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     ktr — kernel tracing facility


     options KTR
     options ALQ
     options KTR_ALQ
     options KTR_CPUMASK=0x3
     options KTR_ENTRIES=8192
     options KTR_VERBOSE


     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 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 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.  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.