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SubjectRe: Weird timer handler used with LAPIC selected
From
On Fri, May 7, 2010 at 4:10 AM, limp <johnkyr83@hotmail.com> wrote:
> Hi all,
>
> I have configured my kernel (2.6.31.6) so that I get periodic ticks from the
> timer interrupt (i.e. tickless is not configured) with a rate determined by
> the HZ value. I have also selected the "Local APIC support on uniprocessors"
> option so that the LAPIC timer is used as a tick source. The problem is that
> the handler used for the timer is not the "local_apic_timer_interrupt"
> handler as I was excepted but the "tick_handle_periodic" handler. This can
> be verified by seeing the output of /proc/timer_list:
>
> Tick Device: mode:     0
> Per CPU device: 0
> Clock Event Device: lapic
>  max_delta_ns:   1006457290
>  min_delta_ns:   1799
>  mult:           35797637
>  shift:          32
>  mode:           1
>  next_event:     9223372036854775807 nsecs
>  set_next_event: lapic_next_event
>  set_mode:       lapic_timer_setup
>  event_handler:  tick_handle_periodic

You might want to read the code in further detail. The clockevent
device is an abstraction on top of the actual hardware, so the
event_handler method is not mapped into the irq handler directly.

So the local_apic_timer_interrupt() is called by the apic hardware
interrupt, it then calls lapic clockevent device's event_handler()
method, which is set to tick_handle_periodic(). Since you're in
_PERIODIC mode, not _ONESHOT (hrtimers/nohz), this seems correct to
me.



> Is there an explanation for that? The board in which I am running the kernel
> is a Kontron 986LCD-M/mITX one with an Intel Celeron M 440 installed on it.
> I have verified that Intel Celeron M Processors have a LAPIC and
> consequently, a LAPIC timer.
>
> --------------------------------------------
> Another weird thing that I noticed is that by running the same kernel but
> configured for "Core 2/newer Xeon" processors, the timer handler used is the
> "hrtimer_interrupt" handler. Again, this is verified by /proc/timer_list:
>
> Tick Device: mode:     1
> Per CPU device: 0
> Clock Event Device: lapic
> event_handler:  hrtimer_interrupt

Yep. On this system I'm guessing you have hrtimers or nohz/tickless enabled?


> I have put some printks inside "local_apic_timer_interrupt" handler which
> should never be executed as the handler selected for the timer is
> "hrtimer_interrupt" and not "local_apic_timer_interrupt" as verified.

Nope. Again, local_apic_timer_interrupt is the actual irq hook. It
then calls the lapic clockevent device's event_handler() method, which
is hrtimer_interrupt.

thanks
-john
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