's cpu
	usage is added to cpu's curr_runnable_sum and prev_runnable_sum
	counters.

===========
7. TUNABLES
===========

*** 7.1 sched_mostly_idle_nr_run

Appears at: /sys/devices/system/cpu/cpuX/sched_mostly_idle_nr_run

Default value: 3

If a CPU has this many runnable tasks (or less), it is considered
"mostly idle." A mostly idle CPU is a preferred destination for a
waking task. To be mostly idle a CPU must not have
more than sched_mostly_idle_nr_run runnable tasks and must not be more
than sched_mostly_idle_load percent busy.

*** 7.2 sched_mostly_idle_load

Appears at: /sys/devices/system/cpu/cpuX/sched_mostly_idle_load

Default value: 20

This tunable is a percentage. If a CPU is busier than this, it cannot
be considered "mostly idle." A mostly idle CPU is a preferred
destination for a waking task. To be mostly idle a CPU must not have
more than sched_mostly_idle_nr_run runnable tasks and must not be more
than sched_mostly_idle_load percent busy.


*** 7.3 sched_spill_load

Appears at: /proc/sys/kernel/sched_spill_load

Default value: 100

CPU selection criteria for fair-sched class tasks is the lowest power cpu where
they can fit. When the most power-efficient cpu where a task can fit is
overloaded (aggregate demand of tasks currently queued on it exceeds
sched_spill_load), a task can be placed on a higher-performance cpu, even though
the task strictly doesn't need one. This applies to non-small tasks.

*** 7.4 sched_spill_nr_run

Appears at: /proc/sys/kernel/sched_spill_nr_run

Default value: 10

The intent of this tunable is similar to sched_spill_load, except it applies to
nr_running count of a cpu. A non-small task can spill over to a
higher-performance cpu when the most power-efficient cpu where it can normally
fit has more tasks than sched_spill_nr_run.

*** 7.5 sched_upmigrate

Appears at: /proc/sys/kernel/sched_upmigrate

Default value: 80

This tunable is a percentage. If a task consumes more than this much
of a CPU, the CPU is considered too small for the task and the
scheduler will try to find a bigger CPU to place the task on.

*** 7.6 sched_downmigrate

Appears at: /proc/sys/kernel/sched_downmigrate

Default value: 60

This tunable is a percentage. It exists to control hysteresis. Lets say a task
migrated to a high-performance cpu when it crossed 80% demand on a
power-efficient cpu. We don't let it come back to a power-efficient cpu until
its demand *in reference to the power-efficient cpu* drops less than 60%
(sched_down_migrate).

*** 7.7 sched_small_task

Appears at: /proc/sys/kernel/sched_small_task

Default value: 10

This tunable is a percentage. If a task consumes this much or less of
the minimum capacity CPU in the system, it is considered a "small
task." The scheduler will not attempt to find an idle CPU for small
tasks - they may be woken up on busy CPUs.

*** 7.8 sched_init_task_load

Appears at: /proc/sys/kernel/sched_init_task_load

Default value: 15

This tunable is a percentage. When a task is first created it has no
history, so the task load tracking mechanism cannot determine a
historical load value to assign to it. This tunable specifies the
initial load value for newly created tasks. Also see Sec 2.8 on per-task
'initial task load' attribute.

*** 7.9 sched_upmigrate_min_nice

Appears at: /proc/sys/kernel/sched_upmigrate_min_nice

Default value: 15

A task whose nice value is greater than this tunable value will never
be considered as a "big" task (it will not be allowed to run on a
high-performance CPU).

*** 7.10 sched_enable_power_aware

Appears at: /proc/sys/kernel/sched_enable_power_aware

Default value: 1

Controls whether or not per-CPU power values are used in determining
task placement. If this is disabled, tasks are simply placed on the
smallest capacity CPU that will adequately meet the task's needs as
determined by the task load tracking mechanism. If this is enabled,
after a set of CPUs are determined which will meet the task's
performance needs, a CPU is selected which is reported to have the
lowest power consumption at that time.

