Re: [RFC PATCH v5 2/3] docs: scheduler: Add scheduler overview documentation

[John Mathew]

On Fri, May 29, 2020 at 5:48 PM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Thu, 14 May 2020 at 11:26, john mathew <john.mathew@xxxxxxxxxx> wrote:
> >
> > From: John Mathew <john.mathew@xxxxxxxxxx>
> >
> > Add documentation for
> >  -scheduler overview
> >  -scheduler state transtion
> >  -CFS overview
> >  -scheduler data structs
> >
> > Add rst for scheduler APIs and modify sched/core.c
> > to add kernel-doc comments.
> >
> > Suggested-by: Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx>
> > Co-developed-by: Mostafa Chamanara <mostafa.chamanara@xxxxxxxxxxxx>
> > Signed-off-by: Mostafa Chamanara <mostafa.chamanara@xxxxxxxxxxxx>
> > Co-developed-by: Oleg Tsymbal <oleg.tsymbal@xxxxxxxxxx>
> > Signed-off-by: Oleg Tsymbal <oleg.tsymbal@xxxxxxxxxx>
> > Signed-off-by: John Mathew <john.mathew@xxxxxxxxxx>
> > ---
> >  Documentation/scheduler/cfs-overview.rst      | 102 +++++++
> >  Documentation/scheduler/index.rst             |   2 +
> >  Documentation/scheduler/overview.rst          | 288 ++++++++++++++++++
> >  Documentation/scheduler/sched-cas.rst         |  92 ++++++
> >  .../scheduler/sched-data-structs.rst          | 182 +++++++++++
> >  Documentation/scheduler/sched-features.rst    |   1 +
> >  Documentation/scheduler/scheduler-api.rst     |  31 ++
> >  kernel/sched/core.c                           |  28 +-
> >  kernel/sched/sched.h                          | 169 +++++++++-
> >  9 files changed, 888 insertions(+), 7 deletions(-)
> >  create mode 100644 Documentation/scheduler/cfs-overview.rst
> >  create mode 100644 Documentation/scheduler/sched-cas.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..34f336b8ec86
> > --- /dev/null
> > +++ b/Documentation/scheduler/cfs-overview.rst
> > @@ -0,0 +1,102 @@
> > +.. 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**:
>
> The title is misleading because the thermal pressure is only one
> source of reduction of the CPU capacity. In fact thermal has just
> reused what was already in place for RT, DL and IRQ.
> "Tracking available capacity" is more correct IMHO
Changed in v7
>
> > +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.
> > +Thermal pressure on a CPU means the maximum possible capacity is
> > +unavailable due to thermal events.
> > +
> > +** Optimizations to NUMA balancing**:
> > +When gathering NUMA statistics, information about whether a core is Idle
> > +is also cached. In case of an imbalance, instead of doing a second scan of
> > +the node runqueues, the idle core is used as the migration target. When
> > +doing so multiple tasks can attempt to select an idle CPU but fail, because
> > +a NUMA balance is active on that CPU. In this case an alternative idle CPU
> > +scanned. Another optimization is to terminate the search for swap candidate
> > +when a reasonable one is found instead of searching all the CPUs on the
> > +target domain.
> > +
> > +**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**:
> > +Skipped looping through all the threads of a core to evaluate if the
> > +core is idle or not. If a thread of a core is not idle, evaluation of
> > +other threads of the core can be skipped.
> > +
> > +**Load balance aggressively for SCHED_IDLE CPUs**:
> > +Newly-woken task is preferred to be  enqueued on a SCHED_IDLE CPU instead
> > +of other busy or idle CPUs. Also load balancer is made to migrate tasks more
> > +aggressively to a SCHED_IDLE CPU. 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. Also the the busy_factor
> > +used with the balance interval to prevent frequent load balancing
> > +is ignored for such CPU's.
> > +
> > +**Load balancing algorithm Reworked**:
> > +Some heuristics in the load balancing algorithm became meaningless because
> > +of the rework of the scheduler's metrics like the introduction of PELT.
> > +Those heuristics were removed. The new load balancing algorithm also 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.
> > +* 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.
> > +
> > +**Energy-aware wake-ups speeded up**:
> > +Algorithmic complexity of the EAS was reduced from O(n^2) to O(n).
> > +Previous algorithm resulted in prohibitively high wake-up latencies on
> > +systems with complex energy models, such as systems with per-CPU DVFS.
> > +The EAS wake-up path was re-factored to compute the energy 'delta' on a
> > +per-performance domain basis, rather than the whole system.
> > +
> > +**Selection of an energy-efficient CPU on task wake-up**:
> > +An Energy efficient CPU is found 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. Energy Model (EM) is
> > +used for this. 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**:
> > +A task which ends up on a CPU which doesn't suit its compute demand is
> > +identified as a misfit task in asymmetric CPU capacity systems. These
> > +'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**:
> > +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 9bdccea74af9..f311abe5b711 100644
> > --- a/Documentation/scheduler/index.rst
> > +++ b/Documentation/scheduler/index.rst
> > @@ -17,6 +17,8 @@ specific implementation differences.
