sched/doc: Document capacity aware scheduling

Add some documentation detailing the concepts, requirements and
implementation of capacity aware scheduling across the different scheduler
classes.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200731192016.7484-3-valentin.schneider@arm.com

1 files changed, 439 insertions, 0 deletions

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+=========================
+Capacity Aware Scheduling
+=========================
+
+1. CPU Capacity
+===============
+
+1.1 Introduction
+----------------
+
+Conventional, homogeneous SMP platforms are composed of purely identical
+CPUs. Heterogeneous platforms on the other hand are composed of CPUs with
+different performance characteristics - on such platforms, not all CPUs can be
+considered equal.
+
+CPU capacity is a measure of the performance a CPU can reach, normalized against
+the most performant CPU in the system. Heterogeneous systems are also called
+asymmetric CPU capacity systems, as they contain CPUs of different capacities.
+
+Disparity in maximum attainable performance (IOW in maximum CPU capacity) stems
+from two factors:
+
+- not all CPUs may have the same microarchitecture (µarch).
+- with Dynamic Voltage and Frequency Scaling (DVFS), not all CPUs may be
+  physically able to attain the higher Operating Performance Points (OPP).
+
+Arm big.LITTLE systems are an example of both. The big CPUs are more
+performance-oriented than the LITTLE ones (more pipeline stages, bigger caches,
+smarter predictors, etc), and can usually reach higher OPPs than the LITTLE ones
+can.
+
+CPU performance is usually expressed in Millions of Instructions Per Second
+(MIPS), which can also be expressed as a given amount of instructions attainable
+per Hz, leading to::
+
+  capacity(cpu) = work_per_hz(cpu) * max_freq(cpu)
+
+1.2 Scheduler terms
+-------------------
+
+Two different capacity values are used within the scheduler. A CPU's
+``capacity_orig`` is its maximum attainable capacity, i.e. its maximum
+attainable performance level. A CPU's ``capacity`` is its ``capacity_orig`` to
+which some loss of available performance (e.g. time spent handling IRQs) is
+subtracted.
+
+Note that a CPU's ``capacity`` is solely intended to be used by the CFS class,
+while ``capacity_orig`` is class-agnostic. The rest of this document will use
+the term ``capacity`` interchangeably with ``capacity_orig`` for the sake of
+brevity.
+
+1.3 Platform examples
+---------------------
+
+1.3.1 Identical OPPs
+~~~~~~~~~~~~~~~~~~~~
+
+Consider an hypothetical dual-core asymmetric CPU capacity system where
+
+- work_per_hz(CPU0) = W
+- work_per_hz(CPU1) = W/2
+- all CPUs are running at the same fixed frequency
+
+By the above definition of capacity:
+
+- capacity(CPU0) = C
+- capacity(CPU1) = C/2
+
+To draw the parallel with Arm big.LITTLE, CPU0 would be a big while CPU1 would
+be a LITTLE.
+
+With a workload that periodically does a fixed amount of work, you will get an
+execution trace like so::
+
+ CPU0 work ^
+           |     ____                ____                ____
+           |    |    |              |    |              |    |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+ CPU1 work ^
+           |     _________           _________           ____
+           |    |         |         |         |         |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+CPU0 has the highest capacity in the system (C), and completes a fixed amount of
+work W in T units of time. On the other hand, CPU1 has half the capacity of
+CPU0, and thus only completes W/2 in T.
+
+1.3.2 Different max OPPs
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Usually, CPUs of different capacity values also have different maximum
+OPPs. Consider the same CPUs as above (i.e. same work_per_hz()) with:
+
+- max_freq(CPU0) = F
+- max_freq(CPU1) = 2/3 * F
+
+This yields:
+
+- capacity(CPU0) = C
+- capacity(CPU1) = C/3
+
+Executing the same workload as described in 1.3.1, which each CPU running at its
+maximum frequency results in::
+
+ CPU0 work ^
+           |     ____                ____                ____
+           |    |    |              |    |              |    |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+                            workload on CPU1
+ CPU1 work ^
+           |     ______________      ______________      ____
+           |    |              |    |              |    |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+1.4 Representation caveat
+-------------------------
+
+It should be noted that having a *single* value to represent differences in CPU
+performance is somewhat of a contentious point. The relative performance
+difference between two different µarchs could be X% on integer operations, Y% on
+floating point operations, Z% on branches, and so on. Still, results using this
+simple approach have been satisfactory for now.
