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

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Date:	Mon, 7 Dec 2009 18:05:02 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Ioannis Kyriakopoulos <johnkyr83@...mail.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: timer interrupt stucks using tickless kernel

On Mon, Dec 7, 2009 at 5:33 PM, Ioannis Kyriakopoulos
<johnkyr83@...mail.com> wrote:
> Thanks for the valuable help so far.
>
>>> 2.   When performing cat proc/interrupts, I see that:
>>>           0:        157   IO-APIC-edge      timer
>>>      How the system timer is attached on IO-APIC at the time that the
>>> IO-APIC
>>>       is disabled on kernel configurations? Basically, the timer is not the
>>> only
>>>       peripheral attached to IO-APIC as can be seen by /proc/interrupts.
>>>
>>> Here is the output of /proc/interrupts :
>>>
>>>            CPU0
>>>   0:        157   IO-APIC-edge      timer
>>>   1:          2   IO-APIC-edge      i8042
>>>   4:        544   IO-APIC-edge      serial
>>>   8:          1   IO-APIC-edge      rtc0
>>>   9:          0   IO-APIC-fasteoi   acpi
>>>  12:          4   IO-APIC-edge      i8042
>>>  14:          4   IO-APIC-edge      ata_piix
>>>  15:          0   IO-APIC-edge      ata_piix
>>>  16:          0   IO-APIC-fasteoi   uhci_hcd:usb5
>>>  18:          0   IO-APIC-fasteoi   uhci_hcd:usb4
>>>  19:         45   IO-APIC-fasteoi   ata_piix, uhci_hcd:usb3
>>>  22:        149   IO-APIC-fasteoi   HDA Intel
>>>  23:         72   IO-APIC-fasteoi   ehci_hcd:usb1, uhci_hcd:usb2
>>>  27:          0   PCI-MSI-edge      eth0
>>> NMI:          0   Non-maskable interrupts
>>> LOC:    1865130   Local timer interrupts
>>            ^^^
>>This value should be incrementing at HZ freq.
>
> Are you sure about that? ASAIK the Local timer interrupts (generated
> by the LAPIC timer) are only used for timekeeping.

Actually local timer interrupts are more often used for per-cpu
scheduler ticks (as well as running per-cpu timers).  Timekeeping is
done from only one cpu (usually 0, but that varies depending on
hotplug, etc).

> 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?

> 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?

> 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?

>>Is there something your actually having problems with here? Ignoring
>>which interrupt line is used for the timer event, the system is
>>functioning properly, right?
>
> Yes, apart from that, everything is working OK but it's really important
> for me to get an answer on these issues.

Ok. I hope these aren't homework questions :)

thanks
-john
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