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