+
+2. Task utilization
+===================
+
+2.1 Introduction
+----------------
+
+Capacity aware scheduling requires an expression of a task's requirements with
+regards to CPU capacity. Each scheduler class can express this differently, and
+while task utilization is specific to CFS, it is convenient to describe it here
+in order to introduce more generic concepts.
+
+Task utilization is a percentage meant to represent the throughput requirements
+of a task. A simple approximation of it is the task's duty cycle, i.e.::
+
+  task_util(p) = duty_cycle(p)
+
+On an SMP system with fixed frequencies, 100% utilization suggests the task is a
+busy loop. Conversely, 10% utilization hints it is a small periodic task that
+spends more time sleeping than executing. Variable CPU frequencies and
+asymmetric CPU capacities complexify this somewhat; the following sections will
+expand on these.
+
+2.2 Frequency invariance
+------------------------
+
+One issue that needs to be taken into account is that a workload's duty cycle is
+directly impacted by the current OPP the CPU is running at. Consider running a
+periodic workload at a given frequency F::
+
+  CPU work ^
+           |     ____                ____                ____
+           |    |    |              |    |              |    |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+This yields duty_cycle(p) == 25%.
+
+Now, consider running the *same* workload at frequency F/2::
+
+  CPU work ^
+           |     _________           _________           ____
+           |    |         |         |         |         |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+This yields duty_cycle(p) == 50%, despite the task having the exact same
+behaviour (i.e. executing the same amount of work) in both executions.
+
+The task utilization signal can be made frequency invariant using the following
+formula::
+
+  task_util_freq_inv(p) = duty_cycle(p) * (curr_frequency(cpu) / max_frequency(cpu))
+
+Applying this formula to the two examples above yields a frequency invariant
+task utilization of 25%.
+
+2.3 CPU invariance
+------------------
+
+CPU capacity has a similar effect on task utilization in that running an
+identical workload on CPUs of different capacity values will yield different
+duty cycles.
+
+Consider the system described in 1.3.2., i.e.::
+
+- capacity(CPU0) = C
+- capacity(CPU1) = C/3
+
+Executing a given periodic workload on each CPU at their maximum frequency would
+result in::
+
+ CPU0 work ^
+           |     ____                ____                ____
+           |    |    |              |    |              |    |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+ CPU1 work ^
+           |     ______________      ______________      ____
+           |    |              |    |              |    |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+
+IOW,
+
+- duty_cycle(p) == 25% if p runs on CPU0 at its maximum frequency
+- duty_cycle(p) == 75% if p runs on CPU1 at its maximum frequency
+
+The task utilization signal can be made CPU invariant using the following
+formula::
+
+  task_util_cpu_inv(p) = duty_cycle(p) * (capacity(cpu) / max_capacity)
+
+with ``max_capacity`` being the highest CPU capacity value in the
+system. Applying this formula to the above example above yields a CPU
+invariant task utilization of 25%.
+
+2.4 Invariant task utilization
+------------------------------
+
+Both frequency and CPU invariance need to be applied to task utilization in
+order to obtain a truly invariant signal. The pseudo-formula for a task
+utilization that is both CPU and frequency invariant is thus, for a given
+task p::
+
+                                     curr_frequency(cpu)   capacity(cpu)
+  task_util_inv(p) = duty_cycle(p) * ------------------- * -------------
+                                     max_frequency(cpu)    max_capacity
+
+In other words, invariant task utilization describes the behaviour of a task as
+if it were running on the highest-capacity CPU in the system, running at its
+maximum frequency.
