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Subject: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation
Date:   Thu,  7 May 2020 21:05:52 +0300
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Add documentation for
 -scheduler overview
 -scheduler state transtion
 -CFS overview
 -scheduler data structs

Add rst for scheduler APIs and modify sched/core.c
to add kernel-doc comments.

Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Co-developed-by: Mostafa Chamanara <mostafa.chamanara@basemark.com>
Signed-off-by: Mostafa Chamanara <mostafa.chamanara@basemark.com>
Co-developed-by: Oleg Tsymbal <oleg.tsymbal@unikie.com>
Signed-off-by: Oleg Tsymbal <oleg.tsymbal@unikie.com>
Signed-off-by: John Mathew <john.mathew@unikie.com>
---
 Documentation/scheduler/cfs-overview.rst      | 113 ++++++++
 Documentation/scheduler/index.rst             |   7 +-
 Documentation/scheduler/overview.rst          | 266 ++++++++++++++++++
 .../scheduler/sched-data-structs.rst          | 253 +++++++++++++++++
 Documentation/scheduler/scheduler-api.rst     |  31 ++
 kernel/sched/core.c                           |  28 +-
 kernel/sched/sched.h                          | 169 ++++++++++-
 7 files changed, 858 insertions(+), 9 deletions(-)
 create mode 100644 Documentation/scheduler/cfs-overview.rst
 create mode 100644 Documentation/scheduler/sched-data-structs.rst
 create mode 100644 Documentation/scheduler/scheduler-api.rst

diff --git a/Documentation/scheduler/cfs-overview.rst b/Documentation/scheduler/cfs-overview.rst
new file mode 100644
index 000000000000..b717f2d3e340
--- /dev/null
+++ b/Documentation/scheduler/cfs-overview.rst
@@ -0,0 +1,113 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+=============
+CFS Overview
+=============
+
+Linux 2.6.23 introduced a modular scheduler core and a Completely Fair
+Scheduler (CFS) implemented as a scheduling module. A brief overview of the
+CFS design is provided in :doc:`sched-design-CFS`
+
+In addition there have been many improvements to the CFS, a few of which are
+
+**Thermal Pressure**:
+cpu_capacity initially reflects the maximum possible capacity of a CPU.
+Thermal pressure on a CPU means this maximum possible capacity is
+unavailable due to thermal events. Average thermal pressure for a CPU
+is now subtracted from its maximum possible capacity so that cpu_capacity
+reflects the remaining maximum capacity.
+
+**Use Idle CPU for NUMA balancing**:
+Idle CPU is used as a migration target instead of comparing tasks.
+Information on an idle core is cached while gathering statistics
+and this is used to avoid a second scan of the node runqueues if load is
+not imbalanced. Preference is given to an idle core rather than an
+idle SMT sibling to avoid packing HT siblings due to linearly scanning
+the node cpumask. Multiple tasks can attempt to select an idle CPU but
+fail because a NUMA balance is active on that CPU, in this case instead of
+failing, an alternative idle CPU scanned.
+
+**Asymmetric CPU capacity wakeup scan**:
+Previous assumption that CPU capacities within an SD_SHARE_PKG_RESOURCES
+domain (sd_llc) are homogeneous didn't hold for newer generations of big.LITTLE
+systems (DynamIQ) which can accommodate CPUs of different compute capacity
+within a single LLC domain. A new idle sibling helper function was added
+which took CPU capacity into account. The policy is to pick the first idle
+CPU which is big enough for the task (task_util * margin < cpu_capacity).
+If no idle CPU is big enough, the idle CPU with the highest capacity is
+picked.
+
+**Optimized idle core selection**:
+Previously all threads of a core were looped through to evaluate if the
+core is idle or not. This was unnecessary. If a thread of a core is not
+idle, skip evaluating other threads of a core. Also while clearing the
+cpumask, bits of all CPUs of a core can be cleared in one shot.
+
+**Load balance aggressively for SCHED_IDLE CPUs**:
+The fair scheduler performs periodic load balance on every CPU to check
+if it can pull some tasks from other busy CPUs. The duration of this
+periodic load balance is set to scheduler domain's balance_interval and
+multiplied by a busy_factor (set to 32 by default) for the busy CPUs. This
+multiplication is done for busy CPUs to avoid doing load balance too
+often and rather spend more time executing actual task. While that is
+the right thing to do for the CPUs busy with SCHED_OTHER or SCHED_BATCH
+tasks, it may not be the optimal thing for CPUs running only SCHED_IDLE
+tasks. With the recent enhancements in the fair scheduler around SCHED_IDLE
+CPUs, it is now preferred to enqueue a newly-woken task to a SCHED_IDLE
+CPU instead of other busy or idle CPUs. The same reasoning is applied
+to the load balancer as well to make it migrate tasks more aggressively
+to a SCHED_IDLE CPU, as that will reduce the scheduling latency of the
+migrated (SCHED_OTHER) tasks. Fair scheduler now does the next
+load balance soon after the last non-SCHED_IDLE task is dequeued from a
+runqueue, i.e. making the CPU SCHED_IDLE.
