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

     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.