Documentation/trace/osnoise-tracer.rst - linux - Git at Google

 ==============
 OSNOISE Tracer
 ==============

 In the context of high-performance computing (HPC), the Operating System
 Noise (*osnoise*) refers to the interference experienced by an application
 due to activities inside the operating system. In the context of Linux,
 NMIs, IRQs, SoftIRQs, and any other system thread can cause noise to the
 system. Moreover, hardware-related jobs can also cause noise, for example,
 via SMIs.

 hwlat_detector is one of the tools used to identify the most complex
 source of noise: *hardware noise*.

 In a nutshell, the hwlat_detector creates a thread that runs
 periodically for a given period. At the beginning of a period, the thread
 disables interrupt and starts sampling. While running, the hwlatd
 thread reads the time in a loop. As interrupts are disabled, threads,
 IRQs, and SoftIRQs cannot interfere with the hwlatd thread. Hence, the
 cause of any gap between two different reads of the time roots either on
 NMI or in the hardware itself. At the end of the period, hwlatd enables
 interrupts and reports the max observed gap between the reads. It also
 prints a NMI occurrence counter. If the output does not report NMI
 executions, the user can conclude that the hardware is the culprit for
 the latency. The hwlat detects the NMI execution by observing
 the entry and exit of a NMI.

 The osnoise tracer leverages the hwlat_detector by running a
 similar loop with preemption, SoftIRQs and IRQs enabled, thus allowing
 all the sources of *osnoise* during its execution. Using the same approach
 of hwlat, osnoise takes note of the entry and exit point of any
 source of interferences, increasing a per-cpu interference counter. The
 osnoise tracer also saves an interference counter for each source of
 interference. The interference counter for NMI, IRQs, SoftIRQs, and
 threads is increased anytime the tool observes these interferences' entry
 events. When a noise happens without any interference from the operating
 system level, the hardware noise counter increases, pointing to a
 hardware-related noise. In this way, osnoise can account for any
 source of interference. At the end of the period, the osnoise tracer
 prints the sum of all noise, the max single noise, the percentage of CPU
 available for the thread, and the counters for the noise sources.

 Usage
 -----

 Write the ASCII text "osnoise" into the current_tracer file of the
 tracing system (generally mounted at /sys/kernel/tracing).

 For example::

         [root@f32 ~]# cd /sys/kernel/tracing/
         [root@f32 tracing]# echo osnoise > current_tracer

 It is possible to follow the trace by reading the trace file::

         [root@f32 tracing]# cat trace
         # tracer: osnoise
         #
         #                                _-----=> irqs-off
         #                               / _----=> need-resched
         #                              | / _---=> hardirq/softirq
         #                              || / _--=> preempt-depth                            MAX
         #                              || /                                             SINGLE     Interference counters:
         #                              ||||               RUNTIME      NOISE   % OF CPU  NOISE    +-----------------------------+
         #           TASK-PID      CPU# ||||   TIMESTAMP    IN US       IN US  AVAILABLE  IN US     HW    NMI    IRQ   SIRQ THREAD
         #              | |         |   ||||      |           |             |    |            |      |      |      |      |      |
                    <...>-859     [000] ....    81.637220: 1000000        190  99.98100       9     18      0   1007     18      1
                    <...>-860     [001] ....    81.638154: 1000000        656  99.93440      74     23      0   1006     16      3
                    <...>-861     [002] ....    81.638193: 1000000       5675  99.43250     202      6      0   1013     25     21
                    <...>-862     [003] ....    81.638242: 1000000        125  99.98750      45      1      0   1011     23      0
                    <...>-863     [004] ....    81.638260: 1000000       1721  99.82790     168      7      0   1002     49     41
                    <...>-864     [005] ....    81.638286: 1000000        263  99.97370      57      6      0   1006     26      2
                    <...>-865     [006] ....    81.638302: 1000000        109  99.98910      21      3      0   1006     18      1
                    <...>-866     [007] ....    81.638326: 1000000       7816  99.21840     107      8      0   1016     39     19

 In addition to the regular trace fields (from TASK-PID to TIMESTAMP), the
 tracer prints a message at the end of each period for each CPU that is
 running an osnoise/ thread. The osnoise specific fields report:

  - The RUNTIME IN US reports the amount of time in microseconds that
    the osnoise thread kept looping reading the time.
  - The NOISE IN US reports the sum of noise in microseconds observed
    by the osnoise tracer during the associated runtime.
  - The % OF CPU AVAILABLE reports the percentage of CPU available for
    the osnoise thread during the runtime window.
  - The MAX SINGLE NOISE IN US reports the maximum single noise observed
    during the runtime window.
  - The Interference counters display how many each of the respective
    interference happened during the runtime window.