+
+**Load balancing algorithm Reworked**:
+The load balancing algorithm contained some heuristics which became
+meaningless since the rework of the scheduler's metrics like the
+introduction of PELT. The new load balancing algorithm fixes several
+pending wrong tasks placement
+
+ * the 1 task per CPU case with asymmetric system
+ * the case of CFS task preempted by other class
+ * the case of tasks not evenly spread on groups with spare capacity
+
+Also the load balance decisions have been consolidated in the 3 separate
+functions.
+
+**Energy-aware wake-ups speeded up**:
+EAS computes the energy impact of migrating a waking task when deciding
+on which CPU it should run. However, the previous approach had high algorithmic
+complexity, which resulted in prohibitively high wake-up latencies on
+systems with complex energy models, such as systems with per-CPU DVFS. On
+such systems, the algorithm complexity was O(n^2). To address this,
+the EAS wake-up path was re-factored to compute the energy 'delta' on a
+per-performance domain basis, rather than system-wide, which brings the
+complexity down to O(n).
+
+**Selection of an energy-efficient CPU on task wake-up**:
+If an Energy Model (EM) is available and if the system isn't overutilized,
+waking tasks are re-routed into an energy-aware placement algorithm.
+The selection of an energy-efficient CPU for a task is achieved by estimating
+the impact on system-level active energy resulting from the placement of the
+task on the CPU with the highest spare capacity in each performance domain.
+This strategy spreads tasks in a performance domain and avoids overly
+aggressive task packing. The best CPU energy-wise is then selected if it
+saves a large enough amount of energy with respect to prev_cpu.
+
+**Consider misfit tasks when load-balancing**:
+On asymmetric CPU capacity systems load intensive tasks can end up on
+CPUs that don't suit their compute demand. In this scenario 'misfit'
+tasks are migrated to CPUs with higher compute capacity to ensure better
+throughput. A new group_type: group_misfit_task is added and indicates this
+scenario. Tweaks to the load-balance code are done to make the migrations
+happen. Misfit balancing is done between a source group of lower per-CPU
+capacity and destination group of higher compute capacity. Otherwise, misfit
+balancing is ignored.
+
+**Make schedstats a runtime tunable that is disabled by default**:
+schedstats is very useful during debugging and performance tuning but it
+incurred overhead to calculate the stats. A kernel command-line and sysctl
+tunable was added to enable or disable schedstats on demand (when it's built in).
+It is disabled by default. The benefits are dependent on how
+scheduler-intensive the workload is.
+
diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
index ede1a30a6894..f311abe5b711 100644
--- a/Documentation/scheduler/index.rst
+++ b/Documentation/scheduler/index.rst
@@ -17,10 +17,13 @@ specific implementation differences.
     :maxdepth: 2
 
     overview
+    sched-data-structs
+    cfs-overview
     sched-design-CFS
     sched-features
-    arch-specific.rst
-    sched-debugging.rst
+    arch-specific
+    sched-debugging
+    scheduler-api
 
 .. only::  subproject and html
 
diff --git a/Documentation/scheduler/overview.rst b/Documentation/scheduler/overview.rst
index aee16feefc61..f2cb0c901208 100644
--- a/Documentation/scheduler/overview.rst
+++ b/Documentation/scheduler/overview.rst
@@ -3,3 +3,269 @@
 ====================
 Scheduler overview
 ====================
+
+Linux kernel implements priority-based scheduling. More than one process are
+allowed to run at any given time and each process is allowed to run as if it
+were the only process on the system. The process scheduler coordinates which
+process runs when. In that context, it has the following tasks:
+
+* share CPU cores equally among all currently running processes.
+* pick appropriate process to run next if required, considering scheduling
+  class/policy and process priorities.
+* balance processes between multiple cores in SMP systems.
+
+The scheduler attempts to be responsive for I/O bound processes and efficient
+for CPU bound processes. The scheduler also applies different scheduling
+policies for real time and normal processes based on their respective
+priorities. Higher priorities in the kernel have a numerical smaller
+value. Real time priorities range from 1 (highest) – 99 whereas normal
+priorities range from 100 – 139 (lowest). SCHED_DEADLINE tasks have negative
+priorities, reflecting the fact that any of them has higher priority than
+RT and NORMAL/BATCH tasks.
+
+Process Management
+==================
+
+Each process in the system is represented by struct task_struct. When a
+process/thread is created, the kernel allocates a new task_struct for it.
+The kernel then stores this task_struct in an RCU list. Macro next_task()
+allows a process to obtain its next task and for_each_process() macro enables
+traversal of the list.
+
+Frequently used fields of the task struct are:
+
+*state:* The running state of the task. The possible states are:
+
+* TASK_RUNNING: The task is currently running or in a run queue waiting
+  to run.
+* TASK_INTERRUPTIBLE: The task is sleeping waiting for some event to occur.
+  This task can be interrupted by signals. On waking up the task transitions
+  to TASK_RUNNING.
+* TASK_UNINTERRUPTIBLE: Similar to TASK_INTERRUPTIBLE but does not wake
+  up on signals. Needs an explicit wake-up call to be woken up. Contributes
+  to loadavg.
+* __TASK_TRACED: Task is being traced by another task like a debugger.
+* __TASK_STOPPED: Task execution has stopped and not eligible to run.
+  SIGSTOP, SIGTSTP etc causes this state.  The task can be continued by
+  the signal SIGCONT.
