tracing: Add documentation for hwlat_detector tracer

Added the documentation on how to use th hwlat_detector.
Signed-off-by: Jon Masters <jcm@redhat.com>
[ Various updates and modified to show hwlat as a tracer ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Documentation/trace/hwlat_detector.txt

+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)