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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. It must be
a power of two. The size of the buffer in the currently running kernel
can be found via the read-only 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
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 variable. It can
also be examined and set after booting via the debug.ktr.mask sysctl. By
default the run time mask is set to log only KTR_GEN events. 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. 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 events on all CPUs are enabled.
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 [/v]
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 modifier is specified,
then they are 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.
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.