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Bottomhalf example cleanup and doc update #338

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Sep 4, 2025
Merged

Bottomhalf example cleanup and doc update #338

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7714f63
Drop unused DECLARE_TASKLET_OLD macro
eleanorLYJ Aug 26, 2025
6807fb9
Update documentation on bottom-half handling
eleanorLYJ Aug 26, 2025
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7 changes: 0 additions & 7 deletions examples/bottomhalf.c
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Expand Up @@ -21,13 +21,6 @@
#define NO_GPIO_REQUEST_ARRAY
#endif

/* Macro DECLARE_TASKLET_OLD exists for compatibility.
* See https://lwn.net/Articles/830964/
*/
#ifndef DECLARE_TASKLET_OLD
#define DECLARE_TASKLET_OLD(arg1, arg2) DECLARE_TASKLET(arg1, arg2, 0L)
#endif

static int button_irqs[] = { -1, -1 };

/* Define GPIOs for LEDs.
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18 changes: 14 additions & 4 deletions lkmpg.tex
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Expand Up @@ -2107,11 +2107,21 @@ \subsection{Detecting button presses}
\subsection{Bottom Half}
\label{sec:bottom_half}
Suppose you want to do a bunch of stuff inside of an interrupt routine.
A common way to do that without rendering the interrupt unavailable for a significant duration is to combine it with a tasklet.
A common way to avoid blocking the interrupt for a significant duration
is to defer the time-consuming part to a workqueue.
Comment on lines +2110 to +2111
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Improve the writing for rationale.

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Are you suggesting that the rationale behind the change needs more explanation, or
Is the issue with the tone and professionalism of the writing itself?

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Are you suggesting that the rationale behind the change needs more explanation?

I meant that you can consolidate BH descriptions, such as the removal of tasklet.

This pushes the bulk of the work off into the scheduler.

The example below modifies the previous example to also run an additional task when an interrupt is triggered.

This approach helps speed up the interrupt handling process itself,
allowing the system to respond to the next hardware interrupt more quickly.

Kernel developers generally discourage using tasklets due to their design limitations,
such as memory management issues and unpredictable latencies.
Instead, they recommend more robust mechanisms like workqueues or \textbf{softirqs}.
To address tasklet shortcomings, Linux contributors introduced the BH workqueue,
activated with the \cpp|WQ_BH| flag.
This workqueue retains critical features,
such as execution in atomic (\textbf{softirq}) context on the same CPU and the inability to sleep.

The example below extends the previous code to include an additional task executed in process context when an interrupt is triggered.
\samplec{examples/bottomhalf.c}

\subsection{Threaded IRQ}
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