linux-kernel - Re: [PATCH 1/2] x86: kvm: rate-limit global clock updates

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Date:	Thu, 27 Feb 2014 14:11:03 +0100
From:	Andrew Jones <drjones@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	pbonzini@...hat.com
Subject: Re: [PATCH 1/2] x86: kvm: rate-limit global clock updates

On Wed, Feb 26, 2014 at 05:25:26PM -0300, Marcelo Tosatti wrote:
> On Wed, Feb 26, 2014 at 07:15:11PM +0100, Andrew Jones wrote:
> > When we update a vcpu's local clock it may pick up an NTP correction.
> > We can't wait an indeterminate amount of time for other vcpus to pick
> > up that correction, so commit 0061d53daf26f introduced a global clock
> > update. However, we can't request a global clock update on every vcpu
> > load either (which is what happens if the tsc is marked as unstable).
> > The solution is to rate-limit the global clock updates. Marcelo
> > calculated that we should delay the global clock updates no more
> > than 0.1s as follows:
> > 
> > Assume an NTP correction c is applied to one vcpu, but not the other,
> > then in n seconds the delta of the vcpu system_timestamps will be
> > c * n. If we assume a correction of 500ppm (worst-case), then the two
> > vcpus will diverge 100us in 0.1s, which is a considerable amount.
> 
> 100us -> 50us.

doh, will fix.

> 
> > Signed-off-by: Andrew Jones <drjones@...hat.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  1 +
> >  arch/x86/kvm/x86.c              | 33 +++++++++++++++++++++++++++++----
> >  2 files changed, 30 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index e714f8c08ccf2..9aa09d330a4b5 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -598,6 +598,7 @@ struct kvm_arch {
> >  	bool use_master_clock;
> >  	u64 master_kernel_ns;
> >  	cycle_t master_cycle_now;
> > +	struct delayed_work kvmclock_update_work;
> >  
> >  	struct kvm_xen_hvm_config xen_hvm_config;
> >  
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 4cca45853dfeb..a2d30de597b7d 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1628,20 +1628,43 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> >   * the others.
> >   *
> >   * So in those cases, request a kvmclock update for all vcpus.
> > - * The worst case for a remote vcpu to update its kvmclock
> > - * is then bounded by maximum nohz sleep latency.
> > + * We need to rate-limit these requests though, as they can
> > + * considerably slow guests that have a large number of vcpus.
> > + * The time for a remote vcpu to update its kvmclock is bound
> > + * by the delay we use to rate-limit the updates.
> >   */
> >  
> > -static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
> > +#define KVMCLOCK_UPDATE_DELAY msecs_to_jiffies(100)
> > +
> > +static void kvmclock_update_fn(struct work_struct *work)
> >  {
> >  	int i;
> > -	struct kvm *kvm = v->kvm;
> > +	struct delayed_work *dwork = to_delayed_work(work);
> > +	struct kvm_arch *ka = container_of(dwork, struct kvm_arch,
> > +					   kvmclock_update_work);
> > +	struct kvm *kvm = container_of(ka, struct kvm, arch);
> >  	struct kvm_vcpu *vcpu;
> >  
> >  	kvm_for_each_vcpu(i, vcpu, kvm) {
> >  		set_bit(KVM_REQ_CLOCK_UPDATE, &vcpu->requests);
> >  		kvm_vcpu_kick(vcpu);
> >  	}
> > +	kvm_put_kvm(kvm);
> > +}
> 
> Can cancel_work_sync on vm shutdown instead of get/put kvm ?
> 
> (somewhat annoying for vm to not go down immediatelly).

OK

> 
> > +static void kvm_schedule_kvmclock_update(struct kvm *kvm)
> > +{
> > +	kvm_get_kvm(kvm);
> > +	schedule_delayed_work(&kvm->arch.kvmclock_update_work,
> > +					KVMCLOCK_UPDATE_DELAY);
> > +}
> > +
> > +static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
> > +{
> > +	struct kvm *kvm = v->kvm;
> > +
> > +	set_bit(KVM_REQ_CLOCK_UPDATE, &v->requests);
> > +	kvm_schedule_kvmclock_update(kvm);
> >  }
> >  
> >  static bool msr_mtrr_valid(unsigned msr)
> > @@ -7019,6 +7042,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >  
> >  	pvclock_update_vm_gtod_copy(kvm);
> >  
> > +	INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
> > +
> >  	return 0;
> >  }
> >  
> > -- 
> > 1.8.1.4
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