[RFC PATCH v3 0/3] Add scheduler overview documentation

[RFC PATCH v3 0/3] Add scheduler overview documentation

[John Mathew]

This patch series updates the scheduler documentation to add more topics
wrt to scheduler overview. New sections are added to provide a brief
overview of the kernel structs used by the scheduler, scheduler invocation
and context switch.
The previous version was reviewed at 
Link: https://lkml.org/lkml/2020/5/6/630

version 3:
 Fix spelling, spacing and typo errors.

version 2:
- Remove :c:func: directive as it was redundant
- Limit document width (line symbol count) to 75
- Replace dot file with ASCII art
- Describe prepare_task_switch(), ASID use, 
  kernel/user transtion, MIPS FPU affinity correctly
- Add missing references to files
- Removed internal APIs from scheduler API reference
- Described rq struct member as kernel-doc comments
- Replaced CFS history with CFS current status
- Added documentation for sched_class fields
- Refined explanation of context swtich functionality
- Replace CFS history with recent changes
- Added kernel-doc comments for struct rq

John Mathew (3):
  docs: scheduler: Restructure scheduler documentation.
  docs: scheduler: Add scheduler overview documentation
  docs: scheduler: Add introduction to scheduler context-switch

 Documentation/scheduler/arch-specific.rst     |  14 +
 Documentation/scheduler/cfs-overview.rst      | 113 ++++++++
 Documentation/scheduler/context-switching.rst | 125 ++++++++
 Documentation/scheduler/index.rst             |  33 ++-
 .../scheduler/mips-context-switch.rst         |  89 ++++++
 Documentation/scheduler/overview.rst          | 271 ++++++++++++++++++
 .../scheduler/sched-data-structs.rst          | 253 ++++++++++++++++
 Documentation/scheduler/sched-debugging.rst   |  14 +
 Documentation/scheduler/sched-features.rst    |  20 ++
 Documentation/scheduler/scheduler-api.rst     |  31 ++
 .../scheduler/x86-context-switch.rst          |  65 +++++
 kernel/sched/core.c                           |  28 +-
 kernel/sched/sched.h                          | 169 ++++++++++-
 13 files changed, 1205 insertions(+), 20 deletions(-)
 create mode 100644 Documentation/scheduler/arch-specific.rst
 create mode 100644 Documentation/scheduler/cfs-overview.rst
 create mode 100644 Documentation/scheduler/context-switching.rst
 create mode 100644 Documentation/scheduler/mips-context-switch.rst
 create mode 100644 Documentation/scheduler/overview.rst
 create mode 100644 Documentation/scheduler/sched-data-structs.rst
 create mode 100644 Documentation/scheduler/sched-debugging.rst
 create mode 100644 Documentation/scheduler/sched-features.rst
 create mode 100644 Documentation/scheduler/scheduler-api.rst
 create mode 100644 Documentation/scheduler/x86-context-switch.rst

-- 
2.17.1

[RFC PATCH v3 1/3] docs: scheduler: Restructure scheduler documentation.

[John Mathew]

Add new sections to enable addition of new documentation on
the scheduler. Existing documentation is moved under the related
new sections. The sections are
  - overview
  - sched-features
  - arch-specific.rst
  - sched-debugging.rst

Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: John Mathew <john.mathew@unikie.com>
---
 Documentation/scheduler/arch-specific.rst   | 11 ++++++++
 Documentation/scheduler/index.rst           | 29 ++++++++++++---------
 Documentation/scheduler/overview.rst        |  5 ++++
 Documentation/scheduler/sched-debugging.rst | 14 ++++++++++
 Documentation/scheduler/sched-features.rst  | 20 ++++++++++++++
 5 files changed, 66 insertions(+), 13 deletions(-)
 create mode 100644 Documentation/scheduler/arch-specific.rst
 create mode 100644 Documentation/scheduler/overview.rst
 create mode 100644 Documentation/scheduler/sched-debugging.rst
 create mode 100644 Documentation/scheduler/sched-features.rst

diff --git a/Documentation/scheduler/arch-specific.rst b/Documentation/scheduler/arch-specific.rst
new file mode 100644
index 000000000000..c9c34863d994
--- /dev/null
+++ b/Documentation/scheduler/arch-specific.rst
@@ -0,0 +1,11 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Architecture Specific Scheduler Implementation Differences
+==========================================================
+
+.. class:: toc-title
+
+	   Table of contents
+
+.. toctree::
+   :maxdepth: 2
diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
index 69074e5de9c4..ede1a30a6894 100644
--- a/Documentation/scheduler/index.rst
+++ b/Documentation/scheduler/index.rst
@@ -1,23 +1,26 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
 ===============
 Linux Scheduler
 ===============
 
-.. toctree::
-    :maxdepth: 1
+This documentation outlines the Linux kernel scheduler with its concepts,
+details about the scheduler design and its data structures and architecture
+specific implementation differences.
 
