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<table class="head">
<tr>
<td class="head-ltitle">COUNTER(9)</td>
<td class="head-vol">Kernel Developer's Manual</td>
<td class="head-rtitle">COUNTER(9)</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">counter</code> —
<span class="Nd">SMP-friendly kernel counter implementation</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="In">#include
<<a class="In">sys/types.h</a>></code>
<br/>
<code class="In">#include <<a class="In">sys/systm.h</a>></code>
<br/>
<code class="In">#include <<a class="In">sys/counter.h</a>></code></p>
<p class="Pp"><var class="Ft">counter_u64_t</var>
<br/>
<code class="Fn">counter_u64_alloc</code>(<var class="Fa" style="white-space: nowrap;">int
wait</var>);</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_u64_free</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>);</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_u64_add</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>, <var class="Fa" style="white-space: nowrap;">int64_t v</var>);</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_enter</code>();</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_exit</code>();</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_u64_add_protected</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>, <var class="Fa" style="white-space: nowrap;">int64_t v</var>);</p>
<p class="Pp"><var class="Ft">uint64_t</var>
<br/>
<code class="Fn">counter_u64_fetch</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>);</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_u64_zero</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>);</p>
<p class="Pp"><var class="Ft">struct counter_rate *</var>
<br/>
<code class="Fn">counter_rate_alloc</code>(<var class="Fa" style="white-space: nowrap;">int
flags</var>, <var class="Fa" style="white-space: nowrap;">int
period</var>);</p>
<p class="Pp"><var class="Ft">int64_t</var>
<br/>
<code class="Fn">counter_ratecheck</code>(<var class="Fa" style="white-space: nowrap;">struct
counter_rate *cr</var>, <var class="Fa" style="white-space: nowrap;">int64_t
limit</var>);</p>
<p class="Pp"><var class="Ft">uint64_t</var>
<br/>
<code class="Fn">counter_rate_get</code>(<var class="Fa" style="white-space: nowrap;">struct
counter_rate *cr</var>);</p>
<p class="Pp"><var class="Ft">void</var>
<br/>
<code class="Fn">counter_rate_free</code>(<var class="Fa" style="white-space: nowrap;">struct
counter_rate *cr</var>);</p>
<p class="Pp"><code class="Fn">COUNTER_U64_SYSINIT</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>);</p>
<p class="Pp"><code class="Fn">COUNTER_U64_DEFINE_EARLY</code>(<var class="Fa" style="white-space: nowrap;">counter_u64_t
c</var>);</p>
<p class="Pp"><code class="In">#include
<<a class="In">sys/sysctl.h</a>></code></p>
<p class="Pp"><code class="Fn">SYSCTL_COUNTER_U64</code>(<var class="Fa" style="white-space: nowrap;">parent</var>,
<var class="Fa" style="white-space: nowrap;">nbr</var>,
<var class="Fa" style="white-space: nowrap;">name</var>,
<var class="Fa" style="white-space: nowrap;">access</var>,
<var class="Fa" style="white-space: nowrap;">ptr</var>,
<var class="Fa" style="white-space: nowrap;">descr</var>);</p>
<p class="Pp"><code class="Fn">SYSCTL_ADD_COUNTER_U64</code>(<var class="Fa" style="white-space: nowrap;">ctx</var>,
<var class="Fa" style="white-space: nowrap;">parent</var>,
<var class="Fa" style="white-space: nowrap;">nbr</var>,
<var class="Fa" style="white-space: nowrap;">name</var>,
<var class="Fa" style="white-space: nowrap;">access</var>,
<var class="Fa" style="white-space: nowrap;">ptr</var>,
<var class="Fa" style="white-space: nowrap;">descr</var>);</p>
<p class="Pp"><code class="Fn">SYSCTL_COUNTER_U64_ARRAY</code>(<var class="Fa" style="white-space: nowrap;">parent</var>,
<var class="Fa" style="white-space: nowrap;">nbr</var>,
<var class="Fa" style="white-space: nowrap;">name</var>,
