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<table class="head">
<tr>
<td class="head-ltitle">KTR(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">KTR(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">ktr</code> — <span class="Nd">kernel
tracing facility</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="Cd">options KTR</code>
<br/>
<code class="Cd">options ALQ</code>
<br/>
<code class="Cd">options KTR_ALQ</code>
<br/>
<code class="Cd">options KTR_COMPILE=(KTR_LOCK|KTR_INTR|KTR_PROC)</code>
<br/>
<code class="Cd">options KTR_CPUMASK=0x3</code>
<br/>
<code class="Cd">options KTR_ENTRIES=8192</code>
<br/>
<code class="Cd">options KTR_MASK=(KTR_INTR|KTR_PROC)</code>
<br/>
<code class="Cd">options KTR_VERBOSE</code></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The <code class="Nm">ktr</code> 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 <code class="Nm">ktr</code>
is “<code class="Li">options KTR</code>”.</p>
<p class="Pp">The <code class="Dv">KTR_ENTRIES</code> 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 <var class="Va">debug.ktr.entries</var>. By
default the buffer contains 1024 entries.</p>
<section class="Ss">
<h2 class="Ss" id="Event_Masking"><a class="permalink" href="#Event_Masking">Event
Masking</a></h2>
<p class="Pp">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 <code class="Dv">KTR_COMPILE</code> option to limit which events are
actually compiled into the kernel. The default value for this option is for
all events to be enabled.</p>
<p class="Pp">Secondly, the actual events logged while the kernel runs can be
further masked via the run time event mask. The
<code class="Dv">KTR_MASK</code> option sets the default value of the run
time event mask. The runtime event mask can also be set by the
<a class="Xr">loader(8)</a> via the <var class="Va">debug.ktr.mask</var>
environment variable. It can also be examined and set after booting via the
<var class="Va">debug.ktr.mask</var> sysctl. By default the run time mask is
set to block any tracing. The definitions of the event mask bits can be
found in
<code class="In"><<a class="In">sys/ktr_class.h</a>></code>.</p>
<p class="Pp">Furthermore, there is a CPU event mask whose default value can be
changed via the <code class="Dv">KTR_CPUMASK</code> option. When two or more
parameters to <code class="Dv">KTR_CPUMASK</code>, 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 <a class="Xr">loader(8)</a> via the
<var class="Va">debug.ktr.cpumask</var> environment variable. It can also be
examined and set after booting via the
<var class="Va">debug.ktr.cpumask</var> sysctl. By default, only CPUs
specified in <code class="Dv">KTR_CPUMASK</code> will log events. See
<span class="Pa">sys/conf/NOTES</span> for more information.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Verbose_Mode"><a class="permalink" href="#Verbose_Mode">Verbose
Mode</a></h2>
<p class="Pp">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 <var class="Va">debug.ktr.verbose</var> environment variable, or it can
be examined and set after booting via the
<var class="Va">debug.ktr.verbose</var> 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 <code class="Dv">KTR_VERBOSE</code> option
sets the flag to one.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Examining_the_Events"><a class="permalink" href="#Examining_the_Events">Examining
the Events</a></h2>
<p class="Pp">The KTR buffer can be examined from within
<a class="Xr">ddb(4)</a> via the <code class="Ic">show ktr</code>
[<code class="Cm">/vV</code>] command. This command displays the contents of
the trace buffer one page at a time. At the
“<code class="Li">--more--</code>” 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
<code class="Cm">/v</code> modifier is specified, then they are displayed in
addition to the normal output. If the <code class="Cm">/V</code> 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.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Logging_ktr_to_Disk"><a class="permalink" href="#Logging_ktr_to_Disk">Logging
ktr to Disk</a></h2>
<p class="Pp">The <code class="Dv">KTR_ALQ</code> option can be used to log
<code class="Nm">ktr</code> entries to disk for post analysis using the
<a class="Xr">ktrdump(8)</a> utility. This option depends on the
<code class="Dv">ALQ</code> 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.</p>
<dl class="Bl-tag">
<dt id="debug.ktr.alq_file"><var class="Va">debug.ktr.alq_file</var></dt>
<dd>displays or sets the file that <code class="Nm">ktr</code> will log to. By
default its value is <span class="Pa">/tmp/ktr.out</span>. If the file
name is changed while <code class="Nm">ktr</code> is enabled it will not
take effect until the next invocation.</dd>
<dt id="debug.ktr.alq_enable"><var class="Va">debug.ktr.alq_enable</var></dt>
<dd>enables logging of <code class="Nm">ktr</code> 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.</dd>
<dt id="debug.ktr.alq_max"><var class="Va">debug.ktr.alq_max</var></dt>
<dd>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.</dd>
<dt id="debug.ktr.alq_depth"><var class="Va">debug.ktr.alq_depth</var></dt>
<dd>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 <code class="Dv">KTR_ENTRIES</code> option.</dd>
<dt id="debug.ktr.alq_failed"><var class="Va">debug.ktr.alq_failed</var></dt>
<dd>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
<code class="Nm">ktr</code> messages outpaces the depth of the queue.</dd>
<dt id="debug.ktr.alq_cnt"><var class="Va">debug.ktr.alq_cnt</var></dt>
<dd>records the number of entries that have currently been written to
disk.</dd>
</dl>
</section>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">ktrdump(8)</a>, <a class="Xr">alq(9)</a>,
<a class="Xr">ktr(9)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The KTR kernel tracing facility first appeared in
<span class="Ux">BSD/OS 3.0</span> and was imported into
<span class="Ux">FreeBSD 5.0</span>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">March 26, 2021</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
