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Re: [Xen-devel] cpu_down() but no cpu_up() in drivers/xen/cpu_hotplug.c

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] cpu_down() but no cpu_up() in drivers/xen/cpu_hotplug.c ?
From: Dietmar Hahn <dietmar.hahn@xxxxxxxxxxxxxx>
Date: Tue, 11 May 2010 13:30:14 +0200
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
Delivery-date: Tue, 11 May 2010 04:31:09 -0700
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Am 11.05.2010 schrieb Ian Campbell:
> On Tue, 2010-05-11 at 10:34 +0100, Jan Beulich wrote:
> > Jeremy,
> > 
> > how is pv-ops bringing up CPUs set to online in xenstore? Is this not
> > an automatic process (i.e. requires writing to respective online node
> > in sysfs), like in the traditional tree?
> The original commit which added CPU hotplug to pvops says:
>     xen: implement CPU hotplugging
>     Note the changes from 2.6.18-xen CPU hotplugging:
>     A vcpu_down request from the remote admin via Xenbus both hotunplugs the
>     CPU, and disables it by removing it from the cpu_present map, and removing
>     its entry in /sys.
>     A vcpu_up request from the remote admin only re-enables the CPU, and does
>     not immediately bring the CPU up. A udev event is emitted, which can be
>     caught by the user if he wishes to automatically re-up CPUs when 
> available,
>     or implement a more complex policy.
>     Signed-off-by: Alex Nixon <alex.nixon@xxxxxxxxxx>
>     Acked-by: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
>     Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
> I'm not sure how the decision was reached to implement it this way,
> perhaps for consistency with CPU hotplug on other
> platforms/architectures?
> FWIW I use a udev rule to bring up CPUs as they are added, which is
> equivalent to the old behaviour:
>         ACTION=="add", SUBSYSTEM=="cpu", RUN+="/bin/sh -c '[ ! -e 
> /sys$devpath/online ] || echo 1 > /sys$devpath/online'"
> Ian.

Maybe it would be good then to have a comment somewhere in the tree with
this udev rule as a hint?


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