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

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] cpu_down() but no cpu_up() in drivers/xen/cpu_hotplug.c ?
From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date: Tue, 11 May 2010 10:45:27 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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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

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'"


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