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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Software Interrupt Context: Softirqs and Tasklets</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Unreliable Guide To Hacking The Linux Kernel" /><link rel="up" href="ch02.html" title="Chapter 2. The Players" /><link rel="prev" href="ch02s02.html" title="Hardware Interrupts (Hard IRQs)" /><link rel="next" href="ch03.html" title="Chapter 3. Some Basic Rules" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Software Interrupt Context: Softirqs and Tasklets</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch02s02.html">Prev</a> </td><th width="60%" align="center">Chapter 2. The Players</th><td width="20%" align="right"> <a accesskey="n" href="ch03.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Software Interrupt Context: Softirqs and Tasklets"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="basics-softirqs"></a>Software Interrupt Context: Softirqs and Tasklets</h2></div></div></div><p>
Whenever a system call is about to return to userspace, or a
hardware interrupt handler exits, any 'software interrupts'
which are marked pending (usually by hardware interrupts) are
run (<code class="filename">kernel/softirq.c</code>).
</p><p>
Much of the real interrupt handling work is done here. Early in
the transition to <acronym class="acronym">SMP</acronym>, there were only 'bottom
halves' (BHs), which didn't take advantage of multiple CPUs. Shortly
after we switched from wind-up computers made of match-sticks and snot,
we abandoned this limitation and switched to 'softirqs'.
</p><p>
<code class="filename">include/linux/interrupt.h</code> lists the
different softirqs. A very important softirq is the
timer softirq (<code class="filename">include/linux/timer.h</code>): you can
register to have it call functions for you in a given length of
time.
</p><p>
Softirqs are often a pain to deal with, since the same softirq
will run simultaneously on more than one CPU. For this reason,
tasklets (<code class="filename">include/linux/interrupt.h</code>) are more
often used: they are dynamically-registrable (meaning you can have
as many as you want), and they also guarantee that any tasklet
will only run on one CPU at any time, although different tasklets
can run simultaneously.
</p><div class="caution" title="Caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>
The name 'tasklet' is misleading: they have nothing to do with 'tasks',
and probably more to do with some bad vodka Alexey Kuznetsov had at the
time.
</p></div><p>
You can tell you are in a softirq (or tasklet)
using the <code class="function">in_softirq()</code> macro
(<code class="filename">include/linux/interrupt.h</code>).
</p><div class="caution" title="Caution" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Caution</h3><p>
Beware that this will return a false positive if a bh lock (see below)
is held.
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