181 lines
9.6 KiB
ReStructuredText
181 lines
9.6 KiB
ReStructuredText
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OSNOISE Tracer
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==============
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In the context of high-performance computing (HPC), the Operating System
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Noise (*osnoise*) refers to the interference experienced by an application
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due to activities inside the operating system. In the context of Linux,
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NMIs, IRQs, SoftIRQs, and any other system thread can cause noise to the
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system. Moreover, hardware-related jobs can also cause noise, for example,
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via SMIs.
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hwlat_detector is one of the tools used to identify the most complex
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source of noise: *hardware noise*.
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In a nutshell, the hwlat_detector creates a thread that runs
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periodically for a given period. At the beginning of a period, the thread
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disables interrupt and starts sampling. While running, the hwlatd
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thread reads the time in a loop. As interrupts are disabled, threads,
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IRQs, and SoftIRQs cannot interfere with the hwlatd thread. Hence, the
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cause of any gap between two different reads of the time roots either on
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NMI or in the hardware itself. At the end of the period, hwlatd enables
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interrupts and reports the max observed gap between the reads. It also
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prints a NMI occurrence counter. If the output does not report NMI
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executions, the user can conclude that the hardware is the culprit for
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the latency. The hwlat detects the NMI execution by observing
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the entry and exit of a NMI.
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The osnoise tracer leverages the hwlat_detector by running a
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similar loop with preemption, SoftIRQs and IRQs enabled, thus allowing
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all the sources of *osnoise* during its execution. Using the same approach
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of hwlat, osnoise takes note of the entry and exit point of any
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source of interferences, increasing a per-cpu interference counter. The
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osnoise tracer also saves an interference counter for each source of
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interference. The interference counter for NMI, IRQs, SoftIRQs, and
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threads is increased anytime the tool observes these interferences' entry
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events. When a noise happens without any interference from the operating
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system level, the hardware noise counter increases, pointing to a
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hardware-related noise. In this way, osnoise can account for any
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source of interference. At the end of the period, the osnoise tracer
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prints the sum of all noise, the max single noise, the percentage of CPU
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available for the thread, and the counters for the noise sources.
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Usage
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-----
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Write the ASCII text "osnoise" into the current_tracer file of the
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tracing system (generally mounted at /sys/kernel/tracing).
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For example::
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[root@f32 ~]# cd /sys/kernel/tracing/
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[root@f32 tracing]# echo osnoise > current_tracer
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It is possible to follow the trace by reading the trace file::
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[root@f32 tracing]# cat trace
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# tracer: osnoise
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#
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# _-----=> irqs-off
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# / _----=> need-resched
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# | / _---=> hardirq/softirq
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# || / _--=> preempt-depth MAX
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# || / SINGLE Interference counters:
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# |||| RUNTIME NOISE % OF CPU NOISE +-----------------------------+
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# TASK-PID CPU# |||| TIMESTAMP IN US IN US AVAILABLE IN US HW NMI IRQ SIRQ THREAD
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# | | | |||| | | | | | | | | | |
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<...>-859 [000] .... 81.637220: 1000000 190 99.98100 9 18 0 1007 18 1
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<...>-860 [001] .... 81.638154: 1000000 656 99.93440 74 23 0 1006 16 3
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<...>-861 [002] .... 81.638193: 1000000 5675 99.43250 202 6 0 1013 25 21
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<...>-862 [003] .... 81.638242: 1000000 125 99.98750 45 1 0 1011 23 0
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<...>-863 [004] .... 81.638260: 1000000 1721 99.82790 168 7 0 1002 49 41
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<...>-864 [005] .... 81.638286: 1000000 263 99.97370 57 6 0 1006 26 2
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<...>-865 [006] .... 81.638302: 1000000 109 99.98910 21 3 0 1006 18 1
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<...>-866 [007] .... 81.638326: 1000000 7816 99.21840 107 8 0 1016 39 19
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In addition to the regular trace fields (from TASK-PID to TIMESTAMP), the
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tracer prints a message at the end of each period for each CPU that is
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running an osnoise/ thread. The osnoise specific fields report:
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- The RUNTIME IN US reports the amount of time in microseconds that
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the osnoise thread kept looping reading the time.
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- The NOISE IN US reports the sum of noise in microseconds observed
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by the osnoise tracer during the associated runtime.
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- The % OF CPU AVAILABLE reports the percentage of CPU available for
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the osnoise thread during the runtime window.
