Re: [Xen-devel] [PATCH v6 11/13] xen: support the Null scheduler

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 2 Jul 2018, Julien Grall wrote:
> Hi Stefano,
> 
> On 06/29/2018 07:38 PM, Stefano Stabellini wrote:
> > On Thu, 28 Jun 2018, Roger Pau Monné wrote:
> > > On Thu, Jun 28, 2018 at 09:27:08AM +0200, Dario Faggioli wrote:
> > > > On Thu, 2018-06-14 at 13:20 -0700, Stefano Stabellini wrote:
> > > > > On Thu, 14 Jun 2018, Andrew Cooper wrote:
> > > > > > On 14/06/18 14:40, Jan Beulich wrote:
> > > > > > I don't think its reasonable to alter the support status with this
> > > > > issue
> > > > > > outstanding.
> > > > > 
> > > > > I completely missed this report, probably because I haven't paid
> > > > > attention to PV-shim. Do you have any more information about this?
> > > > > The
> > > > > report is a bit vague. If I can't repro it, I can't fix it.
> > > > > 
> > > > > Couldn't it be that is normal because after a while you ran out of
> > > > > pcpus?
> > > > > 
> > > > > Dario, do you have any opinion on this?
> > > > > 
> > > > The issue that I know of is that the null scheduler does not properly
> > > > support CPU hotplug/hotunplug.
> > > > 
> > > > This is an issue on, let's say, baremetal, if you use null, and try to
> > > > do CPU hotplug/hotunplug. When trying to use null as the scheduler of
> > > > the shim, we run into that same issue, even if not specifically doing
> > > > CPU hotplug/hotunplug (because the shim use the same path for CPU
> > > > bringup, IIRC).
> > > 
> > > The shim uses CPU hotplug/unplug when the guest brings up/down a
> > > vCPU using the VCPUOP_{up/down} hypercall.
> > > 
> > > The best description of the issue I could find is:
> > > 
> > > https://lists.xenproject.org/archives/html/xen-devel/2018-01/msg01085.html
> > 
> > OK, thanks for the explanation. We don't support CPU hotplug on ARM, so
> > we could mark the NULL scheduler as supported on the ARM architecture
> > today? Once you implement CPU hotplug support in NULL, we could mark it
> > as supported on x86 too.
> Well, Mirela paved the way to support CPU hotplug (should be merged soon). She
> is looking at suspend/resume which is IHMO an extension of hotplug case. So
> are you sure this could never happen on Arm?

I thought that suspend/resume didn't actually require the same kind of
scheduler support that CPU hotplug needs. If suspend/resume ends up
not working with scheduler NULL, then that is a problem.

Real CPU hotplug is unlikely though -- do you know of any ARM platforms
that support it? Doesn't it require an actual board with physically
pluggable CPUs?

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel