RE: [Qemu-devel] rtc: lost some interrupts at 1024Hz.

[Armistead, Jason]

Brad Campbell wrote:

>The VM behaves perfectly until I do a suspend/resume of the host. When the
machine comes back, most 
>of the time (not all the time however) it ends up flooding my logs with
this message.
>
>Jul  8 17:26:03 localhost kernel: [43299223.910000] rtc: lost some
interrupts at 1024Hz.
>Jul  8 17:26:03 localhost kernel: [43299223.930000] rtc: lost some
interrupts at 1024Hz.
>Jul  8 17:26:03 localhost kernel: [43299223.950000] rtc: lost some
interrupts at 1024Hz.
>Jul  8 17:26:03 localhost kernel: [43299223.970000] rtc: lost some
interrupts at 1024Hz.

Some clues from Google's search results, searching for "rtc: lost some
interrupts at 1024Hz":

http://lkml.org/lkml/2006/4/25/107

Also look at Google's groups.  Lots of discussion about these error messages
there

I suspect (without diving into source code) that Linux tracks timer
interrupts (in old PC DOS days, 18 ticks per second from a programmable
interval timer) versus the hardware RTC timer ticks, and when there are less
interrupts occurring than expected, it thinks the hardware is on the blink
or else there is some higher priority interrupt handler that is running for
longer than 2 RTC interrupts.

I get the impression that Linux media player style applications which need
critical timer resources actually increase the interrupt rate from 18 per
second to something a lot higher, in your case 1024 Hz.  The kernel may be
trying to ensure that the interrupts are being handled in a timely fashion,
and to know that, all it can do is compare against the RTC chip.  If you
expect 1024 interrupts per second, but in a two second period (from the HW
RTC clock) there have only been say 1500 processed instead of 1024 i.e. 1500
times the interrupt handler has run, then clearly, around 548 interrupts
went missing, blocked by some higher interrupt priority source.

I found some sources for the RTC driver at

http://www.kernel.org/pub/scm/linux/kernel/bkcvs/linux-2.5/drivers/char/rtc.
c,v

and this seems to confirm what I expected.  Look in function rtc_dropped_irq
and the comments preceding it.

Cheers

Jason