[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [XEN PATCH 1/2] hvm: Support more than 32 VCPUS when migrating.
On 09/04/14 16:34, Konrad Rzeszutek Wilk wrote: > 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'. I tested stop_machine() on all CPUs during suspend once and it was awful: 100s of ms of additional downtime. Perhaps a hand-rolled IPI-and-park-in-handler would be quicker the full stop_machine(). David _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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