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

[Keir Fraser <keir.xen@gmail.com>]

On 16/03/2011 10:36, "Juergen Gross" <juergen.gross@ts.fujitsu.com> wrote:

> On 03/16/11 11:13, Keir Fraser wrote:
>> On 16/03/2011 06:34, "Juergen Gross"<juergen.gross@ts.fujitsu.com>  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.

Ah no, if it's more subtle than it appears, and it's not a straighjtforward
post-condition, then I wouldn't bother.

 -- Keir

> 
> Juergen