Re: [Xen-devel] [PATCH] Avoid endless loop for vcpu migration

To:	Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
Subject:	Re: [Xen-devel] [PATCH] Avoid endless loop for vcpu migration
From:	Keir Fraser <keir.xen@xxxxxxxxx>
Date:	Tue, 15 Mar 2011 08:50:20 +0000
Cc:	xen-devel@xxxxxxxxxxxxxxxxxxx
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On 15/03/2011 08:46, "Juergen Gross" <juergen.gross@xxxxxxxxxxxxxx> wrote:

>> It's not critical, and not affecting correctness. But with updating
>> idle_bias on the first invocation you're (on the right hardware)
>> basically guaranteeing the second invocation to return a
>> different CPU. That way, your loop will be run minimally three
>> times on such systems. I already find it odd to require two
>> iterations when previously this was a strait code path.
> 
> This was wrong. It was always required to hold the schedule lock of the
> picked cpu as well, otherwise a race with cpu hotplug would be possible.

What would that race be? CPU offlining is done in stop_machine context.

 -- Keir



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Re: [Xen-devel] [PATCH] Avoid endless loop for vcpu migration