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

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Message-ID: <6d6a94c50609250049l75b2f070q81583b90d8fcfaec@mail.gmail.com>
Date:	Mon, 25 Sep 2006 15:49:57 +0800
From:	Aubrey <aubreylee@...il.com>
To:	"Arnd Bergmann" <arnd@...db.de>
Cc:	"Luke Yang" <luke.adi@...il.com>, linux-kernel@...r.kernel.org,
	"Andrew Morton" <akpm@...l.org>
Subject: Re: [PATCH 1/4] Blackfin: arch patch for 2.6.18

On 9/25/06, Arnd Bergmann <arnd@...db.de> 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
> users.
>
> 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.

-Aubrey
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