*** 7.11 sched_ravg_hist_size

Appears at: /proc/sys/kernel/sched_ravg_hist_size

Default value: 5

This tunable controls the number of samples used from task's sum_history[]
array for determination of its demand.

*** 7.12 sched_window_stats_policy

Appears at: /proc/sys/kernel/sched_window_stats_policy

Default value: 2

This tunable controls the policy in how window-based load tracking
calculates an overall demand value based on the windows of CPU
utilization it has collected for a task.

Possible values for this tunable are:
0: Just use the most recent window sample of task activity when calculating
   task demand.
1: Use the maximum value of first M samples found in task's cpu demand
   history (sum_history[] array), where M = sysctl_sched_ravg_hist_size
2: Use the maximum of (the most recent window sample, average of first M
   samples), where M = syctl_sched_ravg_hist_size
3. Use average of first M samples, where M = sysctl_sched_ravg_hist_size

*** 7.13 sched_ravg_window

Appears at: kernel command line argument

Default value: 10000000 (10ms, units of tunable are nanoseconds)

This specifies the duration of each window in window-based load
tracking. By default each window is 10ms long. This quantity must
currently be set at boot time on the kernel command line (or the
default value of 10ms can be used).

*** 7.14 RAVG_HIST_SIZE

Appears at: compile time only (see RAVG_HIST_SIZE in include/linux/sched.h)

Default value: 5

This macro specifies the number of windows the window-based load
tracking mechanism maintains per task. If default values are used for
both this and sched_ravg_window then a total of 50ms of task history
would be maintained in 5 10ms windows.

*** 7.15 sched_account_wait_time

Appears at: /proc/sys/kernel/sched_account_wait_time

Default value: 1

This controls whether a task's wait time is accounted as its demand for cpu
and thus the values found in its sum, sum_history[] and demand attributes.

*** 7.16 sched_freq_account_wait_time

Appears at: /proc/sys/kernel/sched_freq_account_wait_time

Default value: 0

This controls whether a task's wait time is accounted in its curr_window and
prev_window attributes and thus in a cpu's curr_runnable_sum and
prev_runnable_sum counters.

*** 7.17 sched_migration_fixup

Appears at: /proc/sys/kernel/sched_migration_fixup

Default value: 1

This controls whether a cpu's busy time counters are adjusted during task
migration.

*** 7.18 sched_freq_inc_notify

Appears at: /proc/sys/kernel/sched_freq_inc_notify

Default value: 10 * 1024 * 1024 (10 Ghz)

When scheduler detects that cur_freq of a cluster is insufficient to meet
demand, it sends notification to governor, provided (freq_required - cur_freq)
exceeds sched_freq_inc_notify, where freq_required is the frequency calculated
by scheduler to meet current task demand. Note that sched_freq_inc_notify is
specified in kHz units.

*** 7.19 sched_freq_dec_notify

Appears at: /proc/sys/kernel/sched_freq_dec_notify

Default value: 10 * 1024 * 1024 (10 Ghz)

When scheduler detects that cur_freq of a cluster is far greater than what is
needed to serve current task demand, it will send notification to governor.
More specifically, notification is sent when (cur_freq - freq_required)
exceeds sched_freq_dec_notify, where freq_required is the frequency calculated
by scheduler to meet current task demand. Note that sched_freq_dec_notify is
specified in kHz units.

** 7.20 sched_heavy_task

Appears at: /proc/sys/kernel/sched_heavy_task

Default value: 0

This tunable can be used to specify a demand value for tasks above which task
are classified as "heavy" tasks. Task's ravg.demand attribute is used for this
comparison. Scheduler will request a raise in cpu frequency when heavy tasks
wakeup after at least one window of sleep, where window size is defined by
sched_ravg_window. Value 0 will disable this feature.

** 7.21 sched_mostly_idle_freq

Appears at: /sys/devices/system/cpu/cpuX/sched_mostly_idle_freq

Default value: 0

This tunable is intended to achieve task packing behavior based on cluster
frequency. Hence it is strongly advised to have all cpus in a cluster have the
same value for mostly_idle_freq. For more details, see section on "Task
packing" (sec 5.6).

