KTR(4) BSD Kernel Interfaces Manual KTR(4)
ktr -- kernel tracing facility
The ktr facility allows kernel events to be logged while the kernel executes so that they can be examined later when debugging. The only
mandatory option to enable ktr is ``options KTR''.
The KTR_ENTRIES option sets the size of the buffer of events. The size of the buffer in the currently running kernel can be found via the
sysctl debug.ktr.entries. By default the buffer contains 1024 entries.
Event levels can be enabled or disabled to trim excessive and overly verbose logging. First, a mask of events is specified at compile time
via the KTR_COMPILE option to limit which events are actually compiled into the kernel. The default value for this option is for all events
to be enabled.
Secondly, the actual events logged while the kernel runs can be further masked via the run time event mask. The KTR_MASK option sets the
default value of the run time event mask. The runtime event mask can also be set by the loader(8) via the debug.ktr.mask environment vari-
able. It can also be examined and set after booting via the debug.ktr.mask sysctl. By default the run time mask is set to block any trac-
ing. The definitions of the event mask bits can be found in <sys/ktr.h>.
Furthermore, there is a CPU event mask whose default value can be changed via the KTR_CPUMASK option. When two or more parameters to
KTR_CPUMASK, are used, it is important they are not separated by whitespace. A CPU must have the bit corresponding to its logical id set in
this bitmask for events that occur on it to be logged. This mask can be set by the loader(8) via the debug.ktr.cpumask environment variable.
It can also be examined and set after booting via the debug.ktr.cpumask sysctl. By default, only CPUs specified in KTR_CPUMASK will log
events. See sys/conf/NOTES for more information.
By default, events are only logged to the internal buffer for examination later, but if the verbose flag is set then they are dumped to the
kernel console as well. This flag can also be set from the loader via the debug.ktr.verbose environment variable, or it can be examined and
set after booting via the debug.ktr.verbose sysctl. If the flag is set to zero, which is the default, then verbose output is disabled. If
the flag is set to one, then the contents of the log message and the CPU number are printed to the kernel console. If the flag is greater
than one, then the filename and line number of the event are output to the console in addition to the log message and the CPU number. The
KTR_VERBOSE option sets the flag to one.
Examining the Events
The KTR buffer can be examined from within ddb(4) via the show ktr [/vV] command. This command displays the contents of the trace buffer one
page at a time. At the ``--more--'' prompt, the Enter key displays one more entry and prompts again. The spacebar displays another page of
entries. Any other key quits. By default the timestamp, filename, and line number are not displayed with each log entry. If the /v modi-
fier is specified, then they are displayed in addition to the normal output. If the /V modifier is specified, then just the timestamp is
displayed in addition to the normal output. Note that the events are displayed in reverse chronological order. That is, the most recent
events are displayed first.
Logging ktr to Disk
The KTR_ALQ option can be used to log ktr entries to disk for post analysis using the ktrdump(8) utility. This option depends on the ALQ
option. Due to the potentially high volume of trace messages the trace mask should be selected carefully. This feature is configured
through a group of sysctls.
debug.ktr.alq_file displays or sets the file that ktr will log to. By default its value is /tmp/ktr.out. If the file name is changed
while ktr is enabled it will not take effect until the next invocation.
debug.ktr.alq_enable enables logging of ktr entries to disk if it is set to one. Setting this to 0 will terminate logging to disk and
revert to logging to the normal ktr ring buffer. Data is not sent to the ring buffer while logging to disk.
debug.ktr.alq_max is the maximum number of entries that will be recorded to disk, or 0 for infinite. This is helpful for limiting the
number of particularly high frequency entries that are recorded.
debug.ktr.alq_depth determines the number of entries in the write buffer. This is the buffer that holds entries before they are written to
disk and defaults to the value of the KTR_ENTRIES option.
debug.ktr.alq_failed records the number of times we failed to write an entry due to overflowing the write buffer. This may happen if the
frequency of the logged ktr messages outpaces the depth of the queue.
debug.ktr.alq_cnt records the number of entries that have currently been written to disk.
ktrdump(8), alq(9), ktr(9)
The KTR kernel tracing facility first appeared in BSD/OS 3.0 and was imported into FreeBSD 5.0.
October 20, 2012 BSD