 
-    completion
-    sched-arch
-    sched-bwc
-    sched-deadline
-    sched-design-CFS
-    sched-domains
-    sched-energy
-    sched-nice-design
-    sched-rt-group
-    sched-stats
+.. class:: toc-title
+
+	   Table of contents
 
-    text_files
+.. toctree::
+    :maxdepth: 2
+
+    overview
+    sched-design-CFS
+    sched-features
+    arch-specific.rst
+    sched-debugging.rst
 
 .. only::  subproject and html
 
diff --git a/Documentation/scheduler/overview.rst b/Documentation/scheduler/overview.rst
new file mode 100644
index 000000000000..aee16feefc61
--- /dev/null
+++ b/Documentation/scheduler/overview.rst
@@ -0,0 +1,5 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+====================
+Scheduler overview
+====================
diff --git a/Documentation/scheduler/sched-debugging.rst b/Documentation/scheduler/sched-debugging.rst
new file mode 100644
index 000000000000..e332069f99d6
--- /dev/null
+++ b/Documentation/scheduler/sched-debugging.rst
@@ -0,0 +1,14 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Scheduler Debugging Interface
+==============================
+
+.. class:: toc-title
+
+	   Table of contents
+
+.. toctree::
+   :maxdepth: 2
+
+   sched-stats
+   text_files
diff --git a/Documentation/scheduler/sched-features.rst b/Documentation/scheduler/sched-features.rst
new file mode 100644
index 000000000000..1afbd9cc8d52
--- /dev/null
+++ b/Documentation/scheduler/sched-features.rst
@@ -0,0 +1,20 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Scheduler Features
+=====================
+
+.. class:: toc-title
+
+	   Table of contents
+
+.. toctree::
+   :maxdepth: 2
+
+   sched-arch
+   sched-bwc
+   sched-deadline
+   sched-domains
+   sched-energy
+   sched-nice-design
+   sched-rt-group
+   completion
-- 
2.17.1

[RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[John Mathew]

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

[RFC PATCH v3 3/3] docs: scheduler: Add introduction to scheduler context-switch

[John Mathew]

Add documentation for introduction to
 -context-switch
 -x86 context-switch
 -MIPS context switch

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/arch-specific.rst     |   3 +
 Documentation/scheduler/context-switching.rst | 125 ++++++++++++++++++
 Documentation/scheduler/index.rst             |   1 +
 .../scheduler/mips-context-switch.rst         |  89 +++++++++++++
 .../scheduler/x86-context-switch.rst          |  65 +++++++++
 5 files changed, 283 insertions(+)
 create mode 100644 Documentation/scheduler/context-switching.rst
 create mode 100644 Documentation/scheduler/mips-context-switch.rst
 create mode 100644 Documentation/scheduler/x86-context-switch.rst

diff --git a/Documentation/scheduler/arch-specific.rst b/Documentation/scheduler/arch-specific.rst
index c9c34863d994..65dc393b605f 100644
--- a/Documentation/scheduler/arch-specific.rst
+++ b/Documentation/scheduler/arch-specific.rst
@@ -9,3 +9,6 @@ Architecture Specific Scheduler Implementation Differences
 