*** 7.22 sched_cpu_high_irqload

Appears at: /proc/sys/kernel/sched_cpu_high_irqload

Default value: 10000000 (10ms)

The scheduler keeps a decaying average of the amount of irq and softirq activity
seen on each CPU within a ten millisecond window. Note that this "irqload"
(reported in the sched_cpu_load tracepoint) will be higher than the typical load
in a single window since every time the window rolls over, the value is decayed
by some fraction and then added to the irq/softirq time spent in the next
window.

When the irqload on a CPU exceeds the value of this tunable, the CPU is no
longer eligible to be seen as mostly idle. This will affect the task placement
logic described above, causing the scheduler to try and steer tasks away from
the CPU.

** 7.23 sched_prefer_idle

Appears at: /proc/sys/kernel/sched_prefer_idle

Default value: 1

Non-small tasks will prefer to wake up on idle CPUs if this tunable is set to 1.
If the tunable is set to 0, non-small tasks will prefer to wake up on mostly
idle CPUs which are not completely idle, increasing task packing behavior.

** 7.24 sched_min_runtime

Appears at: /proc/sys/kernel/sched_min_runtime

Default value: 200000000 (200ms)

This tunable helps avouid frequent migration of task on account of
energy-awareness. During scheduler tick, a check is made (in migration_needed())
whether the running task needs to be migrated to a "better" cpu, which could
either offer better performance or power. When deciding to migrate task on
account of power, we want to avoid "frequent" migration of task (say every
tick), which could be add more overhead for comparatively little gains. A task's
'run_start' attribute is set when it starts running on a cpu. This information
is used in migration_needed() to avoid "frequent" migrations. Once a task has
been associated with a cpu (in either running or runnable state) for more than
'sched_min_vruntime' ns, it is considered eligible for migration in tick path on
account of energy awareness reasons.

=========================
8. HMP SCHEDULER TRACE POINTS
=========================

*** 8.1 sched_enq_deq_task

Logged when a task is either enqueued or dequeued on a CPU's run queue.

  <idle>-0     [004] d.h4 12700.711665: sched_enq_deq_task: cpu=4 enqueue comm=powertop pid=13227 prio=120 nr_running=1 cpu_load=0 rt_nr_running=0 affine=ff sum_scaled=0 period=48237 demand=13364423

- cpu: the CPU that the task is being enqueued on to or dequeued off of
- enqueue/dequeue: whether this was an enqueue or dequeue event
- comm: name of task
- pid: PID of task
- prio: priority of task
- nr_running: number of runnable tasks on this CPU
- cpu_load: current priority-weighted load on the CPU (note, this is *not*
  the same as CPU utilization or a metric tracked by PELT/window-based tracking)
- rt_nr_running: number of real-time processes running on this CPU
- affine: CPU affinity mask in hex for this task (so ff is a task eligible to
  run on CPUs 0-7)
- sum_scaled: PELT-based task demand scaled by cpu frequency and efficiency (ns)
- period: PELT-based decaying average of the period (1024us, ~1ms) that the
  "sum_scaled" is relative to
- demand: window-based task demand computed based on selected policy (recent,
  max, or average) (ns)

*** 8.2 sched_task_load

Logged when selecting the best CPU to run the task (select_best_cpu()).

<...>-2907 [002] d.s3 66.841363: sched_task_load: 32 (kworker/u16:1): sum=319, sum_scaled=69, period=47541 demand=192442 small=1 boost=0 reason=0

- sum: PELT-based task demand (not normalized for CPU frequency) (ns)
- sum_scaled: PELT-based task demand scaled by cpu frequency and efficiency (ns)
- period: PELT-based decaying average of the period (1024us, ~1ms) that the
  "sum_scaled" is relative to
- demand: window-based task demand computed based on selected policy (recent,
  max, or average) (ns)
- small: whether the task is considered small
- boost: whether boost is in effect
- reason: reason we are picking a new CPU:
  0: no migration - selecting a CPU for a wakeup or new task wakeup
  1: move to big CPU (migration)
  2: move to littlte CPU (migration)
  3: move to power efficient CPU (migration)

*** 8.3 sched_cpu_load

Logged when selecting the best CPU to run a task (select_best_cpu() for fair
class tasks, find_lowest_rq_hmp() for RT tasks) and load balancing
(update_sg_lb_stats()).

