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Re: [Xen-devel] [timer/ticks related] dom0 hang during boot on large 1TB

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [timer/ticks related] dom0 hang during boot on large 1TB system
From: Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
Date: Fri, 18 Dec 2009 11:28:26 -0800
Cc: Kurt Hackel <kurt.hackel@xxxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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On Fri, 18 Dec 2009 09:13:32 +0000
Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:

> On 18/12/2009 08:42, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
> >> This isn't really Xen time code, but unchanged Linux time code. I
> >> don't know which tree you quoted the code from -- 2.6.18 has
> >> similar but not identical. Anyway, I suggest try using the
> >> jiffy-comparison macros from <linux/jiffies.h>: time_before(),
> >> time_after(), etc. These are designed to work even when jiffies
> >> wraps. Feel free to send patch(es) for that, if you test that out
> >> and it works okay.
> > 
> > But regardless of that - shouldn't the page scrubbing really be a
> > background operation these days, and as such be (relatively)
> > performance neutral to the booting of Dom0?
> We synchronously scrub free memory before starting dom0, and then
> subsequently scrub memory only for dying domains. So I don't know what
> scrubbing would be going on during dom0's boot-time calibrations, on
> any version of Xen, actually.
>  -- Keir

Scrubbing has nothing to do with the bug. It's just that the timing is just
right to expose the bug. The system boots fine with lesser memory. Since
hyp does:
       create dom0, page scrub, unpause dom0.

It appears with large scrubbing, this gets delta in dom0 timer_interrupt() 
to be large enough that jiffies wraps. 

Hope that makes sense.


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