+* TASK_PARKED: State to support kthread parking/unparking.
+* TASK_DEAD: If a task dies, then it sets TASK_DEAD in tsk->state and calls
+  schedule one last time. The schedule call will never return.
+* TASK_WAKEKILL: It works like TASK_UNINTERRUPTIBLE with the bonus that it
+  can respond to fatal signals.
+* TASK_WAKING: To handle concurrent waking of the same task for SMP.
+  Indicates that someone is already waking the task.
+* TASK_NOLOAD: To be used along with TASK_UNINTERRUPTIBLE to indicate
+  an idle task which does not contribute to loadavg.
+* TASK_NEW: Set during fork(), to guarantee that no one will run the task,
+  a signal or any other wake event cannot wake it up and insert it on
+  the runqueue.
+
+*exit_state* : The exiting state of the task. The possible states are:
+
+* EXIT_ZOMBIE: The task is terminated and waiting for parent to collect
+  the exit information of the task.
+* EXIT_DEAD: After collecting the exit information the task is put to
+  this state and removed from the system.
+
+*static_prio:* Nice value of a task. The value of this field does
+ not change.  Value ranges from -20 to 19. This value is mapped to nice
+ value and used in the scheduler.
+
+*prio:* Dynamic priority of a task. Previously a function of static
+ priority and tasks interactivity. Value not used by CFS scheduler but used
+ by the RT scheduler. Might be boosted by interactivity modifiers. Changes
+ upon fork, setprio syscalls, and whenever the interactivity estimator
+ recalculates.
+
+*normal_prio:* Expected priority of a task. The value of static_prio
+ and normal_prio are the same for non-real-time processes. For real time
+ processes value of prio is used.
+
+*rt_priority:* Field used by real time tasks. Real time tasks are
+ prioritized based on this value.
+
+*sched_class:* Pointer to sched_class CFS structure.
+
+*sched_entity:* Pointer to sched_entity CFS structure.
+
+*policy:* Value for scheduling policy. The possible values are:
+
+* SCHED_NORMAL: Regular tasks use this policy.
+* SCHED_BATCH: Tasks which need to run longer without preemption
+  use this policy. Suitable for batch jobs.
+* SCHED_IDLE: Policy used by background tasks.
+* SCHED_FIFO & SCHED_RR: These policies for real time tasks. Handled by
+  real time scheduler.
+* SCHED_DEADLINE: Tasks which are activated on a periodic or sporadic fashion
+  use this policy. This policy implements the Earliest Deadline First (EDF)
+  scheduling algorithm. This policy is explained in detail in the
+  :doc:`sched-deadline` documentation.
+
+*nr_cpus_allowed:* Bit field containing tasks affinity towards a set of
+ CPU cores.  Set using sched_setaffinity() system call.
+
+New processes are created using the fork() system call which is described
+at manpage :manpage:`FORK(2)` or the clone system call described at
+:manpage:`CLONE(2)`.
+Users can create threads within a process to achieve parallelism. Threads
+share address space, open files and other resources of the process. Threads
+are created like normal tasks with their unique task_struct, but clone()
+is provided with flags that enable the sharing of resources such as address
+space ::
+
+	clone(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND, 0);
+
+The scheduler schedules task_structs so from scheduler perspective there is
+no difference between threads and processes. Threads are created using
+the system call pthread_create described at :manpage:`PTHREAD_CREATE(3)`
+POSIX threads creation is described at :manpage:`PTHREADS(7)`
+
+The Scheduler Entry Point
+=========================
+
+The main scheduler entry point is an architecture independent schedule()
+function defined in kernel/sched/core.c. Its objective is to find a process in
+the runqueue list and then assign the CPU to it. It is invoked, directly
+or in a lazy (deferred) way from many different places in the kernel. A lazy
+invocation does not call the function by its name, but gives the kernel a
+hint by setting a flag TIF_NEED_RESCHED. The flag is a message to the kernel
+that the scheduler should be invoked as soon as possible because another
+process deserves to run.
+
+Following are some places that notify the kernel to schedule:
+
+* scheduler_tick()
+
+* Running task goes to sleep state : Right before a task goes to sleep,
+  schedule() will be called to pick the next task to run and the change
+  its state to either TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE. For
+  instance, prepare_to_wait() is one of the functions that makes the
+  task go to the sleep state.
+
+* try_to_wake_up()
+
+* yield()
+
+* wait_event()
+
+* cond_resched() : It gives the scheduler a chance to run a higher-priority
+  process.
+
+* cond_resched_lock() : If a reschedule is pending, drop the given lock,
+  call schedule, and on return reacquire the lock.
+
+* do_task_dead()
+
+* preempt_schedule() : The function checks whether local interrupts are
+  enabled and the preempt_count field of current is zero; if both
+  conditions are true, it invokes schedule() to select another process
+  to run.
+
+* preempt_schedule_irq()
+
+Calling functions mentioned above leads to a call to __schedule(). Note
+that preemption must be disabled before it is called and enabled after
+the call using preempt_disable and preempt_enable functions family.
+
+
+The steps during invocation are:
+--------------------------------
+1. Disable preemption to avoid another task preempting the scheduling
+   thread itself.
+2. Retrieve the runqueue of current processor and its lock is obtained to
+   allow only one thread to modify the runqueue at a time.