<idle>-0     [004] d.h3 12700.711541: sched_cpu_load: cpu 0 idle 1 mostly_idle 1 nr_run 0 nr_big 0 nr_small 0 lsf 1945 capacity 1045 cr_avg 0 irqload 4456 fcur 199200 fmax 940800 power_cost 1045 cstate 1

- cpu: the CPU being described
- idle: boolean indicating whether the CPU is idle
- mostly_idle: boolean indicating whether the CPU is mostly idle
- nr_run: number of tasks running on CPU
- nr_big: number of BIG tasks running on CPU
- nr_small: number of small tasks running on CPU
- lsf: load scale factor - multiply normalized load by this factor to determine
  how much load task will exert on CPU
- capacity: capacity of CPU (based on max possible frequency and efficiency)
- cr_avg: cumulative runnable average, instantaneous sum of the demand (either
  PELT or window-based) of all the runnable task on a CPU (ns)
- irqload: decaying average of irq activity on CPU (ns)
- fcur: current CPU frequency (Khz)
- fmax: max CPU frequency (but not maximum _possible_ frequency) (KHz)
- power_cost: cost of running this CPU at the current frequency
- cstate: current cstate of CPU

The power_cost value above differs in how it is calculated depending on the
callsite of this tracepoint. The select_best_cpu() call to this tracepoint
finds the minimum frequency required to satisfy the existing load on the CPU
as well as the task being placed, and returns the power cost of that frequency.
The load balance and real time task placement paths used a fixed frequency
(highest frequency common to all CPUs for load balancing, minimum
frequency of the CPU for real time task placement).

*** 8.4 sched_update_task_ravg

Logged when window-based stats are updated for a task. The update may happen
for a variety of reasons, see section 2.5, "Task Events."

<idle>-0     [004] d.h4 12700.711513: sched_update_task_ravg: wc 12700711473496 ws 12700691772135 delta 19701361 event TASK_WAKE cpu 4 cur_freq 199200 cur_pid 0 task 13227 (powertop) ms 12640648272532 delta 60063200964 demand 13364423 sum 0 irqtime 0 cs 0 ps 495018 cur_window 0 prev_window 0

- wc: wallclock, output of sched_clock(), monotonically increasing time since
  boot (will roll over in 585 years) (ns)
- ws: window start, time when the current window started (ns)
- delta: time since the window started (wc - ws) (ns)
- event: What event caused this trace event to occur (see section 2.5 for more
  details)
- cpu: which CPU the task is running on
- cur_freq: CPU's current frequency in KHz
- curr_pid: PID of the current running task (current)
- task: PID and name of task being updated
- ms: mark start - timestamp of the beginning of a segment of task activity,
  either sleeping or runnable/running (ns)
- delta: time since last event within the window (wc - ms) (ns)
- demand: task demand computed based on selected policy (recent, max, or
  average) (ns)
- sum: the task's run time during current window scaled by frequency and
  efficiency (ns)
- irqtime: length of interrupt activity (ns). A non-zero irqtime is seen
  when an idle cpu handles interrupts, the time for which needs to be
  accounted as cpu busy time
- cs: curr_runnable_sum of cpu (ns). See section 6.1 for more details of this
  counter.
- ps: prev_runnable_sum of cpu (ns). See section 6.1 for more details of this
  counter.
- cur_window: cpu demand of task in its most recently tracked window (ns)
- prev_window: cpu demand of task in the window prior to the one being tracked
  by cur_window

*** 8.5 sched_update_history

Logged when update_task_ravg() is accounting task activity into one or
more windows that have completed. This may occur more than once for a
single call into update_task_ravg(). A task that ran for 24ms spanning
four 10ms windows (the last 2ms of window 1, all of windows 2 and 3,
and the first 2ms of window 4) would result in two calls into
update_history() from update_task_ravg(). The first call would record activity
in completed window 1 and second call would record activity for windows 2 and 3
together (samples will be 2 in second call).

