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

[Tianyang Chen <tiche@xxxxxxxxxxxxxx>]

On 1/18/2016 11:07 AM, Meng Xu wrote:
> On Mon, Jan 18, 2016 at 6:00 AM, Dario Faggioli
> <dario.faggioli@xxxxxxxxxx> wrote:
> > On Mon, 2016-01-18 at 10:47 +0000, George Dunlap wrote:
> > > On Fri, Jan 15, 2016 at 1:04 AM, Tianyang Chen <tiche@xxxxxxxxxxxxxx>
> > >
> > > > If an idle vcpu is picked, the ret.time is set accordingly in both
> > > > credit
> > > > and credit2 by checking whether snext is idle. if so, credit
> > > > returns -1 and
> > > > credit2 returns 2ms. However, there is no corresponding code in the
> > > > RTDS
> > > > scheduler to handle this. When an idle_vcpu is picked, the value of
> > > > ret.time
> > > > would be 0 and the scheduler would be invoked again. What is the
> > > > logic
> > > > behind this?
> > > No real logic, as far as I can tell. :-)  The ret.time return value
> > > tells the generic scheduling code when to set the next scheduler
> > > timer.  According to the comment in xen/common/schedule.c:schedule(),
> > > returning a negative value means "don't bother setting a timer"
> > > (e.g.,
> > > no time limit).  So credit1 does the right thing.
> > It does.
>
> Then the RTDS is doing *incorrectly* right now. :-(
George: Thanks. After looking at idle_loop() it makes sense now. Even 
though an idle vcpu won't tell scheduler timer when to fire next time, 
do_tasklet() checks if all tasklets on the list are finished and then 
raise SCHEDULE_SOFTIRQ.
> > > 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).
>
> Right. Tianyang and I are working on changing it from quantum driven
> model to event-driven (or called timer-driven) model. Tianyang sent
> out the first-version patch, but that version has some problems. He is
> working on the second version now.
>
> Hi Dario,
> Tianyang is working on the second version right now.
> If you could have a quick look at our discussion in that thread and
> points out the "serious" issues in the decision, that will be great!
> We won't repeat the error again and again in the following versions.
> As to the minor issues, we could refine it in the second version.
> (I'm just thinking about how to save your time to have this done. For
> the obvious things that I can handle, I will do it and avoid "wasting"
> you time. For the design choices that we are unclear, we definitely
> need your insights/commands. ;-) )
Dario: I had some discussion with Meng recently and the second version 
will soon come out. You can directly comment on it if that saves you 
some time.
Thanks,
Tianyang

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