+3. The state of the previously executed task when the schedule()
+   was called is examined. If it is not runnable and has not been
+   preempted in kernel mode, it is removed from the runqueue. If the
+   previous task has non-blocked pending signals, its state is set to
+   TASK_RUNNING and left in the runqueue.
+4. Scheduler classes are iterated and the corresponding class hook to
+   pick the next suitable task to be scheduled on the CPU is called.
+   Since most tasks are handled by the sched_fair class, a shortcut to this
+   class is implemented in the beginning of the function.
+5. TIF_NEED_RESCHED and architecture specific need_resched flags are cleared.
+6. If the scheduler class picks a different task from what was running
+   before, a context switch is performed by calling context_switch().
+   Internally, context_switch() switches to the new task's memory map and
+   swaps the register state and stack. If scheduler class picked the same
+   task as the previous task, no task switch is performed and the current
+   task keeps running.
+7. Balance callback list is processed. Each scheduling class can migrate tasks
+   between CPUs to balance load. These load balancing operations are queued
+   on a Balance callback list which get executed when balance_callback() is
+   called.
+8. The runqueue is unlocked and preemption is re-enabled. In case
+   preemption was requested during the time in which it was disabled,
+   schedule() is run again right away.
+
+Scheduler State Transition
+==========================
+
+A very high level scheduler state transition flow with a few states can
+be depicted as follows. ::
+
+                                       *
+                                       |
+                                       | task
+                                       | forks
+                                       v
+                        +------------------------------+
+                        |           TASK_NEW           |
+                        |        (Ready to run)        |
+                        +------------------------------+
+                                       |
+                                       |
+                                       v
+                     +------------------------------------+
+                     |            TASK_RUNNING            |
+   +---------------> |           (Ready to run)           | <--+
+   |                 +------------------------------------+    |
+   |                   |                                       |
+   |                   | schedule() calls context_switch()     | task is preempted
+   |                   v                                       |
+   |                 +------------------------------------+    |
+   |                 |            TASK_RUNNING            |    |
+   |                 |             (Running)              | ---+
+   | event occurred  +------------------------------------+
+   |                   |
+   |                   | task needs to wait for event
+   |                   v
+   |                 +------------------------------------+
+   |                 |         TASK_INTERRUPTIBLE         |
+   |                 |        TASK_UNINTERRUPTIBLE        |
+   +-----------------|           TASK_WAKEKILL            |
+                     +------------------------------------+
+                                       |
+                                       | task exits via do_exit()
+                                       v
+                        +------------------------------+
+                        |          TASK_DEAD           |
+                        |         EXIT_ZOMBIE          |
+                        +------------------------------+
+
+
+Scheduler provides tracepoints tracing all major events of the scheduler.
+The tracepoints are defined in ::
+
+  include/trace/events/sched.h
+
+Using these tracepoints it is possible to model the scheduler state transition
+in an automata model. The following journal paper discusses such modeling:
+
+Daniel B. de Oliveira, Rômulo S. de Oliveira, Tommaso Cucinotta, **A thread
+synchronization model for the PREEMPT_RT Linux kernel**, *Journal of Systems
+Architecture*, Volume 107, 2020, 101729, ISSN 1383-7621,
+https://doi.org/10.1016/j.sysarc.2020.101729.
+
+To model the scheduler efficiently the system was divided in to generators
+and specifications. Some of the generators used were "need_resched",
+"sleepable" and "runnable", "thread_context" and "scheduling context".
+The specifications are the necessary and sufficient conditions to call
+the scheduler. New trace events were added to specify the generators
+and specifications. In case a kernel event referred to more than one
+event, extra fields of the kernel event was used to distinguish between
+automation events. The final model was generated from parallel composition
+of all generators and specifications which composed of 34 events,
+12 generators and 33 specifications. This resulted in 9017 states, and
+20103 transitions.
diff --git a/Documentation/scheduler/sched-data-structs.rst b/Documentation/scheduler/sched-data-structs.rst
new file mode 100644
index 000000000000..ec900367cc0c
--- /dev/null
+++ b/Documentation/scheduler/sched-data-structs.rst
@@ -0,0 +1,253 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+=========================
+Scheduler Data Structures
+=========================
+
+The main parts of the Linux scheduler are:
+
+Runqueue
+~~~~~~~~
+
+:c:type:`struct rq <rq>` is the central data structure of process
+scheduling. It keeps track of tasks that are in a runnable state assigned
+for a particular processor. Each CPU has its own run queue and stored in a
+per CPU array::
+
+    DEFINE_PER_CPU(struct rq, runqueues);
+
+Access to the queue requires locking and lock acquire operations must be
+ordered by ascending runqueue. Macros for accessing and locking the runqueue
+are provided in::
+
+    kernel/sched/sched.h
+
+The runqueue contains scheduling class specific queues and several scheduling
+statistics.
+
+Scheduling entity
+~~~~~~~~~~~~~~~~~
+Scheduler uses scheduling entities which contain sufficient information to
+actually accomplish the scheduling job of a task or a task-group. The
+scheduling entity may be a group of tasks or a single task. Every task is
+associated with a sched_entity structure. CFS adds support for nesting of
+tasks and task groups. Each scheduling entity may be run from its parents
+runqueue. The scheduler traverses the sched_entity hierarchy to pick the
+next task to run on the CPU. The entity gets picked up from the cfs_rq on
+which it is queued and its time slice is divided among all the tasks on its my_q.