> >      :maxdepth: 2
> >
> >      overview
> > +    sched-data-structs
> > +    cfs-overview
> >      sched-design-CFS
> >      sched-features
> >      arch-specific
> > diff --git a/Documentation/scheduler/overview.rst b/Documentation/scheduler/overview.rst
> > index aee16feefc61..7536bec6afce 100644
> > --- a/Documentation/scheduler/overview.rst
> > +++ b/Documentation/scheduler/overview.rst
> > @@ -3,3 +3,291 @@
> >  ====================
> >  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) â 99 whereas normal
> > +priorities range from 100 â 139 (lowest). Scheduler implements many scheduling
> > +classes which encapsulate a particular scheduling policy. Each scheduling
> > +policy implements scheduler handling of tasks that belong to a particular
> > +priority. SCHED_FIFO and SCHED_RR policies handle real time priorities tasks
> > +while SCHED_NORMAL and SCHED_BATCH policies handle tasks with normal priorities.
> > +SCHED_IDLE is also a normal scheduling policy when means its priority can
> > +be set between 100 â 139 range too but they are treated as priority 139.
> > +Their priority doesn't matter since they get minimal weight WEIGHT_IDLEPRI=3.
> > +SCHED_DEADLINE policy tasks have negative priorities, reflecting
> > +the fact that any of them has higher priority than RT and NORMAL/BATCH tasks.
> > +
> > +And then there are the maintenance scheduler classes: idle sched class and
> > +stop sched class. Idle class doesn't manage any user tasks and so doesn't
> > +implement a policy. Its idle tasks 'swapper/X' has priority 120 and and aren't
> > +visible to user space. Idle tasks are responsible for by putting the CPUs
> > +into deep idle states when there is no work to do.
> > +Stop sched class is also used internally by the kernel doesn't implement any
> > +scheduling policy. Stopper tasks 'migration/X' disguise as as a SCHED_FIFO
> > +task with priority 139. Stopper tasks are a mechanism to force a CPU to stop
> > +running everything else and perform a specific task. As this is the
> > +highest-priority class, it can preempt everything else and nothing ever
> > +preempts it. It is used by one CPU to stop another in order to run a specific
> > +function, so it is only available on SMP systems. This class is used by the
> > +kernel for task migration.
> > +
> > +
> > +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 trace events tracing all major events of the scheduler.
> > +The trace events are defined in ::
> > +
> > +  include/trace/events/sched.h
> > +
> > +Using these trace events it is possible to model the scheduler state transition
> > +in an automata model. The following journal paper discusses such modeling:
> > +
> > +Daniel B. de Oliveira, RÃmulo S. de Oliveira, Tommaso Cucinotta, **A thread
> > +synchronization model for the PREEMPT_RT Linux kernel**, *Journal of Systems
> > +Architecture*, Volume 107, 2020, 101729, ISSN 1383-7621,
> > +https://doi.org/10.1016/j.sysarc.2020.101729.
> > +
> > +To model the scheduler efficiently the system was divided in to generators
> > +and specifications. Some of the generators used were "need_resched",
> > +"sleepable" and "runnable", "thread_context" and "scheduling context".
> > +The specifications are the necessary and sufficient conditions to call
> > +the scheduler. New trace events were added to specify the generators
> > +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-cas.rst b/Documentation/scheduler/sched-cas.rst
> > new file mode 100644
> > index 000000000000..fcebc5770803
> > --- /dev/null
> > +++ b/Documentation/scheduler/sched-cas.rst
> > @@ -0,0 +1,92 @@
> > +.. SPDX-License-Identifier: GPL-2.0+
> > +
> > +=========================
> > +Capacity-Aware Scheduling
> > +=========================
> > +
> > +Scheduling load balancing on Asymmetric Multiprocessor systems was improved
> > +through the introduction of Capacity-Aware Scheduling. It identifies the
> > +most efficient CPU to assign a task based on its capacity. This capacity
> > +may be asymmetric due to heterogeneous computing architecture such
> > +as ARM big.LITTLE. Scheduler gets information about asymmetric capacities
> > +when the scheduler domain hierarchy is built using build_sched_domains().
> > +CPU capacities are provided to the scheduler topology code through the
> > +architecture specific implementation of the arch_scale_cpu_capacity().
> > +The SD_ASYM_CPUCAPACITY flag is set by the scheduler topology for a domain
> > +in the hierarchy where all CPU capacities are visible for any cpu's point
> > +of view on asymmetric CPU capacity systems. The scheduler can then take
> > +capacity asymmetry into account when load balancing.
> > +
> > +Initial CPU capacities are derived from the Device Tree and CPU frequency.
> > +For RISC-V & ARM64 it is done in drivers/base/arch_topology.c. A cpu-map
> > +device tree is parsed to obtain the cpu topology and the initial CPU capacity
> > +is set using the CPUFreq subsystem. A callback is registered to the CPUFreq
> > +subsystem to rebuild sched_domains once the CPUFreq is loaded, which is when
> > +a complete view of the capacities of the CPUs (which is a mix of Âarch and
> > +frequencies) is available.