 Note that the example above shows a high number of HW noise samples.
 The reason being is that this sample was taken on a virtual machine,
 and the host interference is detected as a hardware interference.

 Tracer Configuration
 --------------------

 The tracer has a set of options inside the osnoise directory, they are:

  - osnoise/cpus: CPUs at which a osnoise thread will execute.
  - osnoise/period_us: the period of the osnoise thread.
  - osnoise/runtime_us: how long an osnoise thread will look for noise.
  - osnoise/stop_tracing_us: stop the system tracing if a single noise
    higher than the configured value happens. Writing 0 disables this
    option.
  - osnoise/stop_tracing_total_us: stop the system tracing if total noise
    higher than the configured value happens. Writing 0 disables this
    option.
  - tracing_threshold: the minimum delta between two time() reads to be
    considered as noise, in us. When set to 0, the default value will
    be used, which is currently 5 us.
  - osnoise/options: a set of on/off options that can be enabled by
    writing the option name to the file or disabled by writing the option
    name preceded with the 'NO\_' prefix. For example, writing
    NO_OSNOISE_WORKLOAD disables the OSNOISE_WORKLOAD option. The
    special DEAFAULTS option resets all options to the default value.

 Tracer Options
 --------------

 The osnoise/options file exposes a set of on/off configuration options for
 the osnoise tracer. These options are:

  - DEFAULTS: reset the options to the default value.
  - OSNOISE_WORKLOAD: do not dispatch osnoise workload (see dedicated
    section below).
  - PANIC_ON_STOP: call panic() if the tracer stops. This option serves to
    capture a vmcore.
  - OSNOISE_PREEMPT_DISABLE: disable preemption while running the osnoise
    workload, allowing only IRQ and hardware-related noise.
  - OSNOISE_IRQ_DISABLE: disable IRQs while running the osnoise workload,
    allowing only NMIs and hardware-related noise, like hwlat tracer.

 Additional Tracing
 ------------------

 In addition to the tracer, a set of tracepoints were added to
 facilitate the identification of the osnoise source.

  - osnoise:sample_threshold: printed anytime a noise is higher than
    the configurable tolerance_ns.
  - osnoise:nmi_noise: noise from NMI, including the duration.
  - osnoise:irq_noise: noise from an IRQ, including the duration.
  - osnoise:softirq_noise: noise from a SoftIRQ, including the
    duration.
  - osnoise:thread_noise: noise from a thread, including the duration.

 Note that all the values are *net values*. For example, if while osnoise
 is running, another thread preempts the osnoise thread, it will start a
 thread_noise duration at the start. Then, an IRQ takes place, preempting
 the thread_noise, starting a irq_noise. When the IRQ ends its execution,
 it will compute its duration, and this duration will be subtracted from
 the thread_noise, in such a way as to avoid the double accounting of the
 IRQ execution. This logic is valid for all sources of noise.

 Here is one example of the usage of these tracepoints::

        osnoise/8-961     [008] d.h.  5789.857532: irq_noise: local_timer:236 start 5789.857529929 duration 1845 ns
        osnoise/8-961     [008] dNh.  5789.858408: irq_noise: local_timer:236 start 5789.858404871 duration 2848 ns
      migration/8-54      [008] d...  5789.858413: thread_noise: migration/8:54 start 5789.858409300 duration 3068 ns
        osnoise/8-961     [008] ....  5789.858413: sample_threshold: start 5789.858404555 duration 8812 ns interferences 2

 In this example, a noise sample of 8 microseconds was reported in the last
 line, pointing to two interferences. Looking backward in the trace, the
 two previous entries were about the migration thread running after a
 timer IRQ execution. The first event is not part of the noise because
 it took place one millisecond before.

 It is worth noticing that the sum of the duration reported in the
 tracepoints is smaller than eight us reported in the sample_threshold.
 The reason roots in the overhead of the entry and exit code that happens
 before and after any interference execution. This justifies the dual
 approach: measuring thread and tracing.

