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=========================
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Hardware Latency Detector
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=========================
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Introduction
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-------------
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The tracer hwlat_detector is a special purpose tracer that is used to
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detect large system latencies induced by the behavior of certain underlying
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hardware or firmware, independent of Linux itself. The code was developed
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originally to detect SMIs (System Management Interrupts) on x86 systems,
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however there is nothing x86 specific about this patchset. It was
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originally written for use by the "RT" patch since the Real Time
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kernel is highly latency sensitive.
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SMIs are not serviced by the Linux kernel, which means that it does not
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even know that they are occuring. SMIs are instead set up by BIOS code
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and are serviced by BIOS code, usually for "critical" events such as
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management of thermal sensors and fans. Sometimes though, SMIs are used for
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other tasks and those tasks can spend an inordinate amount of time in the
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handler (sometimes measured in milliseconds). Obviously this is a problem if
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you are trying to keep event service latencies down in the microsecond range.
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The hardware latency detector works by hogging one of the cpus for configurable
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amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter
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for some period, then looking for gaps in the TSC data. Any gap indicates a
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time when the polling was interrupted and since the interrupts are disabled,
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the only thing that could do that would be an SMI or other hardware hiccup
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(or an NMI, but those can be tracked).
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Note that the hwlat detector should *NEVER* be used in a production environment.
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It is intended to be run manually to determine if the hardware platform has a
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problem with long system firmware service routines.
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Usage
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------
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Write the ASCII text "hwlat" into the current_tracer file of the tracing system
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(mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to
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redefine the threshold in microseconds (us) above which latency spikes will
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be taken into account.
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Example::
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# echo hwlat > /sys/kernel/tracing/current_tracer
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# echo 100 > /sys/kernel/tracing/tracing_thresh
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The /sys/kernel/tracing/hwlat_detector interface contains the following files:
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- width - time period to sample with CPUs held (usecs)
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must be less than the total window size (enforced)
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- window - total period of sampling, width being inside (usecs)
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By default the width is set to 500,000 and window to 1,000,000, meaning that
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for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs
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(0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will
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change to a default of 10 usecs. If any latencies that exceed the threshold is
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observed then the data will be written to the tracing ring buffer.
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The minimum sleep time between periods is 1 millisecond. Even if width
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is less than 1 millisecond apart from window, to allow the system to not
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be totally starved.
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If tracing_thresh was zero when hwlat detector was started, it will be set
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back to zero if another tracer is loaded. Note, the last value in
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tracing_thresh that hwlat detector had will be saved and this value will
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be restored in tracing_thresh if it is still zero when hwlat detector is
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started again.
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The following tracing directory files are used by the hwlat_detector:
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in /sys/kernel/tracing:
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- tracing_threshold - minimum latency value to be considered (usecs)
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- tracing_max_latency - maximum hardware latency actually observed (usecs)
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- tracing_cpumask - the CPUs to move the hwlat thread across
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- hwlat_detector/width - specified amount of time to spin within window (usecs)
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- hwlat_detector/window - amount of time between (width) runs (usecs)
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The hwlat detector's kernel thread will migrate across each CPU specified in
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tracing_cpumask between each window. To limit the migration, either modify
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tracing_cpumask, or modify the hwlat kernel thread (named [hwlatd]) CPU
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affinity directly, and the migration will stop.
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