Re: [Xen-devel] [PATCH] xen/arm: introduce vwfi parameter

["Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>]

On Tue, Feb 21, 2017 at 07:20:29PM +0000, Julien Grall wrote:
> 
> 
> On 21/02/2017 18:30, Stefano Stabellini wrote:
> >On Tue, 21 Feb 2017, Julien Grall wrote:
> >>Hi Stefano,
> >>
> >>On 21/02/17 17:49, Stefano Stabellini wrote:
> >>>On Tue, 21 Feb 2017, Dario Faggioli wrote:
> >>>>On Tue, 2017-02-21 at 13:46 +0000, George Dunlap wrote:
> >>>>Oh, actually, if --which I only now realize may be what you are
> >>>>referring to, since you're talking about "guest burning its credits"--
> >>>>you let the vCPU put the pCPU to sleep *but*, when it wakes up (or when
> >>>>the scheduler runs again for whatever reason), you charge to it for all
> >>>>the time the the pCPU was actually idle/sleeping, well, that may
> >>>>actually  not break scheduling, or cause disruption to the service of
> >>>>other vCPUs.... But indeed I'd consider it rather counter intuitive a
> >>>>behavior.
> >>>
> >>>How can this be safe? There could be no interrupts programmed to wake up
> >>>the pcpu at all. In fact, I don't think today there would be any, unless
> >>>we set one up in Xen for the specific purpose of interrupting the pcpu
> >>>sleep.
> >>>
> >>>I don't know the inner working of the scheduler, but does it always send
> >>>an interrupt to other pcpu to schedule something?
> >>
> >>You still seem to assume that WFI/WFE is the only way to get a vCPU
> >>unscheduled. If that was the case it would be utterly wrong because you 
> >>cannot
> >>expect a guest to use them.
> >>
> >>>
> >>>What if there are 2 vcpu pinned to the same pcpu? This cannot be fair.
> >>
> >>Why wouldn't it be fair? This is the same situation as a guest vCPU not 
> >>using
> >>WFI/WFE.
> >
> >I read your suggestion as trapping WFI in Xen, then, depending on
> >settings, executing WFI in the Xen trap handler to idle the pcpu. That
> >doesn't work. But I take you suggested not trapping wfi (remove
> >HCR_TWI), executing the instruction in guest context. That is what we
> >used to do in the early days (before a780f750). It should be safe and
> >possibly even quick. I'll rerun the numbers and let you know.
> 
> My first suggestion was to emulate WFI in Xen, which I agree is not safe :).
> 
> I think not trapping WFI will have the best performance but may impact the
> credit of the vCPU as mentioned by Dario and George.

I agree, wfi in guest context or at least with everything prepared to return to
the current guest would be great.

An option to enable this would work fine for our use-cases. Or if we could
at runtime detect that it's the best approach given scheduling (i.e
exclusive vCPU/pCPU pinning) even better.

Cheers,
Edgar


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