+
+Virtual Runtime
+~~~~~~~~~~~~~~~~~
+Virtual Run Time or vruntime is the amount of time a task has spent running
+on the CPU. It is updated periodically by scheduler_tick(). Tasks are stored
+in the CFS scheduling class rbtree sorted by vruntime. scheduler_tick() calls
+corresponding hook of CFS which first updates the runtime statistics of the
+currently running task and checks if the current task needs to be preempted.
+vruntime of the task based on the formula ::
+
+    vruntime += delta_exec * (NICE_0_LOAD/curr->load.weight);
+
+where:
+
+* delta_exec is the time in nanoseconds spent by the task since the last time
+  vruntime was updated.
+* NICE_0_LOAD is the load of a task with normal priority.
+* curr is the shed_entity instance of the cfs_rq struct of the currently
+  running task.
+* load.weight: sched_entity load_weight. load_weight is the encoding of
+  the tasks priority and vruntime. The load of a task is the metric
+  indicating the number of CPUs needed to make satisfactory progress on its
+  job. Load of a task influences the time a task spends on the CPU and also
+  helps to estimate the overall CPU load which is needed for load balancing.
+  Priority of the task is not enough for the scheduler to estimate the
+  vruntime of a process. So priority value must be mapped to the capacity of
+  the standard CPU which is done in the array :c:type:`sched_prio_to_weight[]`.
+  The array contains mappings for the nice values from -20 to 19. Nice value
+  0 is mapped to 1024. Each entry advances by approximately 1.25 which means
+  for every increment in nice value the task gets 10% less CPU and vice versa.
+
+Scheduler classes
+~~~~~~~~~~~~~~~~~
+It is an extensible hierarchy of scheduler modules. The modules encapsulate
+scheduling policy details. They are called from the core code which is
+independent. Scheduling classes are implemented through the sched_class
+structure. dl_sched_class for deadline scheduler, fair_sched_class for CFS
+and rt_sched_class for RT are implementations of this class.
+
+The important methods of scheduler class are:
+
+enqueue_task and dequeue_task
+    These functions are used to put and remove tasks from the runqueue
+    respectively. The function takes the runqueue, the task which needs to
+    be enqueued/dequeued and a bit mask of flags. The main purpose of the
+    flags is to describe why the enqueue or dequeue is being called.
+    The different flags used are described in ::
+
+        kernel/sched/sched.h
+
+    enqueue_task and dequeue_task are called for following purposes.
+
+    - When waking a newly created task for the first time. Called with
+      ENQUEUE_NOCLOCK
+    - When migrating a task from one CPU's runqueue to another. Task will be
+      first dequeued from its old runqueue, new CPU will be added to the
+      task struct,  runqueue of the new CPU will be retrieved and task is
+      then enqueued on this new runqueue.
+    - When do_set_cpus_allowed() is called to change a tasks CPU affinity. If
+      the task is queued on a runqueue, it is first dequeued with the
+      DEQUEUE_SAVE and DEQUEUE_NOCLOCK flags set. The set_cpus_allowed()
+      function of the corresponding scheduling class will be called.
+      enqueue_task() is then called with ENQUEUE_RESTORE and ENQUEUE_NOCLOCK
+      flags set.
+    - When changing the priority of a task using rt_mutex_setprio(). This
+      function implements the priority inheritance logic of the RT mutex
+      code. This function changes the effective priority of a task which may
+      in turn change the scheduling class of the task. If so enqueue_task is
+      called with flags corresponding to each class.
+    - When user changes the nice value of the task. If the task is queued on
+      a runqueue, it first needs to be dequeued, then its load weight and
+      effective priority needs to be set. Following which the task is
+      enqueued with ENQUEUE_RESTORE and ENQUEUE_NOCLOCK flags set.
+    - When __sched_setscheduler() is called. This function enables changing
+      the scheduling policy and/or RT priority of a thread. If the task is
+      on a runqueue, it will be first dequeued, changes will be made and
+      then enqueued.
+    - When moving tasks between scheduling groups. The runqueue of the tasks
+      is changed when moving between groups. For this purpose if the task
+      is running on a queue, it is first dequeued with DEQUEUE_SAVE, DEQUEUE_MOVE
+      and DEQUEUE_NOCLOCK flags set, followed by which scheduler function to
+      change the tsk->se.cfs_rq and tsk->se.parent and then task is enqueued
+      on the runqueue with the same flags used in dequeue.
+
+pick_next_task
+    Called by __schedule() to pick the next best task to run.
+    Scheduling class structure has a pointer pointing to the next scheduling
+    class type and each scheduling class is linked using a singly linked list.
+    The __schedule() function iterates through the corresponding
+    functions of the scheduler classes in priority order to pick up the next
+    best task to run. Since tasks belonging to the idle class and fair class
+    are frequent, the scheduler optimizes the picking of next task to call
+    the pick_next_task_fair() if the previous task was of the similar
+    scheduling class.
+
+put_prev_task
+    Called by the scheduler when a running task is being taken off a CPU.
