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Re: [Xen-devel] [PATCH 0/3] VMX: Properly handle pi descriptor and per-cpu blocking list

On Mon, 2016-05-23 at 02:51 -0600, Jan Beulich wrote:
> > > > On 23.05.16 at 10:44, <feng.wu@xxxxxxxxx> wrote:
> > > 
> > > vCPU-s currently having their v->processor set to the pCPU being
> > > hot removed would simply get migrated elsewhere. If that's not
> > > accompanied by respective PI blocking list adjustments, then I
> > > can
> > > only refer you back to the original series' review process, where
> > > it was (iirc) more than once that it was pointed out that getting
> > > out
> > > of sync v->processor and the PI list a vCPU sits on may casue
> > > issues. And if there is an issue here, I'm not sure what a good
> > > solution would be.
> > Okay, here is my understanding about the blocking and vCPU
> > migration things, if vCPU is blocking and its v->processor is pCPU
> > 0,
> > at some point, pCPU0 is removed from the system, it will also
> > migrate the vCPU to another pCPU, say, pCPU1 as the response
> > to pCPU0 being unplugged, hence, v->processor = pCPU1, right?
> Yes.
See, for instance, cpu_disable_scheduler() in schedule.c. What we do is
go over all the vcpus of all domains of either the system or the
cpupool, and force the ones that we found with v->processor set to the
pCPU that is going down, to perform migration (system_state will be
different than SYS_STATE_suspend, and we hence take the 'else' branch).

Considering that the pCPU is no longer part of the relevant bitmask-s
during the migration, the vCPUs will figure out that they just can't
stay there, and move somewhere else.

Note, however, that this happens for running and runnable vCPUs. If a
vCPU is blocker, there is nothing to do, and in fact, nothing happens
(as vcpu_sleep_nosync() and vcpu_wake() are NOP in that case). For
those vCPUs, as soon as they wake up, they'll figure out that their own
v->processor is not there any longer, and will move somewhere else.

> > If that is the case, I think the current code may have issues in
> > the
> > above case, since 'NDST' filed in PI descriptor is still vCPU0 even
> > it is removed from the system. I need to consider how to handle
> > this case. 
Exactly. I don't think you can do that, for instance, from within
cpu_disable_scheduler(), or anythign that gets called from there, as it
basically deals with running vCPUs, while you're interested in blocked

I wonder whether you can register your own notifier, and, inside it,
scan the blocked list and fixup things... (just the first idea that
came up in my mind)

> > But this is not an issue  in non pCPU hotplug scenario.
> > 
It's probably an issue even if you remove a cpu from a cpupool (and
even a more "interesting" one, if you also manage to add it to another
pool, in the meantime) isn't it?

<<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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