[PATCH 1/3] KVM: PPC: Book3S HV: exit halt polling on need_resched() as well

[PATCH 1/3] KVM: PPC: Book3S HV: exit halt polling on need_resched() as well

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

Inspired by commit 262de4102c7bb8 (kvm: exit halt polling on need_resched() 
as well), due to PPC implements an arch specific halt polling logic, we should 
add the need_resched() checking there as well.

Cc: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/powerpc/kvm/book3s_hv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 28a80d2..6199397 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3936,7 +3936,8 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
 				break;
 			}
 			cur = ktime_get();
-		} while (single_task_running() && ktime_before(cur, stop));
+		} while (single_task_running() && !need_resched() &&
+			 ktime_before(cur, stop));
 
 		spin_lock(&vc->lock);
 		vc->vcore_state = VCORE_INACTIVE;
-- 
2.7.4

[PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

In case of undercomitted scenarios, vCPU can get scheduling easily, 
kvm_vcpu_yield_to adds extra overhead, we can observe a lot of race 
between vcpu->ready is true and yield fails due to p->state is 
TASK_RUNNING. Let's bail out is such scenarios by checking the length 
of current cpu runqueue.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5bd550e..c0244a6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8358,6 +8358,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
 	struct kvm_vcpu *target = NULL;
 	struct kvm_apic_map *map;
 
+	if (single_task_running())
+		goto no_yield;
+
 	vcpu->stat.directed_yield_attempted++;
 
 	rcu_read_lock();
-- 
2.7.4

Re: [PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios

[Sean Christopherson]

On Sat, May 08, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> In case of undercomitted scenarios, vCPU can get scheduling easily, 
> kvm_vcpu_yield_to adds extra overhead, we can observe a lot of race 
> between vcpu->ready is true and yield fails due to p->state is 
> TASK_RUNNING. Let's bail out is such scenarios by checking the length 
> of current cpu runqueue.
> 
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5bd550e..c0244a6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8358,6 +8358,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
>  	struct kvm_vcpu *target = NULL;
>  	struct kvm_apic_map *map;
>  
> +	if (single_task_running())
> +		goto no_yield;
> +

Hmm, could we push the result of kvm_sched_yield() down into the guest?
Currently the guest bails after the first attempt, which is perfect for this
scenario, but it seems like it would make sense to keep trying to yield if there
are multiple preempted vCPUs and the "problem" was with the target.  E.g.

	/*
	 * Make sure other vCPUs get a chance to run if they need to.  Yield at
	 * most once, and stop trying to yield if the VMM says yielding isn't
	 * going to happen.
	 */
	for_each_cpu(cpu, mask) {
		if (vcpu_is_preempted(cpu)) {
			r = kvm_hypercall1(KVM_HC_SCHED_YIELD,
					   per_cpu(x86_cpu_to_apicid, cpu));
			if (r != -EBUSY)
				break;
		}
	}


Unrelated to this patch, but it's the first time I've really looked at the guest
side of directed yield...

Wouldn't it also make sense for the guest side to hook .send_call_func_single_ipi?

>  	vcpu->stat.directed_yield_attempted++;

Shouldn't directed_yield_attempted be incremented in this case?  It doesn't seem
fundamentally different than the case where the target was scheduled in between
the guest's check and the host's processing of the yield request.  In both
instances, the guest did indeed attempt to yield.

>  	rcu_read_lock();
> -- 
> 2.7.4
> 

Re: [PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios

[Wanpeng Li]

On Wed, 12 May 2021 at 05:44, Sean Christopherson <seanjc@google.com> wrote:
>
> On Sat, May 08, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > In case of undercomitted scenarios, vCPU can get scheduling easily,
> > kvm_vcpu_yield_to adds extra overhead, we can observe a lot of race
> > between vcpu->ready is true and yield fails due to p->state is
> > TASK_RUNNING. Let's bail out is such scenarios by checking the length
> > of current cpu runqueue.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> >  arch/x86/kvm/x86.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5bd550e..c0244a6 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8358,6 +8358,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
> >       struct kvm_vcpu *target = NULL;
> >       struct kvm_apic_map *map;
> >
> > +     if (single_task_running())
> > +             goto no_yield;
> > +
>
> Hmm, could we push the result of kvm_sched_yield() down into the guest?
> Currently the guest bails after the first attempt, which is perfect for this
> scenario, but it seems like it would make sense to keep trying to yield if there
> are multiple preempted vCPUs and

It can have a race in case of sustain yield if there are multiple
preempted vCPUs , the vCPU which you intend to yield may have already
completed to handle IPI and be preempted now when the yielded sender
is scheduled again and checks the next preempted candidate.

