KTR(4) FreeBSD Kernel Interfaces Manual KTR(4)
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_class.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
ktrdump(8), alq(9), ktr(9)
HISTORY
The KTR kernel tracing facility first appeared in BSD/OS 3.0 and was
imported into FreeBSD 5.0.
FreeBSD 15.1-STABLE-HBSD March 26, 2021 KTR(4)