 Running osnoise tracer without workload
 ---------------------------------------

 By enabling the osnoise tracer with the NO_OSNOISE_WORKLOAD option set,
 the osnoise: tracepoints serve to measure the execution time of
 any type of Linux task, free from the interference of other tasks.
	==============
	OSNOISE Tracer
	==============

	In the context of high-performance computing (HPC), the Operating System
	Noise (osnoise) refers to the interference experienced by an application
	due to activities inside the operating system. In the context of Linux,
	NMIs, IRQs, SoftIRQs, and any other system thread can cause noise to the
	system. Moreover, hardware-related jobs can also cause noise, for example,
	via SMIs.

	hwlat_detector is one of the tools used to identify the most complex
	source of noise: hardware noise.

	In a nutshell, the hwlat_detector creates a thread that runs
	periodically for a given period. At the beginning of a period, the thread
	disables interrupt and starts sampling. While running, the hwlatd
	thread reads the time in a loop. As interrupts are disabled, threads,
	IRQs, and SoftIRQs cannot interfere with the hwlatd thread. Hence, the
	cause of any gap between two different reads of the time roots either on
	NMI or in the hardware itself. At the end of the period, hwlatd enables
	interrupts and reports the max observed gap between the reads. It also
	prints a NMI occurrence counter. If the output does not report NMI
	executions, the user can conclude that the hardware is the culprit for
	the latency. The hwlat detects the NMI execution by observing
	the entry and exit of a NMI.

	The osnoise tracer leverages the hwlat_detector by running a
	similar loop with preemption, SoftIRQs and IRQs enabled, thus allowing
	all the sources of osnoise during its execution. Using the same approach
	of hwlat, osnoise takes note of the entry and exit point of any
	source of interferences, increasing a per-cpu interference counter. The
	osnoise tracer also saves an interference counter for each source of
	interference. The interference counter for NMI, IRQs, SoftIRQs, and
	threads is increased anytime the tool observes these interferences' entry
	events. When a noise happens without any interference from the operating
	system level, the hardware noise counter increases, pointing to a
	hardware-related noise. In this way, osnoise can account for any
	source of interference. At the end of the period, the osnoise tracer
	prints the sum of all noise, the max single noise, the percentage of CPU
	available for the thread, and the counters for the noise sources.

	Usage
	-----

	Write the ASCII text "osnoise" into the current_tracer file of the
	tracing system (generally mounted at /sys/kernel/tracing).

	For example::

	[root@f32 ~]# cd /sys/kernel/tracing/
	[root@f32 tracing]# echo osnoise > current_tracer

	It is possible to follow the trace by reading the trace file::

	[root@f32 tracing]# cat trace
	# tracer: osnoise
	#
	# _-----=> irqs-off
	# / _----=> need-resched
	# \| / _---=> hardirq/softirq
	# \|\| / _--=> preempt-depth MAX
	# \|\| / SINGLE Interference counters:
	# \|\|\|\| RUNTIME NOISE % OF CPU NOISE +-----------------------------+
	# TASK-PID CPU# \|\|\|\| TIMESTAMP IN US IN US AVAILABLE IN US HW NMI IRQ SIRQ THREAD
	# \| \| \| \|\|\|\| \| \| \| \| \| \| \| \| \| \|
	<...>-859 [000] .... 81.637220: 1000000 190 99.98100 9 18 0 1007 18 1
	<...>-860 [001] .... 81.638154: 1000000 656 99.93440 74 23 0 1006 16 3
	<...>-861 [002] .... 81.638193: 1000000 5675 99.43250 202 6 0 1013 25 21
	<...>-862 [003] .... 81.638242: 1000000 125 99.98750 45 1 0 1011 23 0
	<...>-863 [004] .... 81.638260: 1000000 1721 99.82790 168 7 0 1002 49 41
	<...>-864 [005] .... 81.638286: 1000000 263 99.97370 57 6 0 1006 26 2
	<...>-865 [006] .... 81.638302: 1000000 109 99.98910 21 3 0 1006 18 1
	<...>-866 [007] .... 81.638326: 1000000 7816 99.21840 107 8 0 1016 39 19

	In addition to the regular trace fields (from TASK-PID to TIMESTAMP), the
	tracer prints a message at the end of each period for each CPU that is
	running an osnoise/ thread. The osnoise specific fields report:

	- The RUNTIME IN US reports the amount of time in microseconds that
	the osnoise thread kept looping reading the time.
	- The NOISE IN US reports the sum of noise in microseconds observed
	by the osnoise tracer during the associated runtime.
	- The % OF CPU AVAILABLE reports the percentage of CPU available for
	the osnoise thread during the runtime window.
	- The MAX SINGLE NOISE IN US reports the maximum single noise observed
	during the runtime window.
	- The Interference counters display how many each of the respective
	interference happened during the runtime window.

