74 lines
3.4 KiB
Plaintext
74 lines
3.4 KiB
Plaintext
|
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)
|