Re: [Xen-devel] Fwd: [v3 14/15] Update Posted-Interrupts Descriptor during vCPU scheduling

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2015-07-09 at 15:27 +0100, George Dunlap wrote:
> On 07/09/2015 03:18 PM, Dario Faggioli wrote:
> > On Thu, 2015-07-09 at 14:44 +0100, Jan Beulich wrote:
> >>>>> On 09.07.15 at 14:53, <george.dunlap@xxxxxxxxxxxxx> wrote:
> > 
> >>> Consider the following scenario:
> >>> - v1 blocks on pcpu 0.
> >>>  - vcpu_runstate_change() will do everything necessary for v1 on p0.
> >>> - The scheduler does load balancing and moves v1 to p1, calling
> >>> vcpu_migrate().  Because the vcpu is still blocked,
> >>> vcpu_runstate_change() is not called.
> >>> - A device interrupt is generated.
> >>>
> >>> What happens to the interrupt?  Does everything still work properly, or
> >>> will the device wake-up interrupt go to the wrong pcpu (p0 rather than 
> >>> p1)?
> >>
> >> I think much of this was discussed before, since I also disliked the
> >> hooking into vcpu_runstate_change(). What I remember having
> >> been told is that it really only matters which pCPU's list a vCPU is
> >> on, not what v->processor says. 
> >>
> > Right.
> > 
> > But, as far as I could understand from the patches I've seen, a vcpu
> > ends up in a list when it blocks, and when it blocks there will be a
> > context switch, and hence we can deal with the queueing during the the
> > context switch itself (which is, in part, an arch specific operation
> > already).
> > 
> > What am I missing?
> 
> I think what you're missing is that Jan is answering my question about
> migrating a blocked vcpu, not arguing that vcpu_runstate_change() is the
> right way to go.  At least that's how I understood him. :-)
> 
Eheh, no, I got that... I just wanted to take one more chance to try to
get an answer (from Feng, mainly, but from anyone who knows or has an
idea, in reality! :-D) on why another approach is not possible.

Sorry for abusing the reply for my own mean purposes. :-D

> But regarding context_switch: I think the reason we need more hooks than
> that is that context_switch only changes into and out of running state.
>
Sure.

> There are also changes that need to happen when you change from blocked
> to offline, offline to blocked, blocked to runnable, &c; these don't go
> through context_switch.  
>
Right, those ones. And about them, I could say for the third time that
having a non-runstate biased description of what we need would help to
understand where to do things, whether there are suitable spots already
or we need to add new ones, etc.... But I guess I don't want to be so
repetitive! :-PP

> That's why I was suggesting some architectural
> equivalents to the SCHED_OP() callbacks to be added to vcpu_wake &c.
> 
Yep. If (or at least for those of which) really there is no other place
already, adding these would be TheRightThing for me as well.

> vcpu_runstate_change() is at the moment a nice quiet cul-de-sac that
> just does a little bit of accounting; 
>
Indeed!

> I'd rather not have it suddenly
> become a major thoroughfare for runstate change hooks, if we can avoid
> it. :-)
> 
Yep, that was my understanding of it, and my idea too.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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