Re: RE: [PATCH] rendezvous-based local time calibration WOW!

[John Levon <levon@movementarian.org>]

On Wed, Aug 06, 2008 at 09:09:06AM -0600, Dan Magenheimer wrote:

> Again no guarantees but I think we are now under the magic
> threshold where the skew is smaller than the time required
> for scheduling a VCPU onto a different CPU.  If so,
> consecutive gethrtime's by the same thread in a domain
> should always be monotonic.

Right! That sounds positive.

> The overhead of measuring the inter-CPU stime skew is
> too large to do at every cross-PCPU-schedule so doing
> any kind of adjustment would be difficult.
> But it might make sense for the Xen scheduler to do a
> get_s_time() before and after a cross-PCPU-schedule
> to detect the problem and printk if it occurs
> (possibly rate-limited in case it happens a lot on
> some badly-behaved machine).

If we're doing a get_s_time() before the schedule, don't we merely* have
to ensure that the new s_time is after the last recorded one on the
previous CPU? (Yes, I'm handwaving terribly)

regards
john