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- The MAX SINGLE NOISE IN US reports the maximum single noise observed
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during the runtime window.
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- The Interference counters display how many each of the respective
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interference happened during the runtime window.
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Note that the example above shows a high number of HW noise samples.
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The reason being is that this sample was taken on a virtual machine,
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and the host interference is detected as a hardware interference.
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Tracer Configuration
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--------------------
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The tracer has a set of options inside the osnoise directory, they are:
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- osnoise/cpus: CPUs at which a osnoise thread will execute.
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- osnoise/period_us: the period of the osnoise thread.
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- osnoise/runtime_us: how long an osnoise thread will look for noise.
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- osnoise/stop_tracing_us: stop the system tracing if a single noise
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higher than the configured value happens. Writing 0 disables this
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option.
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- osnoise/stop_tracing_total_us: stop the system tracing if total noise
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higher than the configured value happens. Writing 0 disables this
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option.
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- tracing_threshold: the minimum delta between two time() reads to be
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considered as noise, in us. When set to 0, the default value will
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be used, which is currently 5 us.
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- osnoise/options: a set of on/off options that can be enabled by
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writing the option name to the file or disabled by writing the option
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name preceded with the 'NO\_' prefix. For example, writing
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NO_OSNOISE_WORKLOAD disables the OSNOISE_WORKLOAD option. The
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special DEAFAULTS option resets all options to the default value.
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Tracer Options
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--------------
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The osnoise/options file exposes a set of on/off configuration options for
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the osnoise tracer. These options are:
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- DEFAULTS: reset the options to the default value.
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- OSNOISE_WORKLOAD: do not dispatch osnoise workload (see dedicated
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section below).
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- PANIC_ON_STOP: call panic() if the tracer stops. This option serves to
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capture a vmcore.
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- OSNOISE_PREEMPT_DISABLE: disable preemption while running the osnoise
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workload, allowing only IRQ and hardware-related noise.
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- OSNOISE_IRQ_DISABLE: disable IRQs while running the osnoise workload,
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allowing only NMIs and hardware-related noise, like hwlat tracer.
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Additional Tracing
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------------------
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In addition to the tracer, a set of tracepoints were added to
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facilitate the identification of the osnoise source.
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- osnoise:sample_threshold: printed anytime a noise is higher than
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the configurable tolerance_ns.
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- osnoise:nmi_noise: noise from NMI, including the duration.
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- osnoise:irq_noise: noise from an IRQ, including the duration.
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- osnoise:softirq_noise: noise from a SoftIRQ, including the
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duration.
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- osnoise:thread_noise: noise from a thread, including the duration.
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Note that all the values are *net values*. For example, if while osnoise
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is running, another thread preempts the osnoise thread, it will start a
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thread_noise duration at the start. Then, an IRQ takes place, preempting
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the thread_noise, starting a irq_noise. When the IRQ ends its execution,
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it will compute its duration, and this duration will be subtracted from
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the thread_noise, in such a way as to avoid the double accounting of the
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IRQ execution. This logic is valid for all sources of noise.
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Here is one example of the usage of these tracepoints::
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osnoise/8-961 [008] d.h. 5789.857532: irq_noise: local_timer:236 start 5789.857529929 duration 1845 ns
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osnoise/8-961 [008] dNh. 5789.858408: irq_noise: local_timer:236 start 5789.858404871 duration 2848 ns
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migration/8-54 [008] d... 5789.858413: thread_noise: migration/8:54 start 5789.858409300 duration 3068 ns
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osnoise/8-961 [008] .... 5789.858413: sample_threshold: start 5789.858404555 duration 8812 ns interferences 2
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In this example, a noise sample of 8 microseconds was reported in the last
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line, pointing to two interferences. Looking backward in the trace, the
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two previous entries were about the migration thread running after a
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timer IRQ execution. The first event is not part of the noise because
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it took place one millisecond before.
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It is worth noticing that the sum of the duration reported in the
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tracepoints is smaller than eight us reported in the sample_threshold.
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The reason roots in the overhead of the entry and exit code that happens
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before and after any interference execution. This justifies the dual
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approach: measuring thread and tracing.
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Running osnoise tracer without workload
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---------------------------------------
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By enabling the osnoise tracer with the NO_OSNOISE_WORKLOAD option set,
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the osnoise: tracepoints serve to measure the execution time of
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any type of Linux task, free from the interference of other tasks.
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