CODEWITHSUNDEEP
multithreadinglinux-kernel
iknow34languages
iknow34languages

Reputation: 183

why would kernel will schedule irq handler with another thread?

i come across a problem when i use a irq handler in linux;

i register a handler with a request_irq;

it can be called when the external hardware send a interrupt;

in the irq handler,the code is programmed as follows;

irqreturn_t irq_test_handler(int irq, void *desc)
{
wake_up_process(a_thread_handle);
mdelay(10);
printk("end the handler\n");
return IRQ_HANDLED;
}

a_thread_handle come from function : kthread_create(a_kthread....);

in a_kthread()
{
printk("in a kthread !\n");
}

so in my opinion , when irq happens;

it will print like this;

"

end the handler

in a kthread !

"

but in fact ;

it will print like this:

"

in a kthread !

end the handler

"

Can some one explain this to me ?

Upvotes: 2

Views: 844

Answers (1)

Kellerspeicher
Kellerspeicher

Reputation: 628

What you are trying to do by wake_up_process() is what request_threaded_irq() is providing. Calling request_irq() is the same as request_threaded_irq() but with thread_fn=NULL. If you provide your thread and return IRQ_WAKE_THREAD, your thread will be waked after your handler function completed. I think that was your original intention (see here).

To try if this is a multi core effect set your system to be single core

#!/usr/bin/sudo bash
CPU_DIR="/sys/devices/system/cpu"
NEXT=$(( 1 - $( cat ${CPU_DIR}/cpu1/online ) ))
for d in /sys/devices/system/cpu/cpu*
do
  CPU=${d##*/}
  if [ ${CPU} == "cpuidle" ]; then continue; fi
  if [ ${CPU} == "cpu0" ]; then continue; fi
  echo -n $CPU $( cat $d/online )
  echo "${NEXT}" >$d/online
  echo " => " $( cat $d/online )
done

and try again. First call is going single core, second call back. You can also reboot using kernel parameter nosmp or maxcpus=0 to disable SMP activation entirely (see here).

Just for documentation (see comments below): It turned out to be a multi core effect with the thread running on a different core and thus starting immediately.

Upvotes: 1

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