 .. toctree::
    :maxdepth: 2
+
+   x86-context-switch
+   mips-context-switch
diff --git a/Documentation/scheduler/context-switching.rst b/Documentation/scheduler/context-switching.rst
new file mode 100644
index 000000000000..44a185764d3c
--- /dev/null
+++ b/Documentation/scheduler/context-switching.rst
@@ -0,0 +1,125 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+==========================
+Process context switching
+==========================
+
+Context Switching
+-----------------
+
+Context switching, the switching from a running task to another,
+is done by the context_switch() function defined in kernel/sched.c.
+It is called by __schedule() when a new process has been selected to run.
+The execution flow is as follows:
+
+* prepare_task_switch() performs necessary kernel preparations for the
+  context switch and then calls prepare_arch_switch() for architecture
+  specific context switch preparation. This call must be paired with a
+  subsequent finish_task_switch() after the context switch. The various
+  steps are:
+
+  - Prepare kcov for context switch. Context switch does switch_mm() to the
+    next task's mm, then switch_to() that new task. This means vmalloc'd
+    regions which had previously been faulted in can transiently disappear in
+    the context of the prev task. Functions instrumented by KCOV may try to
+    access a vmalloc'd kcov_area during this window, and result in a recursive
+    fault. This is avoided by setting a new flag: KCOV_IN_CTXSW in kcov_mode
+    prior to switching the mm, and cleared once the new task is live.
+  - Update sched_info statistics for both the prev and next tasks.
+  - Handle perf subsystem context switch from previous task to next.
+    The various steps are:
+
+    * Remove perf events for the task being context-switched out.
+    * Stop each perf event and update the event value in event->count.
+    * Call the context switch callback for PMU with flag indicating
+      schedule out.
+    * Create a PERF_RECORD_MISC_SWITCH_OUT perf event.
+    * Context switch the perf event contexts between the current and next tasks.
+    * Schedule out current cgroup events if cgroup perf events exist on the
+      CPU.
+
+  - Set TIF_NOTIFY_RESUME flag on the current thread for the Restartable
+    sequence mechanism. Restartable sequences allow user-space to perform
+    update operations on per-cpu data without requiring heavy-weight atomic
+    operations.
+  - Fire preempt notifiers. A task can request the scheduler to notify it
+    whenever it is preempted or scheduled back in. This allows the task to
+    swap any special-purpose registers like the FPU or Intel's VT registers.
+  - Claim the next task as running to prevent load balancing run on it.
+
+* arch_start_context_switch() batches the reload of page tables and other
+  process state with the actual context switch code for paravirtualized
+  guests.
+
+* Transfer the real and anonymous address spaces between the switching tasks.
+  Four possible transfer types are:
+
+  * kernel task switching to another kernel task
+  * user task switching to a kernel task
+  * kernel task switching to user task
+  * user task switching to user task
+
+  For a kernel task switching to kernel task enter_lazy_tlb() is called
+  which is an architecture specific implementation to handle a context
+  without an mm. Architectures implement lazy tricks to minimize TLB
+  flushes here. The active address space from the previous task is
+  borrowed (transferred) to the next task.
+
+  For a user task switching to kernel task it will have a real address
+  space and so its anonymous users counter is incremented. This makes
+  sure that the address space will not get freed even after the previous
+  task exits.
+
+  For a user task switching to user task the architecture specific
+  switch_mm_irqs_off() or switch_mm() functions are called.  The main
+  functionality of these calls is to switch the address space between
+  the user space processes.  This includes switching the page table pointers
+  either via retrieved valid ASID for the process or page mapping in the TLB.
+
+  For a kernel task switching to a user task, switch_mm_irqs_off()
+  replaces the address space of prev kernel task with the next from the user
+  task. Same as for exiting process in this case, the context_switch()
+  function saves the pointer to the memory descriptor used by prev in the
+  runqueue’s prev_mm field and resets prev task active address space.
+
+* prepare_lock_switch() releases lockdep of the runqueue lock to handle
+  the special case of the scheduler context switch where the runqueue lock
+  will be released by the next task.
+
+* Architecture specific implementation of switch_to() switches the
+  register state and the stack. This involves saving and restoring stack
+  information and the processor registers and any other
+  architecture-specific state that must be managed and restored on a