+
+Any mention of task utilization in the following sections will imply its
+invariant form.
+
+2.5 Utilization estimation
+--------------------------
+
+Without a crystal ball, task behaviour (and thus task utilization) cannot
+accurately be predicted the moment a task first becomes runnable. The CFS class
+maintains a handful of CPU and task signals based on the Per-Entity Load
+Tracking (PELT) mechanism, one of those yielding an *average* utilization (as
+opposed to instantaneous).
+
+This means that while the capacity aware scheduling criteria will be written
+considering a "true" task utilization (using a crystal ball), the implementation
+will only ever be able to use an estimator thereof.
+
+3. Capacity aware scheduling requirements
+=========================================
+
+3.1 CPU capacity
+----------------
+
+Linux cannot currently figure out CPU capacity on its own, this information thus
+needs to be handed to it. Architectures must define arch_scale_cpu_capacity()
+for that purpose.
+
+The arm and arm64 architectures directly map this to the arch_topology driver
+CPU scaling data, which is derived from the capacity-dmips-mhz CPU binding; see
+Documentation/devicetree/bindings/arm/cpu-capacity.txt.
+
+3.2 Frequency invariance
+------------------------
+
+As stated in 2.2, capacity-aware scheduling requires a frequency-invariant task
+utilization. Architectures must define arch_scale_freq_capacity(cpu) for that
+purpose.
+
+Implementing this function requires figuring out at which frequency each CPU
+have been running at. One way to implement this is to leverage hardware counters
+whose increment rate scale with a CPU's current frequency (APERF/MPERF on x86,
+AMU on arm64). Another is to directly hook into cpufreq frequency transitions,
+when the kernel is aware of the switched-to frequency (also employed by
+arm/arm64).
+
+4. Scheduler topology
+=====================
+
+During the construction of the sched domains, the scheduler will figure out
+whether the system exhibits asymmetric CPU capacities. Should that be the
+case:
+
+- The sched_asym_cpucapacity static key will be enabled.
+- The SD_ASYM_CPUCAPACITY flag will be set at the lowest sched_domain level that
+  spans all unique CPU capacity values.
+
+The sched_asym_cpucapacity static key is intended to guard sections of code that
+cater to asymmetric CPU capacity systems. Do note however that said key is
+*system-wide*. Imagine the following setup using cpusets::
+
+  capacity    C/2          C
+            ________    ________
+           /        \  /        \
+  CPUs     0  1  2  3  4  5  6  7
+           \__/  \______________/
+  cpusets   cs0         cs1
+
+Which could be created via:
+
+.. code-block:: sh
+
+  mkdir /sys/fs/cgroup/cpuset/cs0
+  echo 0-1 > /sys/fs/cgroup/cpuset/cs0/cpuset.cpus
+  echo 0 > /sys/fs/cgroup/cpuset/cs0/cpuset.mems
+
+  mkdir /sys/fs/cgroup/cpuset/cs1
+  echo 2-7 > /sys/fs/cgroup/cpuset/cs1/cpuset.cpus
+  echo 0 > /sys/fs/cgroup/cpuset/cs1/cpuset.mems
+
+  echo 0 > /sys/fs/cgroup/cpuset/cpuset.sched_load_balance
+
+Since there *is* CPU capacity asymmetry in the system, the
+sched_asym_cpucapacity static key will be enabled. However, the sched_domain
+hierarchy of CPUs 0-1 spans a single capacity value: SD_ASYM_CPUCAPACITY isn't
+set in that hierarchy, it describes an SMP island and should be treated as such.
+
+Therefore, the 'canonical' pattern for protecting codepaths that cater to
+asymmetric CPU capacities is to:
+
+- Check the sched_asym_cpucapacity static key
+- If it is enabled, then also check for the presence of SD_ASYM_CPUCAPACITY in
+  the sched_domain hierarchy (if relevant, i.e. the codepath targets a specific
+  CPU or group thereof)
+
+5. Capacity aware scheduling implementation
+===========================================
+
+5.1 CFS
+-------
+
+5.1.1 Capacity fitness
+~~~~~~~~~~~~~~~~~~~~~~
+
+The main capacity scheduling criterion of CFS is::
+
+  task_util(p) < capacity(task_cpu(p))
+
+This is commonly called the capacity fitness criterion, i.e. CFS must ensure a
+task "fits" on its CPU. If it is violated, the task will need to achieve more
+work than what its CPU can provide: it will be CPU-bound.