<var class="Fa" style="white-space: nowrap;">access</var>,
<var class="Fa" style="white-space: nowrap;">ptr</var>,
<var class="Fa" style="white-space: nowrap;">len</var>,
<var class="Fa" style="white-space: nowrap;">descr</var>);</p>
<p class="Pp"><code class="Fn">SYSCTL_ADD_COUNTER_U64_ARRAY</code>(<var class="Fa" style="white-space: nowrap;">ctx</var>,
<var class="Fa" style="white-space: nowrap;">parent</var>,
<var class="Fa" style="white-space: nowrap;">nbr</var>,
<var class="Fa" style="white-space: nowrap;">name</var>,
<var class="Fa" style="white-space: nowrap;">access</var>,
<var class="Fa" style="white-space: nowrap;">ptr</var>,
<var class="Fa" style="white-space: nowrap;">len</var>,
<var class="Fa" style="white-space: nowrap;">descr</var>);</p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp"><code class="Nm">counter</code> is a generic facility to create
counters that can be utilized for any purpose (such as collecting
statistical data). A <code class="Nm">counter</code> is guaranteed to be
lossless when several kernel threads do simultaneous updates. However,
<code class="Nm">counter</code> does not block the calling thread, also no
<a class="Xr">atomic(9)</a> operations are used for the update, therefore
the counters can be used in any non-interrupt context. Moreover,
<code class="Nm">counter</code> has special optimisations for SMP
environments, making <code class="Nm">counter</code> update faster than
simple arithmetic on the global variable. Thus
<code class="Nm">counter</code> is considered suitable for accounting in the
performance-critical code paths.</p>
<dl class="Bl-tag">
<dt id="counter_u64_alloc"><a class="permalink" href="#counter_u64_alloc"><code class="Fn">counter_u64_alloc</code></a>(<var class="Fa">wait</var>)</dt>
<dd>Allocate a new 64-bit unsigned counter. The <var class="Fa">wait</var>
argument is the <a class="Xr">malloc(9)</a> wait flag, should be either
<var class="Va">M_NOWAIT</var> or <var class="Va">M_WAITOK</var>. If
<var class="Va">M_NOWAIT</var> is specified the operation may fail and
return <code class="Dv">NULL</code>.</dd>
<dt id="counter_u64_free"><a class="permalink" href="#counter_u64_free"><code class="Fn">counter_u64_free</code></a>(<var class="Fa">c</var>)</dt>
<dd>Free the previously allocated counter <var class="Fa">c</var>. It is safe
to pass <code class="Dv">NULL</code>.</dd>
<dt id="counter_u64_add"><a class="permalink" href="#counter_u64_add"><code class="Fn">counter_u64_add</code></a>(<var class="Fa">c</var>,
<var class="Fa">v</var>)</dt>
<dd>Add <var class="Fa">v</var> to <var class="Fa">c</var>. The KPI does not
guarantee any protection from wraparound.</dd>
<dt id="counter_enter"><a class="permalink" href="#counter_enter"><code class="Fn">counter_enter</code></a>()</dt>
<dd>Enter mode that would allow the safe update of several counters via
<a class="permalink" href="#counter_u64_add_protected"><code class="Fn" id="counter_u64_add_protected">counter_u64_add_protected</code></a>().
On some machines this expands to <a class="Xr">critical(9)</a> section,
while on other is a nop. See
<a class="Sx" href="#IMPLEMENTATION_DETAILS">IMPLEMENTATION
DETAILS</a>.</dd>
<dt id="counter_exit"><a class="permalink" href="#counter_exit"><code class="Fn">counter_exit</code></a>()</dt>
<dd>Exit mode for updating several counters.</dd>
<dt><code class="Fn">counter_u64_add_protected</code>(<var class="Fa">c</var>,
<var class="Fa">v</var>)</dt>
<dd>Same as <code class="Fn">counter_u64_add</code>(), but should be preceded
by <code class="Fn">counter_enter</code>().</dd>
<dt id="counter_u64_fetch"><a class="permalink" href="#counter_u64_fetch"><code class="Fn">counter_u64_fetch</code></a>(<var class="Fa">c</var>)</dt>
<dd>Take a snapshot of counter <var class="Fa">c</var>. The data obtained is
not guaranteed to reflect the real cumulative value for any moment.</dd>