+  per-process basis.
+
+* finish_task_switch() performs the final steps of the context switch:
+
+  - Emit a warning if the preempt count is corrupted and set the preempt count
+    to FORK_PREEMPT_COUNT.
+  - Reset the pointer to the memory descriptor used by prev which was set in
+    context_switch().
+  - Store the state of the previous task to handle the possibility of a DEAD
+    task.
+  - Do virtual CPU time accounting for the previous task.
+  - Handle perf subsystem context switch from previous task to current:
+
+    - Add perf events for the current task.
+    - Schedule in current cgroup events if cgroup perf events exist on the
+      CPU.
+    - Context switch the perf event contexts between the prev and current
+      tasks.
+    - Clear the PERF_RECORD_MISC_SWITCH_OUT perf event.
+    - Call the context switch callback for PMU with flag indicating
+      schedule in.
+  - Free the task for load balancing run on it.
+  - Unlock the rq lock.
+  - Clear the KCOV_IN_CTXSW in kcov_mode which was set in prepare_task_switch
+    now that the new task is live.
+  - Fire preempt notifiers to notify about task scheduled back in.
+  - If the prev task state indicated that it was dead, the corresponding
+    scheduler class task_dead hook is called. Function-return probe
+    instances associated with the task are removed and put back on the
+    free list. Stack for the task is freed and drop the RCU references.
+  - Evaluate the need for No idle tick due to the context switch and do the
+    idle tick if needed.
+
diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
index f311abe5b711..691fdfe32954 100644
--- a/Documentation/scheduler/index.rst
+++ b/Documentation/scheduler/index.rst
@@ -20,6 +20,7 @@ specific implementation differences.
     sched-data-structs
     cfs-overview
     sched-design-CFS
+    context-switching
     sched-features
     arch-specific
     sched-debugging
diff --git a/Documentation/scheduler/mips-context-switch.rst b/Documentation/scheduler/mips-context-switch.rst
new file mode 100644
index 000000000000..d833ab6d8a76
--- /dev/null
+++ b/Documentation/scheduler/mips-context-switch.rst
@@ -0,0 +1,89 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+==============================================
+MIPS Architecture And Scheduler implementation
+==============================================
+
+Multi-threading in MIPS CPUs
+-----------------------------
+The MIPS architecture defines four coprocessors.
+
+- CP0: supports virtual memory system and exception handling.
+- CP1: reserved for the floating point coprocessor, the FPU.
+- CP2: available for specific implementations.
+- CP3: reserved for floating point operations in the release 1
+  implementation of MIPS64.
+
+MIPS32 and MIPS64 architectures provide support for optional components
+known as Modules or Application Specific Extensions. The MT module
+enables the architecture to support multi-threaded implementations.
+This includes support for virtual processors and lightweight thread
+contexts. Implementation of MT features depends on the individual MIPS
+cores. The virtual processing element (VPE) maintains a complete copy
+of the processor state as seen by the software system which includes
+interrupts, register set, and MMU. This enables a single processor to
+appear to an SMP operating system like two separate cores if it has
+2 VPE's. For example two separate OSes can run on each VPE such as Linux
+and and an RTOS.
+
+A lighter version of VPE enables threading at the user/application
+software level.  It is called Thread Context (TC). TC is the hardware
+state necessary to support a thread of execution. This includes a set
+of general purpose registers (GPRs), a program counter (PC), and some
+multiplier and coprocessor state.  TCs have common execution unit.
+MIPS ISA provides instructions to utilize TC.
+
+The Quality of service block of the MT module allows the allocation of
+processor cycles to threads, and sets relative thread priorities. This
+enables 2 thread prioritization mechanisms. The user can prioritize one
+thread over the other as well as allocate a specific ratio of the cycles
+to specific threads. These mechanisms allocate bandwidth to a set
+of threads effectively. QoS block improves system level determinism
+and predictability. Qos block can be replaced by more application
+specific blocks.
+
+MIPS Context Switch
+-------------------
+
+Context switch behavior specific to MIPS begins in the way
+:c:macro:`switch_to()` macro is implemented. The main steps in the MIPS
+implementation of the macro are:
+
+* Handle the FPU affinity management feature. This feature is enabled
+  by the :c:macro:`CONFIG_MIPS_MT_FPAFF` at build time. The macro checks
+  if the FPU was used in the most recent time slice. In case FPU was not
+  used, the restriction of having to run on a CPU with FPU is removed.
+* Disable the FPU and clear the bit indicating the FPU was used in this
