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Re: [Xen-devel] VCPU hotplug problem

To: Chris Lalancette <clalance@xxxxxxxxxx>, Gabriel Southern <gsouther@xxxxxxx>
Subject: Re: [Xen-devel] VCPU hotplug problem
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Tue, 12 Feb 2008 07:59:18 +0000
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>> It would be a big help for me if anyone else could test to see if they
>> experience the same problem.  I'm not sure if this would qualify as a
>> problem with Linux or with Xen; however, any ideas for fixing it are
>> appreciated as well.
> Hm, it would be helpful if you told us what version of Xen you are
> running and which kernel you are running.  For what it is worth, running
> a RHEL-5.1 PV guest on a RHEL-5.1 dom0 (both x86_64) seems to do the
> right thing for me; xm vcpu-set <dom> 1 updates both /proc/cpuinfo and
> /proc/stat, and xm vcpu-set <dom> 2 updates both again.

The issue is the result of a deliberate change in some kernels (including
our 2.6.18-xen tree) in which fs/proc/proc_misc.c:show_stat() iterates over
each_possible_cpu() rather than each_online_cpu(). This of course means it
prints info for all offline cpus as well as online cpus.

The original reason for this is that some user-space tools, such as older
versions of top, would get quite upset when CPUs are hot-unplugged while
they run. This would cause top to crash! I don't know whether this was fixed
by now in the user-space packages. The issue was originally seen years ago
so I suppose it's fairly likely it has been, and the hack could be removed.

 -- Keir

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