title: What kind of file is the smp_affinity-file?
body: <p>The IRQ affinity can be set by writing a bit mask to <code>/proc/irq/&lt;irqid&gt;/smp_affinity</code>.
  I guess there is a kernel module behind smp_affinity, however, <code>ls</code> tells me it is a normal file:</p>
  <pre><code># ls
  -rw-r--r--   1 root root 0 Feb  9 16:06 smp_affinity
  </code></pre>
  <p>So I wonder, what kind of file <code>/proc/irq/&lt;irqid&gt;/smp_affinity</code> is?</p>