Re: [RFC PATCH v2 2/3] docs: scheduler: Add scheduler overview documentation

[Randy Dunlap]

Hi--

On 5/6/20 7:39 AM, john mathew wrote:
> From: John Mathew <john.mathew@xxxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Co-developed-by: Mostafa Chamanara <mostafa.chamanara@xxxxxxxxxxxx>
> Signed-off-by: Mostafa Chamanara <mostafa.chamanara@xxxxxxxxxxxx>
> Co-developed-by: Oleg Tsymbal <oleg.tsymbal@xxxxxxxxxx>
> Signed-off-by: Oleg Tsymbal <oleg.tsymbal@xxxxxxxxxx>
> Signed-off-by: John Mathew <john.mathew@xxxxxxxxxx>
> ---
>  Documentation/scheduler/cfs-overview.rst      | 110 +++++++
>  Documentation/scheduler/index.rst             |   3 +
>  Documentation/scheduler/overview.rst          | 269 ++++++++++++++++++
>  .../scheduler/sched-data-structs.rst          | 253 ++++++++++++++++
>  Documentation/scheduler/scheduler-api.rst     |  30 ++
>  kernel/sched/core.c                           |  28 +-
>  kernel/sched/sched.h                          | 169 ++++++++++-
>  7 files changed, 855 insertions(+), 7 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
> 
> Request review from Valentin Schneider <valentin.schneider@xxxxxxx> 
> for the section describing Scheduler classes in:
> .../scheduler/sched-data-structs.rst
> 
> diff --git a/Documentation/scheduler/cfs-overview.rst b/Documentation/scheduler/cfs-overview.rst
> new file mode 100644
> index 000000000000..50d94b9bb60e
> --- /dev/null
> +++ b/Documentation/scheduler/cfs-overview.rst
> @@ -0,0 +1,110 @@
> +.. 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 and idle CPU but
> +fail, in this case instead of failing, an alternative idle CPU scanned.

I'm having problems parsing that last sentence above.

> +
> +**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 in to account. The policy is to pick the first idle

                           into

> +CPU which is big enough for the task (task_util * margin < cpu_capacity).

why not <= ?

> +If no idle CPU is big enough, the idle CPU with the highest capacity was

s/was/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.

                                                      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

                                    non-SCHED_IDLE

> +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

s/cfs/CFS/

> +- the case of tasks not evenly spread on groups with spare capacity

Can you make that (above) a proper ReST list?

> +Also the load balance decisions have been consolidated in the 3 separate
> +functions

end with '.' period.

> +
> +**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 can resulted in prohibitively high wake-up latencies on

               drop: can
or say         which can result

> +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 scenarios 'misfit'

                                                      scenario

> +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..b952970d3565 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
> +    scheduler-api.rst

Why do some of these end with ".rst" and others don't?

>  
>  .. only::  subproject and html
>  
> diff --git a/Documentation/scheduler/overview.rst b/Documentation/scheduler/overview.rst
> index aee16feefc61..284d6cf0b2f8 100644
> --- a/Documentation/scheduler/overview.rst
> +++ b/Documentation/scheduler/overview.rst
> @@ -3,3 +3,272 @@
>  ====================
>  Scheduler overview
>  ====================
> +
> +Linux kernel implements priority based scheduling. More than one process are

                           priority-based

> +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) â 99 whereas normal
> +priorities range from 100 â 139 (lowest). SCHED_DEADLINE tasks has negative

                                                                  have

> +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 :c:type:`struct task_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 a RCU

                                                                      an RCU

> +list.  Macro next_task() allow a process to obtain its next task and

                            allows

> +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

             RT

> +   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

                                       non-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 pre-emption

overwhelmingly the kernel spells this as                 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.

      CPU

> +
> +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 the clone()

                                                                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.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

           lazy (deferred)

> +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

end with '.' period.

> +
> +* 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

                                                        __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 pre-emption to avoid another task pre-empting the scheduling

              preemption                        preempting

> +   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
> +   pre-empted in kernel mode, it is removed from the runqueue. If the

      preempted

> +   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 short cut to this

                                                              shortcut

> +   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 CPU's to balance load. These load balancing operations are queued

              CPUs

> +   on a Balance callback list which get executed when the balance_callback()

either                                              when balance_callback()
or                                                  when the balanace_callback() function

> +   is called.
> +8. The runqueue is unlocked and pre-emption is re-enabled. In case

                                   preemption

> +   pre-emption was requested during the time in which it was disabled,

      preemption

> +   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 pre-empted

                                                                            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 trace points tracing all major events of the scheduler.
> +The tracepoints are defined in ::

Can the document be consistent with (2 lines above:) "trace points" and
(1 line above) "tracepoints"?

