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

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 03/15/11 10:01, Jan Beulich wrote:
> > > > On 15.03.11 at 09:46, Juergen Gross<juergen.gross@xxxxxxxxxxxxxx>  wrote:
> > On 03/15/11 08:57, Jan Beulich wrote:
> > > > > > On 15.03.11 at 06:50, Juergen Gross<juergen.gross@xxxxxxxxxxxxxx>   wrote:
> > > > 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.
> > > You're only decreasing the likelihood of a live lock, as the return
> > > value of pick_cpu not only depends on idle_bias.
> > Hmm, then another solution would be to let pick_cpu really return the
> > proposed cpu from the first iteration, if it doesn't contradict the
> > allowed settings. It could be sub-optimal, but I don't think this is
> > critical, as vcpu_migrate is called rarely.
> >
> > Patch attached.
> 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 is no longer valid to be used for that vCPU).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 :-)


Juergen

--
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
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Attachment: vcpu_migrate.patch
Description: Text Data

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