+  quantum for the task for the previous task.
+* If FPU is enabled in the next task, check FCSR for any unmasked
+  exceptions pending, clear them and send a signal.
+* If MIPS DSP modules is enabled, save the DSP context of the previous
+  task and restore the dsp context of the next task.
+* If coprocessor 2 is present set the access allowed field of the
+  coprocessor 2.
+* If coprocessor 2 access allowed field was set in previous task, clear it.
+* Clear the the access allowed field of the coprocessor 2.
+* Clear the llbit on MIPS release 6 such that instruction eretnc can be
+  used unconditionally when returning to userland in entry.S.
+  LLbit is used to specify operation for instructions that provide atomic
+  read-modify-write. LLbit is set when a linked load occurs and is tested
+  by the conditional store.  It is cleared, during other CPU operation,
+  when a store to the location would no longer be atomic. In particular,
+  it is cleared by exception return instructions.  eretnc instruction
+  enables to return from interrupt, exception, or error trap without
+  clearing the LLbit.
+* Clear the global variable ll_bit used by MIPS exception handler.
+* Write the thread pointer to the MIPS userlocal register if the CPU
+  supports this feature. This register is not interpreted by hardware and
+  can be used to share data between privileged and unprivileged software.
+* If hardware watchpoint feature is enabled during build, the watchpoint
+  registers are restored from the next task.
+* Finally the MIPS processor specific implementation of the resume()
+  function is called. It restores the registers of the next task including
+  the stack pointer. The implementation is in assembly in the following
+  architecutre specific files ::
+
+    arch/mips/kernel/r4k_switch.S
+    arch/mips/kernel/r2300_switch.S
+    arch/mips/kernel/octeon_switch.S
+
diff --git a/Documentation/scheduler/x86-context-switch.rst b/Documentation/scheduler/x86-context-switch.rst
new file mode 100644
index 000000000000..68453e99cb2b
--- /dev/null
+++ b/Documentation/scheduler/x86-context-switch.rst
@@ -0,0 +1,65 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+X86 Context Switch
+------------------
+
+The x86 architecture context switching logic is as follows.
+After the switching of MM in the scheduler context_switch() calls the x86
+implementation of :c:macro:`switch_to()`. For x86 arch it is located at ::
+
+    arch/x86/include/asm/switch_to.h
+
+Since kernel 4.9, switch_to() has been split into two parts: a
+`prepare_switch_to()` macro and the inline assembly implementation of
+__switch_to_asm() in the assembly files ::
+
+    arch/x86/entry/entry_64.S
+    arch/x86/entry/entry_32.S
+
+prepare_switch_to() handles the case when stack uses virtual memory. This
+is configured at build time and is enabled in most modern distributions.
+This function accesses the stack pointer to prevent a double fault.
+Switching to a stack that has top-level paging entry that is not
+present in the current MM will result in a page fault which will be promoted
+to double fault and the result is a panic. So it is necessary to probe the
+stack now so that the vmalloc_fault can fix the page tables.
+
+The main steps of the inline assembly function __switch_to_asm() are:
+
+* store the callee saved registers to the old stack which will be switched
+  away from.
+* swap the stack pointers between the old and the new task.
+* move the stack canary value to the current CPU's interrupt stack
+* if return trampoline is enabled, overwrite all entries in the RSB on
+  exiting a guest, to prevent malicious branch target predictions from
+  affecting the host kernel.
+* restore all registers from the new stack previously pushed in reverse
+  order.
+* jump to a C implementation of __switch_to(). The sources are located in::
+
+      arch/x86/kernel/process_64.c
+      arch/x86/kernel/process_32.c
+
+
+The main steps of the C function __switch_to() which is effectively
+the new task running are as follows:
+
+* retrieve the thread :c:type:`struct thread_struct <thread_struct>`
+  and fpu :c:type:`struct fpu <fpu>` structs from the next and previous
+  tasks.
+* get the current CPU TSS :c:type:`struct tss_struct <tss_struct>`.
+* save the current FPU state while on the old task.
+* store the FS and GS segment registers before changing the thread local
+  storage.
+* reload the GDT for the new tasks TLS.
+  Following is effectively arch_end_context_switch().
+* save the ES and DS segments of the previous task and load the same from
+  the nest task.
+* load the FS and GS segment registers.
+* update the current task of the CPU.
+* update the top of stack pointer for the CPU for entry trampoline.
+* initialize FPU state for next task.
+* set sp0 to point to the entry trampoline stack.
+* call _switch_to_xtra() to handle debug registers, I/O
+  bitmaps and speculation mitigation.
+* write the task's CLOSid/RMID to IA32_PQR_MSR.
-- 
2.17.1

