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>Software Interrupt Context: Bottom Halves, Tasklets, softirqs</H1
><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 (<TT
CLASS="FILENAME"
>kernel/softirq.c</TT
>).
</P
><P
> Much of the real interrupt handling work is done here. Early in
the transition to <SPAN
CLASS="ACRONYM"
>SMP</SPAN
>, 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.
</P
><P
> <TT
CLASS="FILENAME"
>include/linux/interrupt.h</TT
> lists the
different BH's. No matter how many CPUs you have, no two BHs will run at
the same time. This made the transition to SMP simpler, but sucks hard for
scalable performance. A very important bottom half is the timer
BH (<TT
CLASS="FILENAME"
>include/linux/timer.h</TT
>): you
can register to have it call functions for you in a given length of time.
</P
><P
> 2.3.43 introduced softirqs, and re-implemented the (now
deprecated) BHs underneath them. Softirqs are fully-SMP
versions of BHs: they can run on as many CPUs at once as
required. This means they need to deal with any races in shared
data using their own locks. A bitmask is used to keep track of
which are enabled, so the 32 available softirqs should not be
used up lightly. (<I
CLASS="EMPHASIS"
>Yes</I
>, people will
notice).
</P
><P
> tasklets (<TT
CLASS="FILENAME"
>include/linux/interrupt.h</TT
>)
are like softirqs, except 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 (unlike different BHs).
</P
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> 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
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><P
> You can tell you are in a softirq (or bottom half, or tasklet)
using the <TT
CLASS="FUNCTION"
>in_softirq()</TT
> macro
(<TT
CLASS="FILENAME"
>include/asm/softirq.h</TT
>).
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> Beware that this will return a false positive if a bh lock (see below)
is held.
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