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Re: cpu_idle and cpu_wait

To: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Subject: Re: cpu_idle and cpu_wait
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 16 Nov 2005 18:42:01 +0000
Cc: linux-mips@linux-mips.org
In-reply-to: <20051117.011906.25910026.anemo@mba.ocn.ne.jp>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20051117.011906.25910026.anemo@mba.ocn.ne.jp>
Sender: linux-mips-bounce@linux-mips.org
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On Thu, Nov 17, 2005 at 01:19:06AM +0900, Atsushi Nemoto wrote:

> Looking at recent change in cpu_idle(), I find an another potential
> problem with cpu_wait (WAIT instruction).
> 
>     48        ATTRIB_NORET void cpu_idle(void)
>     49        {
>     50                /* endless idle loop with no priority at all */
>     51                while (1) {
>     52                        while (!need_resched())
>     53                                if (cpu_wait)
>     54                                        (*cpu_wait)();
>     55                        preempt_enable_no_resched();
>     56                        schedule();
>     57                        preempt_disable();
>     58                }
>     59        }
> 
> If an interrupt raised on line 53 and the interrupt handler woke a
> sleeping thread up, the thread becomes runnable and current thread
> (idle thread) is marked as NEED_RESCHED.
> 
> Since preemption is disabled, the interrupt handler just return to
> current thread (idle thread) without rescheduling.  The idle thread
> then call cpu_wait() and execute WAIT instruction (or something
> similer).  The CPU will stops until next interrupt.  Then the idle
> task checks need_resched() and finally calls schedule().  Therefore,
> wakeup-resume latency will be nearly one TICK on worst case!

Pleassure.

> If this analysis was correct, how to fix this?
> 
> Removing above preempt_enable_no_resched/preempt_disable pair would
> fix it for preemptive kernel, but no point for non-preemptive kernel.
> Replacing them with local_irq_enable/local_irq_disable would fix it
> for both kernel, but there is an question:

Somebody sneaking those lines into kernel.org ...

>       The CPU can surely exit from the WAIT instruction by interrupt
>       even if interrupts disabled?

That's implementation dependent behaviour, unfortunately.

  Ralf

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