	Note that the example above shows a high number of HW noise samples.
	The reason being is that this sample was taken on a virtual machine,
	and the host interference is detected as a hardware interference.

	Tracer Configuration
	--------------------

	The tracer has a set of options inside the osnoise directory, they are:

	- osnoise/cpus: CPUs at which a osnoise thread will execute.
	- osnoise/period_us: the period of the osnoise thread.
	- osnoise/runtime_us: how long an osnoise thread will look for noise.
	- osnoise/stop_tracing_us: stop the system tracing if a single noise
	higher than the configured value happens. Writing 0 disables this
	option.
	- osnoise/stop_tracing_total_us: stop the system tracing if total noise
	higher than the configured value happens. Writing 0 disables this
	option.
	- tracing_threshold: the minimum delta between two time() reads to be
	considered as noise, in us. When set to 0, the default value will
	be used, which is currently 5 us.
	- osnoise/options: a set of on/off options that can be enabled by
	writing the option name to the file or disabled by writing the option
	name preceded with the 'NO\_' prefix. For example, writing
	NO_OSNOISE_WORKLOAD disables the OSNOISE_WORKLOAD option. The
	special DEAFAULTS option resets all options to the default value.

	Tracer Options
	--------------

	The osnoise/options file exposes a set of on/off configuration options for
	the osnoise tracer. These options are:

	- DEFAULTS: reset the options to the default value.
	- OSNOISE_WORKLOAD: do not dispatch osnoise workload (see dedicated
	section below).
	- PANIC_ON_STOP: call panic() if the tracer stops. This option serves to
	capture a vmcore.
	- OSNOISE_PREEMPT_DISABLE: disable preemption while running the osnoise
	workload, allowing only IRQ and hardware-related noise.
	- OSNOISE_IRQ_DISABLE: disable IRQs while running the osnoise workload,
	allowing only NMIs and hardware-related noise, like hwlat tracer.

	Additional Tracing
	------------------

	In addition to the tracer, a set of tracepoints were added to
	facilitate the identification of the osnoise source.

	- osnoise:sample_threshold: printed anytime a noise is higher than
	the configurable tolerance_ns.
	- osnoise:nmi_noise: noise from NMI, including the duration.
	- osnoise:irq_noise: noise from an IRQ, including the duration.
	- osnoise:softirq_noise: noise from a SoftIRQ, including the
	duration.
	- osnoise:thread_noise: noise from a thread, including the duration.

	Note that all the values are net values. For example, if while osnoise
	is running, another thread preempts the osnoise thread, it will start a
	thread_noise duration at the start. Then, an IRQ takes place, preempting
	the thread_noise, starting a irq_noise. When the IRQ ends its execution,
	it will compute its duration, and this duration will be subtracted from
	the thread_noise, in such a way as to avoid the double accounting of the
	IRQ execution. This logic is valid for all sources of noise.

	Here is one example of the usage of these tracepoints::

	osnoise/8-961 [008] d.h. 5789.857532: irq_noise: local_timer:236 start 5789.857529929 duration 1845 ns
	osnoise/8-961 [008] dNh. 5789.858408: irq_noise: local_timer:236 start 5789.858404871 duration 2848 ns
	migration/8-54 [008] d... 5789.858413: thread_noise: migration/8:54 start 5789.858409300 duration 3068 ns
	osnoise/8-961 [008] .... 5789.858413: sample_threshold: start 5789.858404555 duration 8812 ns interferences 2

	In this example, a noise sample of 8 microseconds was reported in the last
	line, pointing to two interferences. Looking backward in the trace, the
	two previous entries were about the migration thread running after a
	timer IRQ execution. The first event is not part of the noise because
	it took place one millisecond before.

	It is worth noticing that the sum of the duration reported in the
	tracepoints is smaller than eight us reported in the sample_threshold.
	The reason roots in the overhead of the entry and exit code that happens
	before and after any interference execution. This justifies the dual
	approach: measuring thread and tracing.

	Running osnoise tracer without workload
	---------------------------------------

	By enabling the osnoise tracer with the NO_OSNOISE_WORKLOAD option set,
	the osnoise: tracepoints serve to measure the execution time of
	any type of Linux task, free from the interference of other tasks.