Documentation/scheduler/schedutil.rst

Source file repositories/reference/linux-study-clean/Documentation/scheduler/schedutil.rst

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Documentation/scheduler/schedutil.rst
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Support Tooling And Documentation
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Support Tooling And Documentation: documentation
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=========
Schedutil
=========

.. note::

   All this assumes a linear relation between frequency and work capacity,
   we know this is flawed, but it is the best workable approximation.


PELT (Per Entity Load Tracking)
===============================

With PELT we track some metrics across the various scheduler entities, from
individual tasks to task-group slices to CPU runqueues. As the basis for this
we use an Exponentially Weighted Moving Average (EWMA), each period (1024us)
is decayed such that y^32 = 0.5. That is, the most recent 32ms contribute
half, while the rest of history contribute the other half.

Specifically:

  ewma_sum(u) := u_0 + u_1*y + u_2*y^2 + ...

  ewma(u) = ewma_sum(u) / ewma_sum(1)

Since this is essentially a progression of an infinite geometric series, the
results are composable, that is ewma(A) + ewma(B) = ewma(A+B). This property
is key, since it gives the ability to recompose the averages when tasks move
around.

Note that blocked tasks still contribute to the aggregates (task-group slices
and CPU runqueues), which reflects their expected contribution when they
resume running.

Using this we track 2 key metrics: 'running' and 'runnable'. 'Running'
reflects the time an entity spends on the CPU, while 'runnable' reflects the
time an entity spends on the runqueue. When there is only a single task these
two metrics are the same, but once there is contention for the CPU 'running'
will decrease to reflect the fraction of time each task spends on the CPU
while 'runnable' will increase to reflect the amount of contention.

For more detail see: kernel/sched/pelt.c


Frequency / CPU Invariance
==========================

Because consuming the CPU for 50% at 1GHz is not the same as consuming the CPU
for 50% at 2GHz, nor is running 50% on a LITTLE CPU the same as running 50% on
a big CPU, we allow architectures to scale the time delta with two ratios, one
Dynamic Voltage and Frequency Scaling (DVFS) ratio and one microarch ratio.

For simple DVFS architectures (where software is in full control) we trivially
compute the ratio as::

	    f_cur
  r_dvfs := -----
            f_max

For more dynamic systems where the hardware is in control of DVFS we use
hardware counters (Intel APERF/MPERF, ARMv8.4-AMU) to provide us this ratio.
For Intel specifically, we use::

	   APERF
  f_cur := ----- * P0
	   MPERF

	     4C-turbo;	if available and turbo enabled
  f_max := { 1C-turbo;	if turbo enabled
	     P0;	otherwise

Annotation

Implementation Notes