linux-kernel - RE: timer interrupt stucks using tickless kernel

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Date:	Tue, 8 Dec 2009 03:38:04 +0000
From:	"Ioannis Kyriakopoulos" <johnkyr83@...mail.com>
To:	"'john stultz'" <johnstul@...ibm.com>
CC:	<linux-kernel@...r.kernel.org>
Subject: RE: timer interrupt stucks using tickless kernel

>> The system tick is
>> generated by IRQ0 which in my second configuration should be triggered
>> by HPET (because I have enabled the HPET timer support).
>
>System tick?

I mean the timer interrupt that drives the scheduler.

>> Also, if the
>> interrupts generated by LAPIC are incremented at HZ freq, why the
>> interrupts generated by the timer (IRQ0) not happen at the same frequency
>>  as well?
>
>Why take two interrupts when you could only take one?

That's exactly my question. Why we have both LAPIC and timer interrupts? If
the LOC are used for driving the scheduler then for what the "timer"
interrupts
(IRQ0) are used?

  0:        157   IO-APIC-edge      timer
LOC:    1865130   Local timer interrupts

>> Furthermore, we don't have an answer on why IO-APIC is used at the time
>> that it is disabled on kernel configurations. The fact that I have
enabled
>> LAPIC timer support doesn't justify the use of the IO-APIC controller
>> by each own.
>
>Honestly, I dunno on the details there.   If  CONFIG_X86_IO_APIC=n, it
>looks like you should not be seeing the IO-APIC-* names in
>/proc/interrupts.  Are you sure your booting the right kernel, or
>CONFIG_X86_IO_APIC is actally on in your .config?

I try to uncheck CONFIG_X86_IO_APIC manually because it's not appeared under
Xconcig/menuconfig but it re-checked automatically when it tries to build.
In order to disable it permanently, should I also uncheck all the configs
that depend on it?

>Ok. I hope these aren't homework questions :)
Don't worry, it's not homework ;)

Thanks,

John K.

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