> +
> +  include/trace/events/sched.h
> +
> +Using these treacepoints it is possible to model the scheduler state transition

               spello

> +in an automata model. The following journal paper discusses such modeling:
> +
> +Daniel B. de Oliveira, RÃmulo 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

Change tab above to space.

> +and specifications.  In case a kernel event referred to more then one
> +event,extra fields of the kernel event was used to distinguish between

   event, extra

> +automation events.  The final model was done parallel composition of all

eh? parse error.

> +generators and specifications composed of 15 events, 7 generators and
> +10 specifications.  This resulted in 149 states and 327 transitions.
> diff --git a/Documentation/scheduler/sched-data-structs.rst b/Documentation/scheduler/sched-data-structs.rst
> new file mode 100644
> index 000000000000..52fe95140a8f
> --- /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
> +is provided in::

   are provided

> +
> +    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

       CPU.

> +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

          CPU.

> +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 pre-empted.

                                                                     preempted.

> +vruntime of the task based on the formula ::
> +
> +    vruntime += delta_exec * (NICE_0_LOAD/curr->load.weight);
> +
> +where:
> +
> +* delta_exec is the time spent by the task since the last time vruntime
> +  was updated.

What unit is the time in?

> +* 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 metri

                                                                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

                                                                  CPU

> +  helps to estimate the overall cpu load which is needed for load balancing.

                                   CPU

> +  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[]`.

                  CPU

> +  The array contains mappings for the nice values from -20 to 19. Nice value
> +  0 is mapped to 1024. Each entry advances by ~1.25 which means if for every

Please use "about" or "approximately" etc. instead of "~" (if that is what is meant here).

> +  increment in nice value the task gets 10% less cpu and vice versa.

                                                    CPU

> +
> +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,
> +fair_sched_class and rt_sched_class class 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 describe why the enqueue or dequeue is being called.

       flags is to describe why

> +    The different flags used are described in ::
> +
> +        kernel/sched/sched.h
> +
> +    enqueue_task and dequeue_task is called for following purposes.

                                     are called

> +
> +    - 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

                                                   CPU

> +      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

                                                       preferably: RT

> +      code. This function changes the effective priority of a task which may
> +      inturn change the scheduling class of the task. If so enqueue_task is

         in turn

> +      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 Äurrently running.
> +
> +    In CFS class this function is used put the currently running task back

                                     used to put

> +    in to the CFS RB tree. When a task is running it is dequeued from the tree

       into                                                                  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 and RT classes additional tree is

None of the current sched documentation uses "DL" for deadline.
It is used in some of the source code.  Anyway, if you keep using it, you
should tell what it means somewhere.

> +    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 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 DL 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 HR
> +    tick timer is done if HR timers are enabled.

Likewise, "HR" is not currently used in any scheduler documentation.
At a minimum it needs a brief explanation.

> +    scheduler_tick() runs at 1/HZ and is called from the  timer interrupt

                                            drop one space ^^

> +    handler of the Kernel internal timers.
> +    hrtick() is called from HR Timers to deliver an accurate preemption tick.

                                                            drop ending period ^^

> +    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

end with '.' period.

> +
> +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() wth  SD_BALANCE_EXEC which is called from execv

                              with SD_BALANCE_EXEC (one less space there)

> +    syscall.
> +    DL class decides the CPU on which the task should be woken up based on
> +    the deadline. and RT class decides based on the RT priority. Fair

       the deadline. RT class decides

> +    scheduling class     balances load by selecting the idlest CPU in the

       fewer spaces  ^^^^^^

> +    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.
> +    DL 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 se at

                                                                       se??

> +    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 pre-empt the currently

                                                  preempt

> +    running task. Called by scheduler,
> +    - when moving queued task to new runqueue
> +    - ttwu()
> +    - when waking up newly created task for the first time.
> +
> +    DL class compare the deadlines of the tasks and calls scheduler function

                compares

> +    resched_curr() if the preemption is needed. In case the deadliines are

                                                               deadlines

> +    equal migratilbility of the tasks is used a criteria for preemption.

             migratability

> +    RT class behaves the same except it uses RT priority for comparison.
> +    Fair class sets the buddy hints before calling resched_curr() to preemempt.

                                                                        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..068cdbdbdcc6
> --- /dev/null
> +++ b/Documentation/scheduler/scheduler-api.rst
> @@ -0,0 +1,30 @@
> +.. 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
> \ No newline at end of file

Please fix that warning.

Thanks. This looks helpful.

-- 
~Randy