+    The behavior of this function depends on individual scheduling classes
+    and called in the following cases.
+
+    - When do_set_cpus_allowed() is called and if the task is currently running.
+    - When scheduler pick_next_task() is called, the put_prev_task() is
+      called with the previous task as function argument.
+    - When rt_mutex_setprio() is called and if the task is currently running.
+    - When user changes the nice value of the task and if the task is
+      currently running.
+    - When __sched_setscheduler() is called and if the task is currently
+      running.
+    - When moving tasks between scheduling groups through the sched_move_task()
+      and if the task is ćurrently running.
+
+    In CFS class this function is used to put the currently running task back
+    into the CFS RB tree. When a task is running it is dequeued from the tree.
+    This is to prevent redundant enqueue's and dequeue's for updating its
+    vruntime. vruntime of tasks on the tree needs to be updated by update_curr()
+    to keep the tree in sync. In DL(deadline) and RT classes additional tree is
+    maintained for facilitating task migration between CPUs through push
+    operation between runqueues for load balancing. Task will be added to
+    this queue if it is present on the scheduling class rq and the task has
+    affinity to more than one CPU.
+
+set_next_task
+    Pairs with the put_prev_task(), this function is called when the next
+    task is set to run on the CPU. This function is called in all the places
+    where put_prev_task is called to complete the 'change'. Change is defined
+    as the following sequence of calls::
+
+         - dequeue task
+         - put task
+         - change the property
+         - enqueue task
+         - set task as current task
+
+    It resets the run time statistics for the entity with
+    the runqueue clock.
+    In case of CFS scheduling class, it will set the pointer to the current
+    scheduling entity to the picked task and accounts bandwidth usage on
+    the cfs_rq. In addition it will also remove the current entity from the
+    CFS runqueue for vruntime update optimization opposite to what was done
+    in put_prev_task.
+    For the SCHED_DEADLINE and RT classes it will
+
+    - dequeue the picked task from the tree of pushable tasks.
+    - update the load average in case the previous task belonged to another
+      class.
+    - queues the function to push tasks from current runqueue to other CPUs
+      which can preempt and start execution. Balance callback list is used.
+
+task_tick
+    Called from scheduler_tick(), hrtick() and sched_tick_remote() to update
+    the current task statistics and load averages. Also restarting the high
+    resolution tick timer is done if high resolution timers are enabled.
+    scheduler_tick() runs at 1/HZ and is called from the timer interrupt
+    handler of the Kernel internal timers.
+    hrtick() is called from high resolution timers to deliver an accurate
+    preemption tick as the regular scheduler tick that runs at 1/HZ can be
+    too coarse when nice levels are used.
+    sched_tick_remote() Gets called by the offloaded residual 1Hz scheduler
+    tick. In order to reduce interruptions to bare metal tasks, it is possible
+    to outsource these scheduler ticks to the global workqueue so that a
+    housekeeping CPU handles those remotely.
+
+select_task_rq
+    Called by scheduler to get the CPU to assign a task to and migrating
+    tasks between CPUs. Flags describe the reason the function was called.
+    Called by try_to_wake_up() with SD_BALANCE_WAKE flag which wakes up a
+    sleeping task.
+    Called by wake_up_new_task() with SD_BALANCE_FORK flag which wakes up a
+    newly forked task.
+    Called by sched_exec() with SD_BALANCE_EXEC which is called from execv
+    syscall.
+    SCHED_DEADLINE class decides the CPU on which the task should be woken
+    up based on the deadline. RT class decides based on the RT priority. Fair
+    scheduling class balances load by selecting the idlest CPU in the
+    idlest group, or under certain conditions an idle sibling CPU if the
+    domain has SD_WAKE_AFFINE set.
+
+balance
+    Called by pick_next_task() from scheduler to enable scheduling classes
+    to pull tasks from runqueues of other CPUs for balancing task execution
+    between the CPUs.
+
+task_fork
+    Called from sched_fork() of scheduler which assigns a task to a CPU.
+    Fair scheduling class updates runqueue clock, runtime statistics and
+    vruntime for the scheduling entity.
+
+yield_task
+    Called from SYSCALL sched_yield to yield the CPU to other tasks.
+    SCHED_DEADLINE class forces the runtime of the task to zero using a special
+    flag and dequeues the task from its trees. RT class requeues the task
+    entities to the end of the run list. Fair scheduling class implements
+    the buddy mechanism. This allows skipping onto the next highest priority
+    scheduling entity at every level in the CFS tree, unless doing so would
+    introduce gross unfairness in CPU time distribution.
+
+check_preempt_curr
+    Check whether the task that woke up should preempt the currently
+    running task. Called by scheduler,
+
+    * when moving queued task to new runqueue
+    * ttwu()
+    * when waking up newly created task for the first time.
+
+    SCHED_DEADLINE class compares the deadlines of the tasks and calls
+    scheduler function resched_curr() if the preemption is needed. In case
+    the deadlines are equal, migratability of the tasks is used a criteria
+    for preemption.
+    RT class behaves the same except it uses RT priority for comparison.
+    Fair class sets the buddy hints before calling resched_curr() to preempt.
+
+Scheduler sets the scheduler class for each task based on its priority.
