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diff --git a/Documentation/trace/osnoise-tracer.rst b/Documentation/trace/osnoise-tracer.rst new file mode 100644 index 000000000000..963def9f97c6 --- /dev/null +++ b/Documentation/trace/osnoise-tracer.rst @@ -0,0 +1,152 @@ +============== +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 options +--------------------- + +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. + +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. |