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KTR(4)		       FreeBSD Kernel Interfaces Manual			KTR(4)

     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 exe-
     cutes so that they	can be examined	later when debugging.  The only	manda-
     tory 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 ver-
     bose 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

     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 mod-
     ifier is specified, then just the timestamp is displayed in addition to
     the normal	output.	 Note that the events are displayed in reverse chrono-
     logical 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	analy-
     sis 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 help-
			   ful 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 mes-
			   sages 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.

FreeBSD	11.0		       October 20, 2012			  FreeBSD 11.0


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