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KTR(4) | Device Drivers Manual | KTR(4) |
NAME¶
ktr — kernel tracing facilitySYNOPSIS¶
options KTRoptions 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. 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 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 theKTR_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 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.
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. TheKTR_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¶
TheKTR_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.
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.January 25, 2005 | Debian |