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

To: "John Levon" <levon@xxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] RE: [PATCH] rendezvous-based local time calibration WOW!
From: "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 6 Aug 2008 09:34:28 -0600
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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Thread-index: Acj32eha3zsxfmStT6C2PvaXwDzXhQ==
> > 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)

Yes, that detects the problem so it can be printk'd.
But what can be done to reliably adjust for it?  Adding
a fixed offset to the new cpu's stime doesn't work because
stime computation is adapted independently and dynamically
on each cpu, so inter-CPU skew "jitters" and adding a
constant may just make the max skew worse.

I'm not saying it can't be done, but I'm pretty sure it
will be messy, so let's make sure it needs to be fixed
before trying to fix it.

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