<idle>-0     [004] d.h4 12700.711489: sched_update_history: 13227 (powertop): runtime 13364423 samples 1 event TASK_WAKE demand 13364423 (hist: 13364423 9871252 2236009 6162476 10282078) cpu 4 nr_big 0 nr_small 0

- runtime: task cpu demand in recently completed window(s). This value is scaled
  to max_possible_freq and max_possible_efficiency. This value is pushed into
  task's demand history array. The number of windows to which runtime applies is
  provided by samples field.
- samples: Number of samples (windows), each having value of runtime, that is
  recorded in task's demand history array.
- event: What event caused this trace event to occur (see section 2.5 for more
  details) - PUT_PREV_TASK, PICK_NEXT_TASK, TASK_WAKE, TASK_MIGRATE,
  TASK_UPDATE
- demand: task demand computed based on selected policy (recent, max, or
  average) (ns)
- hist: last 5 windows of history for the task with the most recent window
  listed first
- cpu: CPU the task is associated with
- nr_big: number of big tasks on the CPU
- nr_small: Number of small tasks on the CPU

*** 8.6 sched_reset_all_windows_stats

Logged when key parameters controlling window-based statistics collection are
changed. This event signifies that all window-based statistics for tasks and
cpus are being reset. Changes to below attributes result in such a reset:

* sched_ravg_window (See Sec 2)
* sched_window_stats_policy (See Sec 2.4)
* sched_account_wait_time (See Sec 7.15)
* sched_ravg_hist_size (See Sec 7.11)
* sched_migration_fixup (See Sec 7.17)
* sched_freq_account_wait_time (See Sec 7.16)

<task>-0     [004] d.h4 12700.711489: sched_reset_all_windows_stats: time_taken 1123 window_start 0 window_size 0 reason POLICY_CHANGE old_val 0 new_val 1

- time_taken: time taken for the reset function to complete (ns)
- window_start: Beginning of first window following change to window size (ns)
- window_size: Size of window. Non-zero if window-size is changing (in ticks)
- reason: Reason for reset of statistics.
- old_val: Old value of variable, change of which is triggering reset
- new_val: New value of variable, change of which is triggering reset

*** 8.7 sched_migration_update_sum

Logged when CONFIG_SCHED_FREQ_INPUT feature is enabled and a task is migrating
to another cpu.

<task>-0     [004] d.h4 12700.711489: sched_migration_update_sum: cpu 0: cs XXX ps YYY pid 1234

- cpu: cpu, away from which or to which, task is migrating
- cs: curr_runnable_sum of cpu (ns). See Sec 6.1 for more details of this
  counter.
- ps: prev_runnable_sum of cpu (ns). See Sec 6.1 for more details of this
  counter.
- pid: PID of migrating task

*** 8.8 sched_get_busy

Logged when scheduler is returning busy time statistics for a cpu.

<task>-0     [004] d.h4 12700.711489: sched_get_busy: cpu 0 load XXX

- cpu: cpu, for which busy time statistic (prev_runnable_sum) is being
  returned (ns)
- load: corresponds to prev_runnable_sum (ns), scaled to fmax of cpu

*** 8.9 sched_freq_alert

Logged when scheduler is alerting cpufreq governor about need to change
frequency

<task>-0     [004] d.h4 12700.711489: sched_freq_alert: cpu 0 old_load=XXX new_load=YYY

- cpu: cpu in cluster that has highest load (prev_runnable_sum)
- old_load: cpu busy time last reported to governor. This is load scaled in
  reference to max_possible_freq and max_possible_efficiency.
- new_load: recent cpu busy time. This is load scaled in
  reference to max_possible_freq and max_possible_efficiency.

*** 8.10 sched_set_boost

Logged when boost settings are being changed

<task>-0     [004] d.h4 12700.711489: sched_set_boost: ref_count=1

- ref_count: A non-zero value indicates boost is in effect