Re: [Xen-devel] Questions about the use of idle_vcpu[]

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jan 18, 2016 at 11:00 AM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
>> It looks like credit2's behavior will probably prevent the processor
>> from going into deeper power-saving states, and rtds' behavior might
>> cause it to essentially busy-wait.
>>
> RTDS behavior is broken in many respect, including this, and in fact,
> Meng and Tianyang are sending patches already to fix it (I'll let you
> guys have my comments shortly :-P).
>
> Credit2, AFAICR, could also avoid _always_ re-setting the timer, but it
> does need to do that at least a few times, even when idle is selected,
> because of the dynamic load tracking mechanism it includes. In fact,
> that is based on a 'decaying average', which in turns relies on
> csched2_schedule() to run and update the statistics, even when the cpu
> is idle. If we don't do that, the load tracking mechanism will never
> see that the cpu (well, it's actually the runqueue) is idle, and the
> load will never go down! :-/

I don't think that's true -- it looks like balance_load() will call
__update_runq_load() on the "other" runqueue before considering it,
and will also call __update_svc_load() on each vcpu before considering
it.  Shouldn't that suffice?

 -George

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