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[Valentin Schneider]

On 07/05/20 19:05, John Mathew wrote:
> 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

<snip>

> +**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.
> +

I'm not sure how useful this cherry-picked git history is, but that's
making me think we may want to have an asymmetric CPU capacity scheduling
section somewhere - I've had to explain what we do here a few times
already, and it's true that unless you look at the git history or at the
code it isn't laid out for you (with the exception of EAS that was blessed
with Quentin's writings).

It would also be an opportunity to have one place to (at least briefly)
describe what the different sched classes do wrt capacity asymmetry - CFS
does one thing, RT now does one thing (see Qais' work), and DL will
hopefully soon follow (see Dietmar's work).

I'd be happy to contribute (some of) that, if it can be deemed useful (I
personally think it might).

> +=========================
> +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
> +~~~~~~~~~~~~~~~~~

That probably should be under CFS specific stuff.

> +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.

There's the whole task_clock() shenanigans there, i.e. depending on your
config knobs that delta may not include IRQ or paravirt time; though that
doesn't necessarily help in understanding vruntime, so it might be best to
leave it at that.

> +* NICE_0_LOAD is the load of a task with normal priority.

s/normal/default/

> +* 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.

Careful not to mix up se.load and se.avg.load_avg. Load balancing decisions
are based (amongst other things) on (sums of) se.avg.load_avg, i.e. the
'load' signal generated via PELT. It is indeed weighted by priority, but
se.load is *not* that load signal.

It's also the first time I see that bit about load being ~how many CPUs
that task needs. That's peculiar; you can't really split a task up to
schedule it on other CPUs, so how would that help? And, again, the load
signal being weighted by priority makes this even weirder.

> +  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.

There's stop (at the top) and idle (at the bottom), but admittedly they
are somewhat special.

> +
> +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.
> +

Those are all what Peter dubs the "change pattern", you may want to just
explain the principle (we need to dequeue tasks to do some operations; and
ofc we need to put them back as they were once that's done) & pick just one
rather than exhaustively list them all.

This still applies to a lot of other places IMO - I think you need less
exhaustive listings and more "what's the gist of that".

> 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

Holy moly! I appreciate the effort, but I'm thinking the targeted audience
might prefer this under the form of comments within the struct definition...

>   */
>  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.

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[Dietmar Eggemann]

On 07/05/2020 20:05, John Mathew wrote:

[...]

> 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.
> +

I agree with what Valentin mentioned already. Instead of describing
recent patch-sets, the functionality which was added (or enhanced) by
them) should be depicted instead.

E.g. in case of 'Thermal Pressure' this would be the "scale CPU
capacity" mechanism for CFS so it knows how much CPU capacity is left
for its use after higher priority sched classes (RT, DL), IRQs and
'Thermal Pressure' have reduced the 'original' CPU capacity.

[...]

> +**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.

What are those 3 separate functions? I guess you refer to the 3
(actually 4) migration types (migrate_task, migrate_util, migrate_load,
(migrate_misfit)).

[...]

> 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.

s/RT/SCHED_FIFO, SCHED_RR
s/NORMAL/SCHED_NORMAL
s/BATCH/SCHED_BATCH

SCHED_IDLE tasks can be set in the 100 – 139 range too but IMHO are
treated as 139 (nice 20). Their priority doesn't matter since they get
minimal weight WEIGHT_IDLEPRI=3 anyway.

And then there are the maintenance sched classes, idle sched class and
its idle tasks 'swapper/X' with priority 120 (was MAX_PRIO) as well as
the stop sched class and its stopper tasks 'migration/X' who disguise as
SCHED_FIFO with priority 139.
Might be that people might find this too detailed though but it helps
when you try to understand how it all works.

[...]

> 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
>  
> +                     +------------------------------------+
> +                     |            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

I would refer to them as trace events.

The scheduler started to export (bare) trace points for PELT and
overutilization (e.g. pelt_cfs_tp) (commit ba19f51fcb54 "sched/debug:
Add new tracepoints to track PELT at rq level"). They are not bound to a
trace event and so they don't expose any internal data structures.

[...]

> +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.

load.weight is replaced by PELT in 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.

SCHED_IDLE get minimal weight (WEIGHT_IDLEPRIO=3)

[...]

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[Dietmar Eggemann]

On 07/05/2020 23:15, Valentin Schneider wrote:
> 
> On 07/05/20 19:05, John Mathew wrote:

[...]

> It would also be an opportunity to have one place to (at least briefly)
> describe what the different sched classes do wrt capacity asymmetry - CFS
> does one thing, RT now does one thing (see Qais' work), and DL will
> hopefully soon follow (see Dietmar's work).
> 
> I'd be happy to contribute (some of) that, if it can be deemed useful (I
> personally think it might).

I like the idea.

Essentially all the code which is guarded by the 'if
(static_branch_unlikely(&sched_asym_cpucapacity)' condition or which
sets it during bring-up.

* 'Cpu capacity < SCHED_LOAD_SCALE for non-big' CPUs setting during
   bringup (necessary dt binding, CPUfreq influence)

* CFS capacity awareness:

  * wakeup - select_idle_capacity() (replaced wake_cap() & slow path to
             cover DynamIQ and classical big.LITTLE)

  * load_balance - misfit handling

* RT & DL capacity awareness

* ... & the relation to EAS (Documentation/scheduler/sched-energy.rst)

This is what we referred to (at least internally) as CAS (Capacity-Aware
Scheduling).