+Tasks assigned with SCHED_NORMAL, SCHED_IDLE and SCHED_BATCH call
+fair_sched_class hooks and tasks assigned with SCHED_RR and
+SCHED_FIFO call rt_sched_class hooks. Tasks assigned with SCHED_DEADLINE
+policy calls dl_sched_class hooks.
diff --git a/Documentation/scheduler/scheduler-api.rst b/Documentation/scheduler/scheduler-api.rst
new file mode 100644
index 000000000000..1fc6bd4c2908
--- /dev/null
+++ b/Documentation/scheduler/scheduler-api.rst
@@ -0,0 +1,31 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+=============================
+Scheduler related functions
+=============================
+
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: __schedule
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: scheduler_tick
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: try_to_wake_up
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: do_task_dead
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: preempt_schedule_irq
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: prepare_task_switch
+
+.. kernel-doc:: kernel/sched/core.c
+	:functions: finish_task_switch
+
+.. kernel-doc:: kernel/sched/sched.h
+	:functions: rq
+
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a2fbf98fd6f..b349ed9b4d92 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3576,9 +3576,13 @@ void arch_set_thermal_pressure(struct cpumask *cpus,
 		WRITE_ONCE(per_cpu(thermal_pressure, cpu), th_pressure);
 }
 
-/*
+/**
+ * scheduler_tick - sched tick timer handler
+ *
  * This function gets called by the timer code, with HZ frequency.
  * We call it with interrupts disabled.
+ *
+ * Return: 0.
  */
 void scheduler_tick(void)
 {
@@ -3959,8 +3963,10 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	BUG();
 }
 
-/*
- * __schedule() is the main scheduler function.
+/**
+ * __schedule() - the main scheduler function.
+ *
+ * @preempt: preemption enabled/disabled
  *
  * The main means of driving the scheduler and thus entering this function are:
  *
@@ -4089,6 +4095,12 @@ static void __sched notrace __schedule(bool preempt)
 	balance_callback(rq);
 }
 
+/**
+ * do_task_dead - handle task exit
+ *
+ * Changes the the task state to TASK_DEAD and calls
+ * schedule to pick next task to run.
+ */
 void __noreturn do_task_dead(void)
 {
 	/* Causes final put_task_struct in finish_task_switch(): */
@@ -4320,7 +4332,8 @@ EXPORT_SYMBOL_GPL(preempt_schedule_notrace);
 
 #endif /* CONFIG_PREEMPTION */
 
-/*
+/**
+ * preempt_schedule_irq - schedule from irq context
  * This is the entry point to schedule() from kernel preemption
  * off of irq context.
  * Note, that this is called and return with irqs disabled. This will
@@ -5618,6 +5631,13 @@ SYSCALL_DEFINE0(sched_yield)
 }
 
 #ifndef CONFIG_PREEMPTION
+/**
+ * _cond_resched - explicit rescheduling
+ *
+ * gives the scheduler a chance to run a higher-priority process
+ *
+ * Return: 1 if reschedule was done, 0 if reschedule not done.
+ */
 int __sched _cond_resched(void)
 {
 	if (should_resched(0)) {
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index db3a57675ccf..21f2953b72c7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -865,12 +865,175 @@ struct uclamp_rq {
 };
 #endif /* CONFIG_UCLAMP_TASK */
 
-/*
- * This is the main, per-CPU runqueue data structure.
+/**
+ * struct rq - This is the main, per-CPU runqueue data structure.
  *
  * Locking rule: those places that want to lock multiple runqueues
  * (such as the load balancing or the thread migration code), lock
  * acquire operations must be ordered by ascending &runqueue.
+ *
+ * @lock:
+ *	lock to be acquired while modifying the runqueue
+ * @nr_running:
+ *	number of runnable tasks on this queue
+ * @nr_numa_running:
+ *	number of tasks running that care about their placement
+ * @nr_preferred_running:
+ *	number of tasks that are optimally NUMA placed
+ * @numa_migrate_on:
+ *	per run-queue variable to check if NUMA-balance is
+ *	active on the run-queue
+ * @last_blocked_load_update_tick:
+ *	tick stamp for decay of blocked load
+ * @has_blocked_load:
+ *	idle CPU has blocked load
+ * @nohz_tick_stopped:
+ *	CPU is going idle with tick stopped
+ * @nohz_flags:
+ *	flags indicating NOHZ idle balancer actions
+ * @nr_load_updates:
+ *	unused
+ * @nr_switches:
+ *	number of context switches
+ * @uclamp:
+ *	utilization clamp values based on CPU's RUNNABLE tasks
+ * @uclamp_flags:
+ *	flags for uclamp actions, currently one flag for idle.
+ * @cfs:
+ *	fair scheduling class runqueue
+ * @rt:
+ *	rt scheduling class runqueue
+ * @dl:
+ *	dl scheduing class runqueue
+ * @leaf_cfs_rq_list:
+ *	list of leaf cfs_rq on this CPU
+ * @tmp_alone_branch:
+ *	reference to add child before its parent in leaf_cfs_rq_list
+ * @nr_uninterruptible:
+ *	global counter where the total sum over all CPUs matters. A task
+ *      can increase this counter on one CPU and if it got migrated
+ *	afterwards it may decrease it on another CPU. Always updated under
+ *	the runqueue lock
+ * @curr:
+ *	points to the currently running task of this rq.
