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16 CPU capacity is a measure of the performance a CPU can reach, normalized against
28 performance-oriented than the LITTLE ones (more pipeline stages, bigger caches,
32 CPU performance is usually expressed in Millions of Instructions Per Second
42 ``capacity_orig`` is its maximum attainable capacity, i.e. its maximum
43 attainable performance level. A CPU's ``capacity`` is its ``capacity_orig`` to
44 which some loss of available performance (e.g. time spent handling IRQs) is
47 Note that a CPU's ``capacity`` is solely intended to be used by the CFS class,
48 while ``capacity_orig`` is class-agnostic. The rest of this document will use
121 performance is somewhat of a contentious point. The relative performance
134 while task utilization is specific to CFS, it is convenient to describe it here
137 Task utilization is a percentage meant to represent the throughput requirements
138 of a task. A simple approximation of it is the task's duty cycle, i.e.::
142 On an SMP system with fixed frequencies, 100% utilization suggests the task is a
143 busy loop. Conversely, 10% utilization hints it is a small periodic task that
151 One issue that needs to be taken into account is that a workload's duty cycle is
152 directly impacted by the current OPP the CPU is running at. Consider running a
224 utilization that is both CPU and frequency invariant is thus, for a given
262 CPU scaling data, which is derived from the capacity-dmips-mhz CPU binding; see
273 have been running at. One way to implement this is to leverage hardware counters
275 AMU on arm64). Another is to directly hook into cpufreq frequency transitions,
276 when the kernel is aware of the switched-to frequency (also employed by
292 The sched_asym_cpucapacity static key is intended to guard sections of code that
293 cater to asymmetric CPU capacity systems. Do note however that said key is
317 Since there *is* CPU capacity asymmetry in the system, the
323 asymmetric CPU capacities is to:
326 - If it is enabled, then also check for the presence of SD_ASYM_CPUCAPACITY in
339 The main capacity scheduling criterion of CFS is::
343 This is commonly called the capacity fitness criterion, i.e. CFS must ensure a
344 task "fits" on its CPU. If it is violated, the task will need to achieve more
356 above. On top of that, uclamp is used to clamp the task utilization values,
370   (EAS), which is described in Documentation/scheduler/sched-energy.rst.
399 - was improperly scheduled from the start (inaccurate initial
401 - was properly scheduled from the start, but suddenly needs more
405 the CPU capacity scheduling criterion is violated, and there may not be any more
427 is followed and CPU capacities are ignored.