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[Valentin Schneider]

On 08/05/20 11:58, Dietmar Eggemann wrote:
> On 07/05/2020 23:15, Valentin Schneider wrote:
>>
>> On 07/05/20 19:05, John Mathew wrote:
>
> [...]
>
>> It would also be an opportunity to have one place to (at least briefly)
>> describe what the different sched classes do wrt capacity asymmetry - CFS
>> does one thing, RT now does one thing (see Qais' work), and DL will
>> hopefully soon follow (see Dietmar's work).
>>
>> I'd be happy to contribute (some of) that, if it can be deemed useful (I
>> personally think it might).
>
> I like the idea.
>
> Essentially all the code which is guarded by the 'if
> (static_branch_unlikely(&sched_asym_cpucapacity)' condition or which
> sets it during bring-up.
>
> * 'Cpu capacity < SCHED_LOAD_SCALE for non-big' CPUs setting during
>    bringup (necessary dt binding, CPUfreq influence)
>
> * CFS capacity awareness:
>
>   * wakeup - select_idle_capacity() (replaced wake_cap() & slow path to
>              cover DynamIQ and classical big.LITTLE)
>
>   * load_balance - misfit handling
>
> * RT & DL capacity awareness
>
> * ... & the relation to EAS (Documentation/scheduler/sched-energy.rst)
>
> This is what we referred to (at least internally) as CAS (Capacity-Aware
> Scheduling).

Yeah, something like that. I'll sharpen my quill and have a go at it,
though it won't be for right now with OSPM and the other ongoing stuff.

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[Tao Zhou]

Hi John,

On Thu, May 07, 2020 at 09:05:52PM +0300, John Mathew wrote:
> 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

s/shed_entity/sched_entity/

> +  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.

s/ćurrently/currently

Thanks,
Tau

> +
> +    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
> 

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[John Mathew]

On Fri, May 8, 2020 at 12:15 AM Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
>
> On 07/05/20 19:05, John Mathew wrote:
> > 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
>
> <snip>
>
> > +**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.
> > +
>
> I'm not sure how useful this cherry-picked git history is, but that's
> making me think we may want to have an asymmetric CPU capacity scheduling
> section somewhere - I've had to explain what we do here a few times
> already, and it's true that unless you look at the git history or at the
> code it isn't laid out for you (with the exception of EAS that was blessed
> with Quentin's writings).
Added a new rst for Capacity Aware scheduling. It contains a brief description
according to Dietmar. That will be a good place for you to contribute.
>
> It would also be an opportunity to have one place to (at least briefly)
> describe what the different sched classes do wrt capacity asymmetry - CFS
> does one thing, RT now does one thing (see Qais' work), and DL will
> hopefully soon follow (see Dietmar's work).
>
> I'd be happy to contribute (some of) that, if it can be deemed useful (I
> personally think it might).
>
> > +=========================
> > +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
> > +~~~~~~~~~~~~~~~~~
>
> That probably should be under CFS specific stuff.
Agree, infact this is already described in sched-design-CFS.rst.
>
> > +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.
>
> There's the whole task_clock() shenanigans there, i.e. depending on your
> config knobs that delta may not include IRQ or paravirt time; though that
> doesn't necessarily help in understanding vruntime, so it might be best to
> leave it at that.
>
> > +* NICE_0_LOAD is the load of a task with normal priority.
>
> s/normal/default/
>
> > +* 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.
>
> Careful not to mix up se.load and se.avg.load_avg. Load balancing decisions
> are based (amongst other things) on (sums of) se.avg.load_avg, i.e. the
> 'load' signal generated via PELT. It is indeed weighted by priority, but
> se.load is *not* that load signal.
>
> It's also the first time I see that bit about load being ~how many CPUs
> that task needs. That's peculiar; you can't really split a task up to
> schedule it on other CPUs, so how would that help? And, again, the load
> signal being weighted by priority makes this even weirder.
This section was removed as sched-design-CFS.rst describes it.
>
> > +  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.
>
> There's stop (at the top) and idle (at the bottom), but admittedly they
> are somewhat special.
Added in v4 patch version.
>
> > +
> > +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.
> > +
>
> Those are all what Peter dubs the "change pattern", you may want to just
> explain the principle (we need to dequeue tasks to do some operations; and
> ofc we need to put them back as they were once that's done) & pick just one
> rather than exhaustively list them all.
Removed describing change pattern in multiple places.
>
> This still applies to a lot of other places IMO - I think you need less
> exhaustive listings and more "what's the gist of that".
>
> > 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
>
> Holy moly! I appreciate the effort, but I'm thinking the targeted audience
> might prefer this under the form of comments within the struct definition...
This was done based on review comments to move describing the rq struct
from documentation for kernel-doc comments so that documentation will
be updated when parameters are changed/removed.
>
> >   */
> >  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.

