Hello,
I need help/advice on a little "performance" problem I have:
I am using linux 3.0.3 with the related RT patch on an ARM9 processor.
I have a RT task which is reading from a driver. The RT task is blocked
on wait_event_interruptible() and the driver interrupt handler is
releasing the waiting task using wake_up_interruptible() when there is
data available.
By default on linux RT, driver IRQ handlers are run in threads with a
priority of 50.
At first I setup my RT task with a priority of 10.
When some data are received by the system, I can see the following
scheduling (using LTTng).
- irq_handler
- my_driver_irq_handler (priority 50)
- my_task (priority 10)
This is all good but I really need "my_task" to be high priority. Once
an event is received, it need to process it immediately without being
interrupted by further events coming on the device.
So now, I set my_task priority to 80. Now when data is received, I can
see the following scheduling (still using LTTng).
- irq_handler
- my_driver_irq_handler (priority 50)
- my_task (priority 80) // this is a short run. I assume the task is
checking the wait() "condition"
- my_driver_irq_handler (priority 80) // here there seems to be an
"automatic" priority inversion to allow the handler to run and finish
the wake_up call
- my_task (priority 80) // run until it blocks again
- my_driver_irq_handler (priority 50) // end of the interrupt handler.
What is good is that now, "my_task" cannot be interrupted by another
interrupt handler processing when it runs. But because of the 2
additional context switches when the interrupt handler wake_up()
"my_task", I now experience an additional latency and this is not expected.
On average the latency performance is better when "my_task" priority is
10 but the real-time behavior is better when "my_task" priority is 80.
The latency difference is mainly due to the additional context switches
we have when the interrupt handler wake_up() "my_task". So the question
I have: Are these additional context switches mandatory? Is there a way
to avoid them.
As an exercise I raised (programatically) the interrupt handler priority
to 99 before doing the wake_up() and lower it to 50 just after the
wake_up. With this change, I now have (with LTTng):
- irq_handler
- my_driver_irq_handler (priority 50 then 99) // run until the priority
is set back to 50
- my_task (priority 80) // run until it blocks again
- my_driver_irq_handler (priority 50) // end of the interrupt handler.
Setting the irq handler priority is fast and cost almost nothing
(compared to context switch). As a result, I don't have the additional
latency cause by the 2 context switches of the previous example and I
get the best of both worlds: A short latency and the expected real-time
behavior (I am not interrupted by later interrupt handler).
But I feel that changing explicitly/dynamically the driver irq handler
priority to get the expected behavior might not be the appropriate
solution.
Am I missing something? Is there a better way to prevent the additional
context switches?
Thanks for any advice/hint you may have.
JC
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