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

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Message-ID: <1f1b08da0912071330i35808790r554f7a3a8d712ca7@mail.gmail.com>
Date:	Mon, 7 Dec 2009 13:30:35 -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 7:25 AM, Ioannis Kyriakopoulos
<johnkyr83@...mail.com> 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?
>
> 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.

Not quite, if High Resolution Timers support was disabled, you would
still use the HPET hardware instead of the pit, but you'd stay in
periodic mode. However, since you have Highres Timers on, the system
is using oneshot mode (which is necessary to trigger interrupts faster
then HZ), and likely has chosen the LAPIC timer as the interrupt
source.

In this case, the timer tick becomes an hrt event, that triggers every
HZ, instead of being caused by a periodic interrupt.

thanks
-john
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