Thanks for your review

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[John Mathew]

On Fri, May 8, 2020 at 1:19 PM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>
> On 07/05/2020 20:05, John Mathew wrote:
>
> [...]
>
> > 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.
> > +
>
> I agree with what Valentin mentioned already. Instead of describing
> recent patch-sets, the functionality which was added (or enhanced) by
> them) should be depicted instead.
>
> E.g. in case of 'Thermal Pressure' this would be the "scale CPU
> capacity" mechanism for CFS so it knows how much CPU capacity is left
> for its use after higher priority sched classes (RT, DL), IRQs and
> 'Thermal Pressure' have reduced the 'original' CPU capacity.
Changed in v4 patch version.
> [...]
>
> > +**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.
>
> What are those 3 separate functions? I guess you refer to the 3
> (actually 4) migration types (migrate_task, migrate_util, migrate_load,
> (migrate_misfit)).

The three functions are
* update_sd_pick_busiest() select the busiest sched_group.
* find_busiest_group() checks if there is an imbalance between local and
busiest group.
* calculate_imbalance() decides what have to be moved.
Added them in v4 of patchset.
>
> [...]
>
> > 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.
>
> s/RT/SCHED_FIFO, SCHED_RR
> s/NORMAL/SCHED_NORMAL
> s/BATCH/SCHED_BATCH
>
Changed in patchset v4.

> SCHED_IDLE tasks can be set in the 100 – 139 range too but IMHO are
> treated as 139 (nice 20). Their priority doesn't matter since they get
> minimal weight WEIGHT_IDLEPRI=3 anyway.
>
> And then there are the maintenance sched classes, idle sched class and
> its idle tasks 'swapper/X' with priority 120 (was MAX_PRIO) as well as
> the stop sched class and its stopper tasks 'migration/X' who disguise as
> SCHED_FIFO with priority 139.
> Might be that people might find this too detailed though but it helps
> when you try to understand how it all works.
>
Added in patchset v4.
> [...]
>
> > 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
> >
> > +                     +------------------------------------+
> > +                     |            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
>
> I would refer to them as trace events.
Changed in patchset v4.
>
> The scheduler started to export (bare) trace points for PELT and
> overutilization (e.g. pelt_cfs_tp) (commit ba19f51fcb54 "sched/debug:
> Add new tracepoints to track PELT at rq level"). They are not bound to a
> trace event and so they don't expose any internal data structures.
>
> [...]
>
> > +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.
>
> load.weight is replaced by PELT in load balancing.
Removed this section as it is already described in sched-design-CFS
>
> > +  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.
>
> SCHED_IDLE get minimal weight (WEIGHT_IDLEPRIO=3)

Thanks for your review.
>
> [...]

Re: [RFC PATCH v3 2/3] docs: scheduler: Add scheduler overview documentation

[John Mathew]

On Fri, May 8, 2020 at 1:58 PM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>
> On 07/05/2020 23:15, Valentin Schneider wrote:
> >
> > On 07/05/20 19:05, John Mathew wrote:
>
> [...]
>
> > It would also be an opportunity to have one place to (at least briefly)
> > describe what the different sched classes do wrt capacity asymmetry - CFS
> > does one thing, RT now does one thing (see Qais' work), and DL will
> > hopefully soon follow (see Dietmar's work).
> >
> > I'd be happy to contribute (some of) that, if it can be deemed useful (I
> > personally think it might).
>
> I like the idea.
>
> Essentially all the code which is guarded by the 'if
> (static_branch_unlikely(&sched_asym_cpucapacity)' condition or which
> sets it during bring-up.
>
> * 'Cpu capacity < SCHED_LOAD_SCALE for non-big' CPUs setting during
>    bringup (necessary dt binding, CPUfreq influence)
>
> * CFS capacity awareness:
>
>   * wakeup - select_idle_capacity() (replaced wake_cap() & slow path to
>              cover DynamIQ and classical big.LITTLE)
>
>   * load_balance - misfit handling
>
> * RT & DL capacity awareness
>
> * ... & the relation to EAS (Documentation/scheduler/sched-energy.rst)
>
> This is what we referred to (at least internally) as CAS (Capacity-Aware
> Scheduling).

Added in pathset v4.

Thanks