NAME
ktr — kernel tracing facility
SYNOPSIS
options KTR
options ALQ
options KTR_ALQ
options KTR_COMPILE=(KTR_LOCK|KTR_INTR|KTR_PROC)
options KTR_CPUMASK=0x3
options KTR_ENTRIES=8192
options KTR_MASK=(KTR_INTR|KTR_PROC)
options KTR_VERBOSE
DESCRIPTION
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
Masking
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 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 block any tracing. 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.
Verbose
Mode
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 modifier 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.
SEE ALSO
HISTORY
The KTR kernel tracing facility first appeared in BSD/OS 3.0 and was imported into FreeBSD 5.0.
BSD October 20, 2012 BSD