Re: [Xen-devel] [PATCH v1 2/5] xen: sched: null: don't assign down vcpus to pcpus

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Jul 16, 2019, at 11:50 AM, Dario Faggioli <dfaggioli@xxxxxxxx> wrote:
> 
> On Mon, 2019-07-15 at 17:06 +0100, George Dunlap wrote:
>> On 8/25/18 1:21 AM, Dario Faggioli wrote:
>>> If a pCPU has been/is being offlined, we don't want it to be
>>> neither
>>> assigned to any pCPU, nor in the wait list.
>> 
>> I take it the first `pCPU` should be `vCPU`?
>> 
> Indeed. :-)
> 
>> Also, English grammar agreement is funny: `neither` needs to agree
>> with
>> `nor`, but the two do *not* agree with the original verb.  That is,
>> the
>> sentence should say:
>> 
>> "...we want it to be neither assigned to pCPU, nor in the wait list".
>> 
> Yep, now that I see it, this rings a bell back from my high-school
> English class! :-O
> 
> Sorry... and thanks. :-)
> 
>> Both here and in the comment.
>> 
> And in patch 3 changelog too, I'm afraid. :-P
> 
>> The other thing is, from a "developmental purity" point of view, I
>> think
>> this series technically has a regression in the middle: cpu offline /
>> online stops working between patch 2 and patch 4.  But I'm inclined
>> in
>> this case not to worry too much. :-)
>> 
> Well, the point is that offlining/onlining does not work before this
> series. System does not crash, but behavior is wrong, as offline vCPUs
> stay assigned to pCPUs (keeping them idle) while online vCPUs are
> "trapped" in the wait list, which is wrong.
> 
> So that's why I don't think there's much value in being consistent with
> such behavior throughout the series... which I guess is why you said
> you "won't worry too much in this case” ?

It’s definitely sub-optimal from a system point of view; but from a guest point 
of view, it does (or should) function.  Before this series, if a guest offline 
and then online vcpus, they should come back.  In the middle of this series, 
once a vcpu is offlined, it can’t be brought back up.  (That is, if I’m reading 
it right.)

But I’m not expecting people to be doing bisections of that particular 
functionality in this particular scheduler very much.  I think the “benefit” of 
avoiding a complicated re-write is well worth the “cost” of having that 
particular bisection fail, on the off chance that anyone tries it.

 -George

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