Xen devel - Jul 2008 - [PATCH] roughly synchronize calibration calls to reduce skew

This patch (based from 18139) roughly synchronizes calls
to local_time_calibration() across multiple processors
by setting each next timer to a "round epoch".  This results
in reduced skew; both max and average skew are reduced by
between a factor of 2 and 3.  Note that timers still
fire at slightly different times because the next
"round epoch" is still relative to local stime which
still has some inter-processor skew.

Making local_time_calibration() precisely concurrent
across processors is a possible next step but since
each processor still has to get the platform timer -- an
operation that is locked and takes several (and sometimes
tens of) microseconds, concurrency will take some more
cleverness.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

==================================Thanks... for the memory
I really could use more / My throughput's on the floor
The balloon is flat / My swap disk's fat / I've OOM's in store
Overcommitted so much
(with apologies to the late great Bob Hope)

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This patch collects additional useful clock skew output
which is printk'd, for example, by "xm debug-key t".

Since this information assists in the decision whether to
select the new clocksource=tsc and softtsc boot options,
it would be nice to have this in 3.3.

By repeating "xm debug-key t; xm dmesg | tail -N" in
dom0 (e.g. with the "watch" command), useful skew info
about the machine can be collected, which will be very
helpful for diagnosing customer problems on misbehaving
machines.

Patch details:
1) Min and max readings from the current sample are useless
so get rid of those but retain the current maxdif (stime skew).
2) Remember max of skew, sum of skews, and a count
so average skew can be displayed.
3) Also record info for cycles (tsc on x86, itc on ia64).
4) Add a hook for arch-dependent info.  Only used for
clocksource on x86 now, but other info will be added later.

(Note, not compiled on ia64... its possible a header file
will need to be include'd somewhere.)

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

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