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From: =?utf-8?Q?Bj=C3=B8rn_Mork?= <bjorn@mork.no>
To: Don Zickus <dzickus@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Norbert Warmuth <nwarmuth@t-online.de>,
        Joseph Salisbury <joseph.salisbury@canonical.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RESEND][PATCH v3] watchdog: Fix disable/enable regression
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	<20121219201336.GK88797@redhat.com> <874njh7nd5.fsf@nemi.mork.no>
Date: Wed, 19 Dec 2012 22:44:44 +0100
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 Mork"'s message of "Wed, 19
	Dec 2012 22:17:42 +0100")
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Bjørn Mork <bjorn@mork.no> writes:
> Don Zickus <dzickus@redhat.com> writes:
>
>> What about the opposite cases?  
>> nmi_watchdog=1
>> echo 1 > /sys/devices/system/cpu/cpu1/online
>
> I don't see why not. But verifying it would be nice.  I thought that it
> would be a simple thing to test using qemu-kvm, but it seems that the
> CPU hotplugging support there isn't quite ready. The guest just dies
> with "Assertion `bus->allow_hotplug' failed."
>
> I'll go digging for alternatives, but if anyone else could verify this
> then I'd appreciate it.

I just realized that you might not really want/need hotplugging, but
just booting with some cores initially offline?

That's easier to test at least:

qmitest:~# cat /proc/cmdline 
initrd=/initrd.img.test root=/dev/sda1 ro console=tty0 console=ttyS0,9600n8 maxcpus=1 BOOT_IMAGE=/vmlinuz.test 
qmitest:~#  grep . /sys/devices/system/cpu/o*line
/sys/devices/system/cpu/offline:1-3
/sys/devices/system/cpu/online:0
qmitest:~# grep . /sys/devices/system/cpu/cpu?/online 
/sys/devices/system/cpu/cpu1/online:0
/sys/devices/system/cpu/cpu2/online:0
/sys/devices/system/cpu/cpu3/online:0
qmitest:~# grep . /sys/devices/system/cpu/o*line
/sys/devices/system/cpu/offline:1-3
/sys/devices/system/cpu/online:0
qmitest:~# grep . /proc/sys/kernel/nmi_watchdog 
1


qmitest:~#  echo 1 > /sys/devices/system/cpu/cpu1/online
qmitest:~#  echo 1 > /sys/devices/system/cpu/cpu2/online
qmitest:~# grep . /sys/devices/system/cpu/cpu?/online 
/sys/devices/system/cpu/cpu1/online:1
/sys/devices/system/cpu/cpu2/online:1
/sys/devices/system/cpu/cpu3/online:0


No problem observed.  As expected.



Bjørn