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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 09:58:37 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] [PATCH] Avoid endless loop for vcpu migration
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On 15/03/2011 09:21, "Juergen Gross" <juergen.gross@xxxxxxxxxxxxxx> wrote:

>> That candidate-is-valid check seems absolutely independent of the
>> particular scheduler used, and hence could be done in the (sole)
>> caller, thus not requiring any change to the scheduler interface.
>> Which at once would eliminate unnecessary calls into pick_cpu (i.e.
>> you'd call it a second time only if the previously selected CPU really
>> is no longer valid to be used for that vCPU).
> True.
> The patch seems to become smaller :-)

By the way, why is the cpu_isset(new_cpu, v->domain->vcpupool->cpu_valid)
check required (after calling pick_cpu, in the cuirrently checked-in code)?
You already check that pick_cpu was called holding the correct pair of
locks, if it has returned a cpu that is not in the pool's cpu_valid mask,
what would make pick_cpu return anything different on the next invocation
thus avoiding an endless loop?

Looks like this question would remain even if this new patch was applied.

 -- Keir

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