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

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Date:	Mon, 7 Dec 2009 10:06:09 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Ioannis Kyriakopoulos <johnkyr83@...mail.com>
cc:	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: timer interrupt stucks using tickless kernel

On Mon, 7 Dec 2009, Ioannis Kyriakopoulos wrote:

> 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've also checked the "high resolution timer support"
> and also SMP support. The posblem is that the timer is getting incremented
> very slowly (way slower than the HZ value), just like it would be if the
> kernel was tickless. Is there an explanation for that? How can I get
> periodic
> timer ticks?

This is what platform? Embedded? Timer is referring to the system time
running slow? kernel boot log would be useful.

> AFAI understand, the HPET timer is used through the IO-APIC controller to
> trigger the processors (N.B. if the "High Resolution Timer Support" option
> wasn't checked, the timer used would  be PIT, right?) and each processor's
> LAPIC timer is used for time keeping. Please correct me if I am wrong.
>
> Another question is how the LAPIC timner's rate is determined? In my case,
> it seams that an interurpt by LAPIC timer is generated very appoximately
> 0.25 sec (that is HZ which is 1000 * 4) but I can't understand why.

You could have 4 processors that each need a timer interrupt at
1000 HZ?

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