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

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Date:	Mon, 25 Sep 2006 17:45:18 +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 Monday 25 September 2006 09:49, Aubrey wrote:
> > 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.
>
> And that works if interrupts are disabled as it should?

No, We are trying to figure out what's going on.

>
> > 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.
>
> It's the same problem as why sleep_on() is wrong and wait_event() is
> right, you can find lots of documentation about that.
>
> Think about a process calling nanosleep() to wait for a timeout.
> It eventually ends up going to sleep and the kernel schedules
> the idle task.
>
> The idle task checks need_resched(), which returns false, so it
> will call the idle instruction. Before it gets there, the timer
> tick happens, which calls the timer softirq. That softirq notices
> that the user process should continue running and calls wake_up(),
> which makes the condition for need_resched() true.
>
> Since we're handling that softirq that interrupted the idle task,
> that task continues what it was last doing and calls the idle instruction.
> This is the point where it goes wrong, because your user task should
> run, but the CPU is sleeping until the next interrupt happens.
>
> Note that you should in the future disable timer ticks during the
> idle function (CONFIG_NO_IDLE_HZ or similar) to save more power, but
> in that case the CPU may be in idle indefinitely after missing the
> one interrupt that should have woken it up.
>
Very clear, thanks a lot.
-Aubrey
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