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[Xen-devel] Re: Concern about c/s 21404: Remove special-case paths for s

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: Concern about c/s 21404: Remove special-case paths for start-of-day SMP bringup
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 19 May 2010 20:15:45 +0100
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I'll add an __initcall() to rendezvous the CPUs and whack their TSCs.


On 19/05/2010 17:24, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> "Notably ganged TSC calibration is removed"
> Really?  We have recently seen machines which generated
> the removed message "CPUX had N usecs TSC skew, fixed it up."
> I'm not sure I fully understand the scope or the purpose
> of all the patches, but since the TSC is the foundation for
> Xen system time which is the foundation for virtually all time
> handling in all guests (because even the non-TSC emulated
> timers are built on Xen system time), I think you are introducing
> a big risk here for an unknown number of existing systems.
> Some of the problems may be obvious because a guest often
> won't boot when clocks are screwy, but other problems may
> not manifest until "later" when under a heavy load that
> causes more frequent vcpu->pcpu switching and may not
> be at all obviously related to clock problems.  (We've
> recently seen that also with a Solaris HV guest running
> on a pre-4.0 Xen where rdtsc is not yet emulated.)
> Note that Linux is *adding* code to attempt to make TSC more
> reliable, while it appears this Xen patch is taking code away.
> If I understand, the purpose is to provide a better
> way to online CPUs so as to handle CPU hotplug better?
> While hotplug is a nice feature, am I correct that the
> feature is supported in a very small number of
> systems?  And is actually used in an even smaller
> number?
> My two cents: Doesn't seem like a good tradeoff to me...

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