Re: [PATCH 1/4] Blackfin: arch patch for 2.6.18

From: Aubrey
Date: Mon Sep 25 2006 - 03:51:23 EST

On 9/25/06, Arnd Bergmann <arnd@xxxxxxxx> wrote:
On Sunday 24 September 2006 05:35, Aubrey wrote:
> > This looks racy. What if you get an interrupt after testing need_resched()
> > but before the idle instruction?
> >
> > Normally, this should look like
> >
> > while(!need_resched()) {
> > local_irq_disable();
> > if (!need_resched())
> > asm volatile("idle");
> > local_irq_enable();
> > }
> >
> > Of course that only works if your idle instruction wakes up on pending
> > interrupts.
> No, that doesn't work on blackfin. Blackfin need interrupt(here is
> timer interrupt) to wake up the core from IDLE mode. Once you disable
> the interrupt, the core will sleep forever.

Ok, then this is something you should take back as feedback to your
CPU designers. With the behavior you describe, the interrupt latency
(until the point where an application runs) can be up to one jiffy
longer than it should be, which is unacceptable for many real-time

Worse, it means that you can not use the CONFIG_NO_IDLE_HZ option in the
future, because you have no way to guarantee that you ever wake up from
idle if you hit the race.

Oh, sorry for my unclear description, any of the peripherals can be configured
to wake up the core from its idled state to process the interrupt on
Blackfin. I should say __if no other interrupts__ here is the timer
interrupt waking up the processor every one jiffy.

I don't understand if interrupt occurs between need_resched() and the
idle instruction, what will become the racy object? Can you comment it
detailed? thanks.

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