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SubjectRe: linux-2.6.0-test2: Never using pm_idle (CPU wasting power)
Christian Vogel <vogel@skunk.physik.uni-erlangen.de> wrote:
>
> Hi,
>
> on a Thinkpad 600X I noticed the CPU getting very hot. It turned
> out that pm_idle was never called (which invokes the ACPI pm_idle
> call in this case) and default_idle was used instead.
>
> /* arch/i386/kernel/process.c, line 723 */
> void cpu_idle (void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> void (*idle)(void) = pm_idle;
> if (!idle)
> idle = default_idle; /* once on bootup */
> irq_stat[smp_processor_id()].idle_timestamp = jiffies;
> while (!need_resched())
> idle();
> schedule(); /* never reached */
> }
> }
>
> The schedule() is never reached (need_resched() is never 0) and
> so the idle-variable is not updated. pm_idle is NULL on the
> first call to cpu_idle on this thinkpad, and so I stay idling
> in the default_idle()-function.
>
> By moving the "void *idle = pm_idle; if(!idle)..." in the inner
> while()-loop the notebook calls pm_idle (as it get's updated by ACPI)
> and stays cool.

Yes, I assume that need_resched() is always false because kernel preemption
cuts in first. Can you please confirm that you're using CONFIG_PREEMPT,
and that the problem goes away if CONFIG_PREEMPT is disabled?

The problem which you identify will also invalidate the idle_timestamp
instrumentation so I think we should move that inside as well.


diff -puN arch/i386/kernel/process.c~acpi-idle-fix arch/i386/kernel/process.c
--- 25/arch/i386/kernel/process.c~acpi-idle-fix Thu Jul 31 15:51:16 2003
+++ 25-akpm/arch/i386/kernel/process.c Thu Jul 31 15:57:27 2003
@@ -139,12 +139,15 @@ void cpu_idle (void)
{
/* endless idle loop with no priority at all */
while (1) {
- void (*idle)(void) = pm_idle;
- if (!idle)
- idle = default_idle;
- irq_stat[smp_processor_id()].idle_timestamp = jiffies;
- while (!need_resched())
+ while (!need_resched()) {
+ void (*idle)(void) = pm_idle;
+
+ if (!idle)
+ idle = default_idle;
+
+ irq_stat[smp_processor_id()].idle_timestamp = jiffies;
idle();
+ }
schedule();
}
}
_


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