Re: [Xen-devel] [XEN PATCH 1/2] hvm: Support more than 32 VCPUS when migrating.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Apr 09, 2014 at 09:37:01AM +0200, Roger Pau Monné wrote:
> On 08/04/14 20:53, Konrad Rzeszutek Wilk wrote:
> > On Tue, Apr 08, 2014 at 08:18:48PM +0200, Roger Pau Monné wrote:
> >> On 08/04/14 19:25, konrad@xxxxxxxxxx wrote:
> >>> From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >>>
> >>> When we migrate an HVM guest, by default our shared_info can
> >>> only hold up to 32 CPUs. As such the hypercall
> >>> VCPUOP_register_vcpu_info was introduced which allowed us to
> >>> setup per-page areas for VCPUs. This means we can boot PVHVM
> >>> guest with more than 32 VCPUs. During migration the per-cpu
> >>> structure is allocated fresh by the hypervisor (vcpu_info_mfn
> >>> is set to INVALID_MFN) so that the newly migrated guest
> >>> can do make the VCPUOP_register_vcpu_info hypercall.
> >>>
> >>> Unfortunatly we end up triggering this condition:
> >>> /* Run this command on yourself or on other offline VCPUS. */
> >>>  if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) )
> >>>
> >>> which means we are unable to setup the per-cpu VCPU structures
> >>> for running vCPUS. The Linux PV code paths make this work by
> >>> iterating over every vCPU with:
> >>>
> >>>  1) is target CPU up (VCPUOP_is_up hypercall?)
> >>>  2) if yes, then VCPUOP_down to pause it.
> >>>  3) VCPUOP_register_vcpu_info
> >>>  4) if it was down, then VCPUOP_up to bring it back up
> >>>
> >>> But since VCPUOP_down, VCPUOP_is_up, and VCPUOP_up are
> >>> not allowed on HVM guests we can't do this. This patch
> >>> enables this.
> >>
> >> Hmmm, this looks like a very convoluted approach to something that could
> >> be solved more easily IMHO. What we do on FreeBSD is put all vCPUs into
> >> suspension, which means that all vCPUs except vCPU#0 will be in the
> >> cpususpend_handler, see:
> >>
> >> http://svnweb.freebsd.org/base/head/sys/amd64/amd64/mp_machdep.c?revision=263878&view=markup#l1460
> > 
> > How do you 'suspend' them? If I remember there is a disadvantage of doing
> > this as you have to bring all the CPUs "offline". That in Linux means using
> > the stop_machine which is pretty big hammer and increases the latency for 
> > migration.
> 
> In order to suspend them an IPI_SUSPEND is sent to all vCPUs except vCPU#0:
> 
> http://fxr.watson.org/fxr/source/kern/subr_smp.c#L289
> 
> Which makes all APs call cpususpend_handler, so we know all APs are
> stuck in a while loop with interrupts disabled:
> 
> http://fxr.watson.org/fxr/source/amd64/amd64/mp_machdep.c#L1459
> 
> Then on resume the APs are taken out of the while loop and the first
> thing they do before returning from the IPI handler is registering the
> new per-cpu vcpu_info area. But I'm not sure this is something that can
> be accomplished easily on Linux.

That is a bit of what the 'stop_machine' would do. It puts all of the
CPUs in whatever function you want. But I am not sure of the latency impact - as
in what if the migration takes longer and all of the CPUs sit there spinning.
Another variant of that is the 'smp_call_function'.

Then when we resume - we need a mailbox that is shared (easily enough
I think) to tell us that the migration has been done - and then need to call
that VCPUOP_register_vcpu_info.

But if the migration has taken quite long - I fear that the watchdogs
might kick in and start complaining about the CPUs stuck. Especially
if we migrating on overcommitted guest.

With this the latency for them to be 'paused', 'initted', 'unpaused' I
think is much much smaller.

Ugh, lets wait with this exercise of using the 'smp_call_function'
sometime at the end of the summer - and see. That functionality
should be shared with the PV code path IMHO.

> 
> I've tried to local-migrate a FreeBSD PVHVM guest with 33 vCPUs on my
> 8-way box, and it seems to be working fine :).

Awesome!
> 
> Roger.
> 

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