| Re: [chrony-dev] Chrony freezing |
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On Fri, 13 Nov 2009, Miroslav Lichvar wrote:
On Thu, Nov 12, 2009 at 10:05:27AM -0800, Bill Unruh wrote:
The problem is that when the UIE flag goes on in the driver for rtc, it
immediately unblocks-- not waiting for the next seconds rollover, and opens
opens the file for reading. Chrony assumes that at this unblocking the time is
at the rollover.
To make sure I understand it correctly, setting UIE caused select to
immediately return that the descriptor is ready and then the read
returned immediately? Or did it block, or returned an error?
Yes, that was what was happening. The select was triggering immediately when
the rtc interrupts were turned on, rather than waiting for the "seconds"
turnover of the rtc clock. This was making the reading of the rtc inaccurate.
Was the second read added to improve accuracy?
Yes. It should have blocked until the next "second" turnover. Unfortunately it
sometimes blocks forever. Now, since I "fixed" the original problem, there
has been quite a lot of work done on the rtc driver, and that problem may have
gone, to be replaced by another.
I don't see any of that here, on a machine with HPET I guess.
You may have a newer kernel with different behaviour. It is the presence of
the HPET which caused the problem-- the bios on hpet machines reroutes the rtc interrupt
essentially making it unavailable. The rtc routine has then replaced the
interrupt with a poll of the rtc to see when the seconds turn over.
What about making always two readings, i.e. don't disable the interrupt
after first one, wait for the interrupt again and use that? (see the patch)
Precisely what it does now. And that freezes the system sometimes.
It only needs to do this in "normal" operation.
--
William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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