Re: Intel Local APIC behavior on low frequencies

[Arjan van de Ven <arjan@infradead.org>]

On Tue, 22 Sep 2009 12:31:24 -0400
pradeep hettiarachchi <pacprnt@gmail.com> wrote:

> Hi,
> 
> Here’s a description of my problem. It would be great if you can find
> some info on this matter.
> 
> 
> 
> I need to find some information about local APIC and CPU frequency
> scaling behavior.  Currently I am modifying  the Linux kernel for my
> research, which involves studying a custom job scheduler I am
> developing at different CPU frequencies.  As we know, if the kernel
> works in tick-less mode (and in hi resolution mode) it uses the local
> APIC as a clock event device and as the clock tick device.
> 
> 
> 
> My working environment is: Linux vanilla kernel 2.6.24, Intel core 2
> Quad Q6600, Intel core 2 duo T6500 processors. Gcc compiler and the
> tool chain.
> 
> The information I am trying to find is:
> 
> 1) I want to find-out the tick count(say for a 100 ms period) of the
> local APIC when the CPU operates at different operating frequencies.
> When I study this matter, I found that the local APIC does not tick
> (does not work accurately) reliably as the clock frequency is varied.
> It works accurately in the maximum frequency of the system. Do you
> know any reason for that? or it is the way it operates?(is that
> normal?)
> 
> (my observation: when I count the LAPIC tick count for the maximum CPU
> frequency, it matches with the number of ticks that the system
> generates at the time of the initial LAPIC calibration, which happens
> at the very beginning of the kernel initialization; however when I try
> to count the number of ticks with scaled frequencies, the LAPIC
> "current counter" difference is very less, some times less than 100
> for 100 ms, which should not be true.)
> 
> I don't have good reason to think that, some other task re-programs
> the LAPIC: I set the LAPIC "initial counter" to a huge value, further
> I assume when the system works in hi resolution mode, the re-program
> happens from the LAPIC handler "hrtimer_interrupt" which I replace
> with a dummy handler.
> 
> Thank you in advance.

the local apic timer is not impacted by the cpu frequency. HOWEVER,
it will stop when the CPU is idle.