> > +
> > +Asymmetric CPU capacity information is used in
> > +
> > +* Energy Aware Scheduling: The scheduler is  able to predict the impact of
> > +  its decisions on the energy consumed by CPUs. Described in :doc:`sched-energy` .
> > +* Optimized task wakeup load balancing by finding idle CPU with enough capacity.
> > +
> > +The different scheduler classes asymmetric use the Asymmetric CPU capacity
> > +information differently.
> > +
> > +CFS Capacity Awareness
> > +======================
> > +
> > +Used to identify misfit tasks:
> > +A load intensive task on a CPU which doesn't meet its compute demand is
> > +identified as a misfit task. 'Misfit' tasks are migrated to CPUs with
> > +higher compute capacity to ensure better throughput. CFS frequently updates
> > +the misfit status of the current task by comparing its utilization vs the
> > +CPU capacity using task_fits_capacity(). If the utilization is more than the
> > +CPU capacity the calculated misfit load is updated to the runqueue
> > +rq->misfit_task_load. This misfit load is then checked by the load
> > +balancing operations to migrate the task to a CPU of higher capacity.
> > +
> > +Modified wakeup logic to support DynamIQ systems:
> > +When the scheduler class calls select_task_rq_fair to select a runqueue for
> > +a waking task, load balancing is performed 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. In DynamIQ systems Last Level Cache (LLC)
> > +domain of a CPU spans all CPUs in the system. This may include CPU's of
> > +different capacities. So in select_idle_sibling() an idle sibling is picked
> > +based on CPU capacity for asymmetric CPU capacity systems and for symmetric
> > +systems use LLC domain is used. 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. For asymmetric CPU capacity systems select_idle_sibling() operates
> > +on the sd_asym_cpucapacity sched_domain pointer, which is guaranteed to span
> > +all known CPU capacities in the system. This works for both "legacy"
> > +big.LITTLE (LITTLEs & bigs split at MC, joined at DIE) and for newer
> > +DynamIQ systems (e.g. LITTLEs and bigs in the same MC domain).
> > +
> > +
> > +RT Capacity Awareness
> > +=====================
> > +
> > +Since RT tasks doesn't have a per task utilization signal RT tasks uses uclamp
> > +to guarantee a minimum performance point. Utilization clamping is a mechanism
> > +which allows to "clamp" (i.e. filter) the utilization generated by RT and
> > +FAIR tasks within a range defined by user-space. It exposes to user-space a
> > +new set of per-task attributes the scheduler can use as hints about the
> > +expected/required utilization for a task. RT is made capacity aware
> > +by ensuring that the capacity of the CPU is >= uclamp_min value. This check
> > +is done in the rt_task_fits_capacity()
> > +
> > +DL Capacity Awareness
> > +=====================
> > +
> > +TBD
> > +
> > +
> > +
> > +
> > +
> > +
> > +
> > +
> > +
> > diff --git a/Documentation/scheduler/sched-data-structs.rst b/Documentation/scheduler/sched-data-structs.rst
> > new file mode 100644
> > index 000000000000..a16408676b71
> > --- /dev/null
> > +++ b/Documentation/scheduler/sched-data-structs.rst
> > @@ -0,0 +1,182 @@
> > +.. 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.
> > +
> > +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 to change a property of a task. This is referred to as
> > +    change pattern.  Change is defined as the following sequence of calls::
> > +
> > +         * dequeue_task()
> > +         * put_prev_task()
> > +         * change a property
> > +         * enqueue_task()
> > +         * set_next_task()
> > +
> > +    The enqueue_task function takes the runqueue, the task which needs to
> > +    be enqueued/dequeued and a bit mask of flags as parameters. 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
> > +
> > +    Some places where the enqueue_task and dequeue_task are called for
> > +    changing task properties are
> > +
> > +    * When migrating a task from one CPU's runqueue to another.
> > +    * When changing a tasks CPU affinity.
> > +    * When changing the priority of a task.
> > +    * When changing the nice value of the task.
> > +    * When changing the scheduling policy and/or RT priority of a thread.
> > +
> > +pick_next_task
> > +    Called by the scheduler to pick the next best task to run. The scheduler
> > +    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.
> > +    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 SCHED_DEADLINE and RT classes additional tree
> > +    is maintained to push tasks from the current CPU to another CPU where the
> > +    task can preempt and start executing. 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 pattern'. In case
> > +    of CFS scheduling class, it will set 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 the vruntime
> > +    update optimization, opposite to what was done in put_prev_task.
> > +    For the SCHED_DEADLINE and RT classes it will remove the task from the
> > +    tree of pushable tasks trigger the balance callback to push another task
> > +    which is non running on the current CPU for execution on another CPU.
> > +
> > +    * 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/sched-features.rst b/Documentation/scheduler/sched-features.rst
> > index 1afbd9cc8d52..e576c7d9e556 100644
> > --- a/Documentation/scheduler/sched-features.rst
> > +++ b/Documentation/scheduler/sched-features.rst
> > @@ -17,4 +17,5 @@ Scheduler Features
> >     sched-energy
> >     sched-nice-design
> >     sched-rt-group
> > +   sched-cas
> >     completion
> > 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
> >