+ * @idle:
+ *	points to the idle task of this rq
+ * @stop:
+ *	points to the stop task of this rq
+ * @next_balance:
+ *	shortest next balance before updating nohz.next_balance
+ * @prev_mm:
+ *	real address space of the previous task
+ * @clock_update_flags:
+ *	RQCF clock_update_flags bits
+ * @clock:
+ *	sched_clock() value for the queue
+ * @clock_task:
+ *	clock value minus irq handling time
+ * @clock_pelt:
+ *	clock which scales with current capacity when something is
+ *	running on rq and synchronizes with clock_task when rq is idle
+ * @lost_idle_time:
+ *	idle time lost when utilization of a rq has reached the
+ *	maximum value
+ * @nr_iowait:
+ *	account the idle time that we could have spend running if it
+ *	were not for IO
+ * @membarrier_state:
+ *	copy of membarrier_state from the mm_struct
+ * @rd:
+ *	root domain, each exclusive cpuset essentially defines an island
+ *	domain by fully partitioning the member CPUs from any other cpuset
+ * @sd:
+ *	a domain heirarchy of CPU groups to balance process load among them
+ * @cpu_capacity:
+ *	information about CPUs heterogeneity used for CPU performance
+ *	scaling
+ * @cpu_capacity_orig:
+ *	original capacity of a CPU before being altered by
+ *	rt tasks and/or IRQ
+ * @balance_callback:
+ *	queue to hold load balancing push and pull operations
+ * @idle_balance:
+ *	flag to do the nohz idle load balance
+ * @misfit_task_load:
+ *	set whenever the current running task has a utilization
+ *	greater than 80% of rq->cpu_capacity. A non-zero value
+ *	in this field enables misfit load balancing
+ * @active_balance:
+ *	synchronizes accesses to ->active_balance_work
+ * @push_cpu:
+ *	idle cpu to push the running task on to during active load
+ *	balancing.
+ * @active_balance_work:
+ *	callback scheduled to run on one or multiple cpus
+ *	with maximum priority monopolozing those cpus.
+ * @cpu:
+ *	CPU of this runqueue
+ * @online:
+ *	Used by scheduling classes to support CPU hotplug
+ * @cfs_tasks:
+ *	an MRU list used for load balancing, sorted (except
+ *	woken tasks) starting from recently given CPU time tasks
+ *	toward tasks with max wait time in a run-queue
+ * @avg_rt:
+ *	track the utilization of RT tasks for a  more accurate
+ *	view of the utilization of the CPU when overloaded by CFS and
+ *	RT tasks
+ * @avg_dl:
+ *	track the utilization of DL tasks as CFS tasks can be preempted
+ *	by DL tasks and the CFS's utilization might no longer describe
+ *	the real utilization level
+ * @avg_irq:
+ *	track the the utilization of interrupt to give a more accurate
+ *	level of utilization of CPU taking into account the time spent
+ *	under interrupt context when rqs' clock is updated
+ * @avg_thermal:
+ *	tracks thermal pressure which is the reduction in maximum
+ *	possible capacity due to thermal events
+ * @idle_stamp:
+ *	time stamp at which idle load balance started for this rq.
+ *	Used to find the idlest CPU, when multiple idle CPUs are in
+ *	the same state
+ * @avg_idle:
+ *	average idle time for this rq
+ * @max_idle_balance_cost:
+ *	used to determine avg_idle's max value
+ * @prev_irq_time:
+ *	updated to account time consumed when a previous
+ *	update_rq_clock() happened inside a {soft,}irq region
+ * @prev_steal_time:
+ *	to account how much elapsed time was spent in steal
+ * @prev_steal_time_rq:
+ *	for fine granularity task steal time accounting by
+ *	making update_rq_clock() aware of steal time
+ * @calc_load_update:
+ *	sample window for global load-average calculations
+ * @calc_load_active:
+ *	fold any nr_active delta into a global accumulate
+ * @hrtick_csd:
+ *	call_single_data used to set hrtick timer state on a specific CPU
+ * @hrtick_timer:
+ *	HR-timer to deliver an accurate preemption tick
+ * @rq_sched_info:
+ *	runqueue specific latency stats
+ * @rq_cpu_time:
+ *	runqueue specific accumulated per-task cpu runtime
+ * @yld_count:
+ *	runqueue specific sys_sched_yield() stats
+ * @sched_count:
+ *	runqueue specific __schedule() stats
+ * @sched_goidle:
+ *	runqueue specific idle scheduling class stats
+ * @ttwu_count:
+ *	runqueue specific idle ttwu stats , both remote and local
+ * @ttwu_local:
+ *	ttwu count for the CPU of the rq
+ * @wake_list:
+ *	list which stores tasks being woken up remotely by ttwu
+ * @idle_state:
+ *	cpuidle state pointer of the CPU of this rq used to make a
+ *	better decision when balancing tasks
  */
 struct rq {
 	/* runqueue lock: */
@@ -1136,7 +1299,7 @@ static inline u64 rq_clock_task(struct rq *rq)
 	return rq->clock_task;
 }
 
-/**
+/*
  * By default the decay is the default pelt decay period.
  * The decay shift can change the decay period in
  * multiples of 32.
-- 
2.17.1