Re: [Xen-devel] [Patch] remove unneeded test for cpu in cpupool

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 03/16/11 11:13, Keir Fraser wrote:
> On 16/03/2011 06:34, "Juergen Gross"<juergen.gross@xxxxxxxxxxxxxx>  wrote:
>
> > Hi,
> >
> > attached patch removes an unneeded check in vcpu_migrate().
> >
> > Andre, could you please give it a try on your 48 core machine with
> >
> > xl cpupool-numa-split
> >
> > I'd like to make sure the check is really not needed. On my machines it worked
> > okay, but your machine seems to trigger races more easily.
> Could you BUG_ON failing this test when we exit the retry loop? It seems a
> suitable loop post-condition.
Not easily. The cpu_valid mask of a cpupool might change between the call of
pick_cpu and the test. In the normal case this is okay, as removing a cpu
from a cpupool (which is the critical case) will call cpu_disable_scheduler()
after removing the cpu from the mask, which will migrate the vcpu away.

Using a saved copy of cpu_valid for the test isn't working either, if a cpu
is added to and removed from the cpupool in a very short time. I would have
to use a generation count for the cpu_valid mask and do the check only if
the count didn't change. I think this is little bit of overkill here, but if
you'd prefer it, I can do a patch.


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
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Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
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