+
+Furthermore, uclamp lets userspace specify a minimum and a maximum utilization
+value for a task, either via sched_setattr() or via the cgroup interface (see
+Documentation/admin-guide/cgroup-v2.rst). As its name imply, this can be used to
+clamp task_util() in the previous criterion.
+
+5.1.2 Wakeup CPU selection
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+CFS task wakeup CPU selection follows the capacity fitness criterion described
+above. On top of that, uclamp is used to clamp the task utilization values,
+which lets userspace have more leverage over the CPU selection of CFS
+tasks. IOW, CFS wakeup CPU selection searches for a CPU that satisfies::
+
+  clamp(task_util(p), task_uclamp_min(p), task_uclamp_max(p)) < capacity(cpu)
+
+By using uclamp, userspace can e.g. allow a busy loop (100% utilization) to run
+on any CPU by giving it a low uclamp.max value. Conversely, it can force a small
+periodic task (e.g. 10% utilization) to run on the highest-performance CPUs by
+giving it a high uclamp.min value.
+
+.. note::
+
+  Wakeup CPU selection in CFS can be eclipsed by Energy Aware Scheduling
+  (EAS), which is described in Documentation/scheduling/sched-energy.rst.
+
+5.1.3 Load balancing
+~~~~~~~~~~~~~~~~~~~~
+
+A pathological case in the wakeup CPU selection occurs when a task rarely
+sleeps, if at all - it thus rarely wakes up, if at all. Consider::
+
+  w == wakeup event
+
+  capacity(CPU0) = C
+  capacity(CPU1) = C / 3
+
+                           workload on CPU0
+  CPU work ^
+           |     _________           _________           ____
+           |    |         |         |         |         |
+           +----+----+----+----+----+----+----+----+----+----+-> time
+                w                   w                   w
+
+                           workload on CPU1
+  CPU work ^
+           |     ____________________________________________
+           |    |
+           +----+----+----+----+----+----+----+----+----+----+->
+                w
+
+This workload should run on CPU0, but if the task either:
+
+- was improperly scheduled from the start (inaccurate initial
+  utilization estimation)
+- was properly scheduled from the start, but suddenly needs more
+  processing power
+
+then it might become CPU-bound, IOW ``task_util(p) > capacity(task_cpu(p))``;
+the CPU capacity scheduling criterion is violated, and there may not be any more
+wakeup event to fix this up via wakeup CPU selection.
+
+Tasks that are in this situation are dubbed "misfit" tasks, and the mechanism
+put in place to handle this shares the same name. Misfit task migration
+leverages the CFS load balancer, more specifically the active load balance part
+(which caters to migrating currently running tasks). When load balance happens,
+a misfit active load balance will be triggered if a misfit task can be migrated
+to a CPU with more capacity than its current one.
+
+5.2 RT
+------
+
+5.2.1 Wakeup CPU selection
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+RT task wakeup CPU selection searches for a CPU that satisfies::
+
+  task_uclamp_min(p) <= capacity(task_cpu(cpu))
+
+while still following the usual priority constraints. If none of the candidate
+CPUs can satisfy this capacity criterion, then strict priority based scheduling
+is followed and CPU capacities are ignored.
+
+5.3 DL
+------
+
+5.3.1 Wakeup CPU selection
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+DL task wakeup CPU selection searches for a CPU that satisfies::
+
+  task_bandwidth(p) < capacity(task_cpu(p))
+
+while still respecting the usual bandwidth and deadline constraints. If
+none of the candidate CPUs can satisfy this capacity criterion, then the
+task will remain on its current CPU.