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

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Avoid endless loop for vcpu migration
From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
Date: Tue, 15 Mar 2011 06:50:59 +0100
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On 03/14/11 16:03, Jan Beulich wrote:
On 14.03.11 at 15:39, Juergen Gross<juergen.gross@xxxxxxxxxxxxxx>  wrote:
On multi-thread multi-core systems an endless loop can occur in vcpu_migrate()
with credit scheduler. Avoid this loop by changing the interface of pick_cpu
to indicate a repeated call in this case.

But you're not changing in any way the loop that doesn't get
exited - did you perhaps read my original description as the
pick function itself looping (which - afaict - it doesn't)?

I'm changing the way the pick_cpu function is reacting on multiple calls in
a loop. If I've understood the idle_bias correctly, updating it in each
loop iteration did result in returning another cpu for each call.
By updating idle_bias only once, it should return the same cpu in subsequent
calls. This should exit the loop in vcpu_migrate.

Further, the change still isn't consistent with idle_bias - the
updating ought to happen on the last iteration (if you need
to call the function more than once), not the first one, which
creates a chicken-and-egg problem for you as you will know
it's the last one only when it returned.

Is it really so important idle_bias is reflecting the last cpu selected?
I was under the impression it should be okay when this is true in most
cases. With my patch idle_bias might be "wrong" if there is a race with
other cpus forcing a selection of a different cpu in the second iteration
of the loop in vcpu_migrate. Is this really critical? I doubt it.


Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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