tracing: Add documentation for hwlat_detector tracer

Introduction:

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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:

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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)