From: Jon Masters <jcm@xxxxxxxxxx> Added the documentation on how to use th hwlat_detector. Signed-off-by: Jon Masters <jcm@xxxxxxxxxx> [ Various updates and modified to show hwlat as a tracer ] Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> --- Documentation/trace/hwlat_detector.txt | 73 ++++++++++++++++++++++++++++++++++ 1 file changed, 73 insertions(+) create mode 100644 Documentation/trace/hwlat_detector.txt diff --git a/Documentation/trace/hwlat_detector.txt b/Documentation/trace/hwlat_detector.txt new file mode 100644 index 000000000000..c02e8ef800cf --- /dev/null +++ b/Documentation/trace/hwlat_detector.txt @@ -0,0 +1,73 @@ +Introduction: +------------- + +The tracer hwlat_detector is a special purpose tracer that is used to +detect large system latencies induced by the behavior of certain underlying +hardware or firmware, independent of Linux itself. The code was developed +originally to detect SMIs (System Management Interrupts) on x86 systems, +however there is nothing x86 specific about this patchset. It was +originally written for use by the "RT" patch since the Real Time +kernel is highly latency sensitive. + +SMIs are not serviced by the Linux kernel, which means that it does not +even know that they are occuring. SMIs are instead set up by BIOS code +and are serviced by BIOS code, usually for "critical" events such as +management of thermal sensors and fans. Sometimes though, SMIs are used for +other tasks and those tasks can spend an inordinate amount of time in the +handler (sometimes measured in milliseconds). Obviously this is a problem if +you are trying to keep event service latencies down in the microsecond range. + +The hardware latency detector works by hogging one of the cpus for configurable +amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter +for some period, then looking for gaps in the TSC data. Any gap indicates a +time when the polling was interrupted and since the interrupts are disabled, +the only thing that could do that would be an SMI or other hardware hiccup +(or an NMI, but those can be tracked). + +Note that the hwlat detector should *NEVER* be used in a production environment. +It is intended to be run manually to determine if the hardware platform has a +problem with long system firmware service routines. + +Usage: +------ + +Write the ASCII text "hwlat" into the current_tracer file of the tracing system +(mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to +redefine the threshold in microseconds (us) above which latency spikes will +be taken into account. + +Example: + + # echo hwlat > /sys/kernel/tracing/current_tracer + # echo 100 > /sys/kernel/tracing/tracing_thresh + +The /sys/kernel/tracing/hwlat_detector interface contains the following files: + +width - time period to sample with CPUs held (usecs) + must be less than the total window size (enforced) +window - total period of sampling, width being inside (usecs) + +By default the width is set to 500,000 and window to 1,000,000, meaning that +for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs +(0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will +change to a default of 10 usecs. If any latencies that exceed the threshold is +observed then the data will be written to the tracing ring buffer. + +The minimum sleep time between periods is 1 millisecond. Even if width +is less than 1 millisecond apart from window, to allow the system to not +be totally starved. + +If tracing_thresh was zero when hwlat detector was started, it will be set +back to zero if another tracer is loaded. Note, the last value in +tracing_thresh that hwlat detector had will be saved and this value will +be restored in tracing_thresh if it is still zero when hwlat detector is +started again. + +The following tracing directory files are used by the hwlat_detector: + +in /sys/kernel/tracing: + + tracing_threshold - minimum latency value to be considered (usecs) + tracing_max_latency - maximum hardware latency actually observed (usecs) + hwlat_detector/width - specified amount of time to spin within window (usecs) + hwlat_detector/window - amount of time between (width) runs (usecs) -- 2.8.1 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
