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* This completes task list in https://github.com/apache/nuttx/issues/11127. * This preserves selected content from cwiki and moves it to new docs. * Most pages are simple copy-paste with a simple RST formatting updates, with minor updates. * Content update / reorganization will follow later on when needed. * Files added (or updated title from cwiki -> current docs): * Documentation/implementation: * index. * cancellation_points. * Asynchronous vs. Synchronous Context Switches -> context_switches.rst. * ARMv7-M Hardfaults, SVCALL, and Debuggers -> hardfatuls.rst. * chip.h FAQ -> chip_h.rst. * Debug Output (SYSLOG) Issues -> syslog.rst. * Detaching File Descriptors -> file_descriptors.rst. * device_nodes.rst. * Dynamic Clocking -> power_management.rst. * ENOTTY ioctl() Return Value -> ioctl.rst. * memory_configurations.rst. * kernel_modules_vs_shared_libraries.rst. * NAKing USB OUT/IN Tokens -> usb.rst. * naming_arch_mcu_board_interfaces.rst. * naming_os_internals.rst. * nuttx_tasking.rst. * oneshot_timers_and_cpu_load.rst. * nuttx_initialization_sequence.rst. * short_time_delays.rst. * Signal Handler Tour -> signal_handlers.rst. * smp.rst. * syslog.rst. * Task Exit Sequence -> nuttx_tasking.rst. * tasks_vs_threads.rst. * tls.rst. * tickless_os.rst. * Why Can't Kernel Threads Have pthreads -> kernel_threads_vs_pthreads.rst. * Documentation/components/filesystem: * smartfs.rst. Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>
126 lines
4 KiB
ReStructuredText
126 lines
4 KiB
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=======================================
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Oneshot Timers and CPU Load Measurement
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=======================================
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Issues with CPU Load Measurement
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================================
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There is been support for CPU load measurement for a long time.
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Enabled with:
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.. code-block:: sh
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CONFIG_SCHED_CPULOAD=y
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CONFIG_SCHED_CPULOAD_TIMECONSTANT=2
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The main problem with the existing logic is that it ran synchronously
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with the system timer. That turns out to be useless for measurement of the
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performance of some tasks.
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Many tasks operate synchronously with the timer interrupt, i.e., timed event
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is the stimulus for task execution.
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Those tasks are always sampled at essentially the same point in the execution
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profile leading to nonsense results.
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In order to get believable results in all cases you would need to:
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* Sample at times are completely random with respect to program behavior, AND
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* Sample at a high rate or for a very long time.
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External Clock
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==============
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In order to work around these things, an option to use an external clock
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was added:
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.. code-block:: sh
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CONFIG_SCHED_CPULOAD_EXTCLK=y
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CONFIG_SCHED_CPULOAD_TICKSPERSEC=77
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This should work well if ``CONFIG_SCHED_CPULOAD_TICKSPERSEC`` is prime
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and less than ``CONFIG_USEC_PER_TICK``.
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However, you were completely on your own one how to implement this
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external clock. But no longer.
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Using a Oneshot Timer to Drive CPU Load Measurement
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===================================================
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Recently, a generic, platform-independent one-shot lower half interface
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was developed. That interface is described in
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``include/nuttx/timers/oneshot.h``. There are implementations of the one shot
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timer lower half available for STM32, STM32L4, SAM4CM, SAMA5D3/4,
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and SAMV71/SAME70.
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And now there is also an OS component that can be initialized and used to
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drive the CPU load sampling from a precision, high rate oneshot timer.
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The implementation of that feature resides at
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``sched/sched/sched_cpuload_oneshot.c`` and is enabled with:
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.. code-block:: sh
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CONFIG_ONESHOT=y
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CONFIG_CPULOAD_ONESHOT=y
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Perhaps with additional configuration options that are likely required
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to enable board-specific oneshot timer lower-half support.
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The oneshot timer must still be configured by board specific logic which
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must call:
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.. code-block:: c
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void sched_oneshot_extclk(FAR struct oneshot_lowerhalf_s *lower);
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To start the CPU load measurement.
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``sched_oneshot_extclk()`` is prototyped in ``include/nuttx/clock.h``.
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There is some example setup code in the NuttX simulation code at
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``boards/sim/sim/sim/src/sim_bringup.c``.
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Entropy
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=======
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Another recent addition to NuttX came from David Alessio.
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David contributed support for ``/dev/urandom`` with a built-in XorShift128
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pseduo-random number generator (PRNG).
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We have detached the XorShift128 implementation from the ``/dev/urandom``
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implementation and moved it to ``/libc/misc`` so that is it available
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for other purposes.
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In particular, there is an option to use the XorShift128 PRNG to add entropy
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to the CPU load measurement of the oneshot timer.
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That feature is enabled with:
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.. code-block:: sh
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CONFIG_CPULOAD_ONESHOT_ENTROPY=n
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for ``n=1..30`` and disabled with ``CONFIG_CPULOAD_ONESHOT_ENTROPY=0``.
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This value represents the number of bits of entropy that will be added
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to the oneshot interval delays.
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The oneshot timer will be set to the following interval each time
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the oneshot timer is restarted is ``CONFIG_CPULOAD_ONESHOT_ENTROPY``:
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.. code-block:: sh
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CPULOAD_ONESHOT_NOMINAL \
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- (CPULOAD_ONESHOT_ENTROPY / 2) \
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+ error \
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+ nrand(CPULOAD_ONESHOT_ENTROPY)
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Where:
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* ``CPULOAD_ONESHOT_NOMINAL`` is ``CONFIG_SCHED_CPULOAD_TICKSPERSEC``
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in units of microseconds.
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* ``CPULOAD_ONESHOT_ENTROPY`` is ``(1 << CONFIG_CPULOAD_ONESHOT_ENTROPY)``.
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* ``error`` is an error value that is retained from interval to interval
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so that although individual intervals are randomized,
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the average will still be equal to ``CONFIG_SCHED_CPULOAD_TICKSPERSEC``.
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If ``CONFIG_CPULOAD_ONESHOT_ENTROPY=0``, then the interval delay will
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always be equal to ``CPULOAD_ONESHOT_NOMINAL``.
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