On Mon, Dec 19, 2011 at 02:59:10PM +0200, Avi Kivity wrote:

On 12/16/2011 11:31 AM, Marcelo Tosatti wrote:

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Slow enough that progress of the watchdog thread is unable to keep up
with timer interrupt processing. This is considered a hang and
should be reported.

It's not a guest hang though!

No, but your host system is in such a load state that for the sake of
system usability you better print out a warning message.

What's the point in printing it in the guest?  The guest can't observe
host conditions.

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I don't see the advantage of preempt notifiers over the simple, paravirt
solution proposed? Note kvmclock is already paravirt.

Right.

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What do you want to be done in preempt notifiers? Measure what to
consider setting this flag?

Preemption while TASK_RUNNING or TASK_UNINTERRUPTIBLE.

Maybe it is good (not sure), need to look into schedstats and think of
cases that would break legitimate guest hangs. And it probably also
affects the position of clearing the flag on the guest side as its 
currently done in Eric's patchset.