Re: [Xen-devel] [PATCH v2 30/48] xen/sched: introduce unit_runnable_state()

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Fri, 2019-08-09 at 16:58 +0200, Juergen Gross wrote:
> Today the vcpu runstate of a new scheduled vcpu is always set to
> "running" even if at that time vcpu_runnable() is already returning
> false due to a race (e.g. with pausing the vcpu).
> 
> With core scheduling this can no longer work as not all vcpus of a
> schedule unit have to be "running" when being scheduled. So the
> vcpu's
> new runstate has to be selected at the same time as the runnability
> of
> the related schedule unit is probed.
> 
So, you're saying that this patch is meant at fixing a race. A race
that, if I understood correctly, it's not there/it's not problematic
right now, but will be when we have more than 1 vcpu in a unit and we
enable core scheduling. Is this the case?

If yes:
- this very patch, at this point in the series, is basically
introducing no functional (or at least behavioral) change, is this
right too?
- can you provide some more detail about the race. When/how it occurs
and how the changes done in credit and rt fix it?

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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