<dt id="counter_u64_zero"><a class="permalink" href="#counter_u64_zero"><code class="Fn">counter_u64_zero</code></a>(<var class="Fa">c</var>)</dt>
<dd>Clear the counter <var class="Fa">c</var> and set it to zero.</dd>
<dt id="counter_rate_alloc"><a class="permalink" href="#counter_rate_alloc"><code class="Fn">counter_rate_alloc</code></a>(<var class="Fa">flags</var>,
<var class="Fa">period</var>)</dt>
<dd>Allocate a new struct counter_rate. <var class="Fa">flags</var> is passed
to <a class="Xr">malloc(9)</a>. <var class="Fa">period</var> is the time
over which the rate is checked.</dd>
<dt id="counter_ratecheck"><a class="permalink" href="#counter_ratecheck"><code class="Fn">counter_ratecheck</code></a>(<var class="Fa">cr</var>,
<var class="Fa">limit</var>)</dt>
<dd>The function is a multiprocessor-friendly version of
<a class="permalink" href="#ppsratecheck"><code class="Fn" id="ppsratecheck">ppsratecheck</code></a>()
which uses <code class="Nm">counter</code> internally. Returns
non-negative value if the rate is not yet reached during the current
period, and a negative value otherwise. If the limit was reached during
the previous period, but was just reset back to zero, then
<code class="Fn">counter_ratecheck</code>() returns number of events since
previous reset.</dd>
<dt id="counter_rate_get"><a class="permalink" href="#counter_rate_get"><code class="Fn">counter_rate_get</code></a>(<var class="Fa">cr</var>)</dt>
<dd>The number of hits to this check within the current period.</dd>
<dt id="counter_rate_free"><a class="permalink" href="#counter_rate_free"><code class="Fn">counter_rate_free</code></a>(<var class="Fa">cr</var>)</dt>
<dd>Free the <var class="Fa">cr</var> counter.</dd>
<dt id="COUNTER_U64_SYSINIT"><a class="permalink" href="#COUNTER_U64_SYSINIT"><code class="Fn">COUNTER_U64_SYSINIT</code></a>(<var class="Fa">c</var>)</dt>
<dd>Define a <a class="Xr">SYSINIT(9)</a> initializer for the global counter
<var class="Fa">c</var>.</dd>
<dt id="COUNTER_U64_DEFINE_EARLY"><a class="permalink" href="#COUNTER_U64_DEFINE_EARLY"><code class="Fn">COUNTER_U64_DEFINE_EARLY</code></a>(<var class="Fa">c</var>)</dt>
<dd>Define and initialize a global counter <var class="Fa">c</var>. It is
always safe to increment <var class="Fa">c</var>, though updates prior to
the <code class="Dv">SI_SUB_COUNTER</code> <a class="Xr">SYSINIT(9)</a>
event are lost.</dd>
<dt id="SYSCTL_COUNTER_U64"><a class="permalink" href="#SYSCTL_COUNTER_U64"><code class="Fn">SYSCTL_COUNTER_U64</code></a>(<var class="Fa">parent</var>,
<var class="Fa">nbr</var>, <var class="Fa">name</var>,
<var class="Fa">access</var>, <var class="Fa">ptr</var>,
<var class="Fa">descr</var>)</dt>
<dd>Declare a static <a class="Xr">sysctl(9)</a> oid that would represent a
<code class="Nm">counter</code>. The <var class="Fa">ptr</var> argument
should be a pointer to allocated <var class="Vt">counter_u64_t</var>. A
read of the oid returns value obtained through
<code class="Fn">counter_u64_fetch</code>(). Any write to the oid zeroes
it.</dd>
<dt id="SYSCTL_ADD_COUNTER_U64"><a class="permalink" href="#SYSCTL_ADD_COUNTER_U64"><code class="Fn">SYSCTL_ADD_COUNTER_U64</code></a>(<var class="Fa">ctx</var>,
<var class="Fa">parent</var>, <var class="Fa">nbr</var>,
<var class="Fa">name</var>, <var class="Fa">access</var>,
<var class="Fa">ptr</var>, <var class="Fa">descr</var>)</dt>
<dd>Create a <a class="Xr">sysctl(9)</a> oid that would represent a
<code class="Nm">counter</code>. The <var class="Fa">ptr</var> argument
should be a pointer to allocated <var class="Vt">counter_u64_t</var>. A
read of the oid returns value obtained through
<code class="Fn">counter_u64_fetch</code>(). Any write to the oid zeroes
it.</dd>
<dt id="SYSCTL_COUNTER_U64_ARRAY"><a class="permalink" href="#SYSCTL_COUNTER_U64_ARRAY"><code class="Fn">SYSCTL_COUNTER_U64_ARRAY</code></a>(<var class="Fa">parent</var>,
<var class="Fa">nbr</var>, <var class="Fa">name</var>,
