Re: [PATCH v2 3/7] KVM-HV: KVM Steal time implementation

[Glauber Costa]

On 06/20/2011 05:56 PM, Marcelo Tosatti wrote:
> On Sun, Jun 19, 2011 at 12:57:53PM +0300, Avi Kivity wrote:
> > On 06/17/2011 01:20 AM, Glauber Costa wrote:
> > > To implement steal time, we need the hypervisor to pass the guest information
> > > about how much time was spent running other processes outside the VM.
> > > This is per-vcpu, and using the kvmclock structure for that is an abuse
> > > we decided not to make.
> > >
> > > In this patchset, I am introducing a new msr, KVM_MSR_STEAL_TIME, that
> > > holds the memory area address containing information about steal time
> > >
> > > This patch contains the hypervisor part for it. I am keeping it separate from
> > > the headers to facilitate backports to people who wants to backport the kernel
> > > part but not the hypervisor, or the other way around.
> > >
> > >
> > >
> > > +#define KVM_STEAL_ALIGNMENT_BITS 5
> > > +#define KVM_STEAL_VALID_BITS ((-1ULL<<   (KVM_STEAL_ALIGNMENT_BITS + 1)))
> > > +#define KVM_STEAL_RESERVED_MASK (((1<<   KVM_STEAL_ALIGNMENT_BITS) - 1 )<<   1)
> >
> > Clumsy, but okay.
> >
> > > +static void record_steal_time(struct kvm_vcpu *vcpu)
> > > +{
> > > +	u64 delta;
> > > +
> > > +	if (vcpu->arch.st.stime&&   vcpu->arch.st.this_time_out) {
> >
> > 0 is a valid value for stime.
> >
> > > +
> > > +		if (unlikely(kvm_read_guest(vcpu->kvm, vcpu->arch.st.stime,
> > > +			&vcpu->arch.st.steal, sizeof(struct kvm_steal_time)))) {
> > > +
> > > +			vcpu->arch.st.stime = 0;
> > > +			return;
> > > +		}
> > > +
> > > +		delta = (get_kernel_ns() - vcpu->arch.st.this_time_out);
> > > +
> > > +		vcpu->arch.st.steal.steal += delta;
> > > +		vcpu->arch.st.steal.version += 2;
> > > +
> > > +		if (unlikely(kvm_write_guest(vcpu->kvm, vcpu->arch.st.stime,
> > > +			&vcpu->arch.st.steal, sizeof(struct kvm_steal_time)))) {
> > > +
> > > +			vcpu->arch.st.stime = 0;
> > > +			return;
> > > +		}
> > > +	}
> > > +
> > > +}
> > > +
> > >
> > > @@ -2158,6 +2206,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > >   			kvm_migrate_timers(vcpu);
> > >   		vcpu->cpu = cpu;
> > >   	}
> > > +
> > > +	record_steal_time(vcpu);
> > >   }
> >
> > This records time spent in userspace in the vcpu thread as steal
> > time.  Is this what we want?  Or just time preempted away?
>
> It also accounts halt time (kvm_vcpu_block) as steal time. Glauber, you
> could instead use the "runnable-state-but-waiting-in-runqueue" field of
> SCHEDSTATS, i forgot the exact name.
I thought about it in the past. I let the idea aside because I didn't 
want to introduce a dependency on SCHEDSTATS. But thinking about it 
again now (and after some days of experimentations with it), I think we 
could have both.

use run_delay (the field you were thinking of) when schedstats are 
available, and fallback to an estimate method like the one we're doing 
when it is not.

Objections ?
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