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. It should
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 com

pile 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 vari

able. 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

Furthermore, there is a CPU event mask whose default value

can be changed
via the KTR_CPUMASK option. A CPU must have the bit corre

sponding 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 boot

ing 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 exam

ined 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 dis

plays 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 infi

nite. 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 op

tion.

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 cur

rently 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.

BSD January 25, 2005

BSD