<var class="Fa">access</var>, <var class="Fa">ptr</var>,
<var class="Fa">len</var>, <var class="Fa">descr</var>)</dt>
<dd>Declare a static <a class="Xr">sysctl(9)</a> oid that would represent an
array of <code class="Nm">counter</code>. The <var class="Fa">ptr</var>
argument should be a pointer to allocated array of
<var class="Vt">counter_u64_t's</var>. The <var class="Fa">len</var>
argument should specify number of elements in the array. A read of the oid
returns len-sized array of <var class="Vt">uint64_t</var> values obtained
through <code class="Fn">counter_u64_fetch</code>(). Any write to the oid
zeroes all array elements.</dd>
<dt id="SYSCTL_ADD_COUNTER_U64_ARRAY"><a class="permalink" href="#SYSCTL_ADD_COUNTER_U64_ARRAY"><code class="Fn">SYSCTL_ADD_COUNTER_U64_ARRAY</code></a>(<var class="Fa">ctx</var>,
<var class="Fa">parent</var>, <var class="Fa">nbr</var>,
<var class="Fa">name</var>, <var class="Fa">access</var>,
<var class="Fa">ptr</var>, <var class="Fa">len</var>,
<var class="Fa">descr</var>)</dt>
<dd>Create a <a class="Xr">sysctl(9)</a> oid that would represent an array of
<code class="Nm">counter</code>. The <var class="Fa">ptr</var> argument
should be a pointer to allocated array of
<var class="Vt">counter_u64_t's</var>. The <var class="Fa">len</var>
argument should specify number of elements in the array. A read of the oid
returns len-sized array of <var class="Vt">uint64_t</var> values obtained
through <code class="Fn">counter_u64_fetch</code>(). Any write to the oid
zeroes all array elements.</dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="IMPLEMENTATION_DETAILS"><a class="permalink" href="#IMPLEMENTATION_DETAILS">IMPLEMENTATION
DETAILS</a></h1>
<p class="Pp">On all architectures <code class="Nm">counter</code> is
implemented using per-CPU data fields that are specially aligned in memory,
to avoid inter-CPU bus traffic due to shared use of the variables between
CPUs. These are allocated using <var class="Va">UMA_ZONE_PCPU</var>
<a class="Xr">uma(9)</a> zone. The update operation only touches the field
that is private to current CPU. Fetch operation loops through all per-CPU
fields and obtains a snapshot sum of all fields.</p>
<p class="Pp">On amd64 a <code class="Nm">counter</code> update is implemented
as a single instruction without lock semantics, operating on the private
data for the current CPU, which is safe against preemption and
interrupts.</p>
<p class="Pp">On i386 architecture, when machine supports the cmpxchg8
instruction, this instruction is used. The multi-instruction sequence
provides the same guarantees as the amd64 single-instruction
implementation.</p>
<p class="Pp">On some architectures updating a counter require a
<a class="Xr">critical(9)</a> section.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="EXAMPLES"><a class="permalink" href="#EXAMPLES">EXAMPLES</a></h1>
<p class="Pp">The following example creates a static counter array exported to
userspace through a sysctl:</p>
<div class="Bd Pp Bd-indent Li">
<pre>#define MY_SIZE 8
static counter_u64_t array[MY_SIZE];
SYSCTL_COUNTER_U64_ARRAY(_debug, OID_AUTO, counter_array, CTLFLAG_RW,
&array[0], MY_SIZE, "Test counter array");</pre>
</div>
</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">atomic(9)</a>, <a class="Xr">critical(9)</a>,
<a class="Xr">locking(9)</a>, <a class="Xr">malloc(9)</a>,
<a class="Xr">ratecheck(9)</a>, <a class="Xr">sysctl(9)</a>,
<a class="Xr">SYSINIT(9)</a>, <a class="Xr">uma(9)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="Nm">counter</code> facility first appeared in
<span class="Ux">FreeBSD 10.0</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">The <code class="Nm">counter</code> facility was written by
<span class="An">Gleb Smirnoff</span> and <span class="An">Konstantin
Belousov</span>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">June 19, 2025</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