> the "problem" was with the target.  E.g.

At the beginning of kvm_sched_yield() we can just get the run queue
length of the source, it can be treated as a hint of under-committed
instead of guarantee of accuracy.

>
>         /*
>          * Make sure other vCPUs get a chance to run if they need to.  Yield at
>          * most once, and stop trying to yield if the VMM says yielding isn't
>          * going to happen.
>          */
>         for_each_cpu(cpu, mask) {
>                 if (vcpu_is_preempted(cpu)) {
>                         r = kvm_hypercall1(KVM_HC_SCHED_YIELD,
>                                            per_cpu(x86_cpu_to_apicid, cpu));
>                         if (r != -EBUSY)
>                                 break;
>                 }
>         }
>
>
> Unrelated to this patch, but it's the first time I've really looked at the guest
> side of directed yield...
>
> Wouldn't it also make sense for the guest side to hook .send_call_func_single_ipi?

reschedule ipi is called by .smp_send_reschedule hook, there are a lot
of researches intend to accelerate idle vCPU reactivation, my original
attemption is to boost synchronization primitive, I believe we need a
lot of benchmarkings to consider inter-VM fairness and performance
benefit for  hooks .send_call_func_single_ipi and
.smp_send_reschedule.

>
> >       vcpu->stat.directed_yield_attempted++;
>
> Shouldn't directed_yield_attempted be incremented in this case?  It doesn't seem
> fundamentally different than the case where the target was scheduled in between
> the guest's check and the host's processing of the yield request.  In both
> instances, the guest did indeed attempt to yield.

Yes, it should be treated as attempted, I move it above the counting
because this patch helps improve successful ratio in under-committed
scenarios and easily shows me how much failure ratio leaves over. I
can move it after the counting in the next version.

    Wanpeng

Re: [PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios

[Sean Christopherson]

On Wed, May 12, 2021, Wanpeng Li wrote:
> On Wed, 12 May 2021 at 05:44, Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Sat, May 08, 2021, Wanpeng Li wrote:
> > > From: Wanpeng Li <wanpengli@tencent.com>
> > >
> > > In case of undercomitted scenarios, vCPU can get scheduling easily,
> > > kvm_vcpu_yield_to adds extra overhead, we can observe a lot of race
> > > between vcpu->ready is true and yield fails due to p->state is
> > > TASK_RUNNING. Let's bail out is such scenarios by checking the length
> > > of current cpu runqueue.
> > >
> > > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > > ---
> > >  arch/x86/kvm/x86.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 5bd550e..c0244a6 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -8358,6 +8358,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
> > >       struct kvm_vcpu *target = NULL;
> > >       struct kvm_apic_map *map;
> > >
> > > +     if (single_task_running())
> > > +             goto no_yield;
> > > +
> >
> > Hmm, could we push the result of kvm_sched_yield() down into the guest?
> > Currently the guest bails after the first attempt, which is perfect for this
> > scenario, but it seems like it would make sense to keep trying to yield if there
> > are multiple preempted vCPUs and
> 
> It can have a race in case of sustain yield if there are multiple
> preempted vCPUs , the vCPU which you intend to yield may have already
> completed to handle IPI and be preempted now when the yielded sender
> is scheduled again and checks the next preempted candidate.

Ah, right, don't want to penalize the happy case.

> > Unrelated to this patch, but it's the first time I've really looked at the guest
> > side of directed yield...
> >
> > Wouldn't it also make sense for the guest side to hook .send_call_func_single_ipi?
> 
> reschedule ipi is called by .smp_send_reschedule hook, there are a lot
> of researches intend to accelerate idle vCPU reactivation, my original
> attemption is to boost synchronization primitive, I believe we need a
> lot of benchmarkings to consider inter-VM fairness and performance
> benefit for  hooks .send_call_func_single_ipi and
> .smp_send_reschedule.

I was thinking of the 2 vCPU case.  If the VM has 2 vCPUs, then this

	/*
	 * Choose the most efficient way to send an IPI. Note that the
	 * number of CPUs might be zero due to concurrent changes to the
	 * provided mask.
	 */
	if (nr_cpus == 1)
		send_call_function_single_ipi(last_cpu);
	else if (likely(nr_cpus > 1))
		arch_send_call_function_ipi_mask(cfd->cpumask_ipi);

means .send_call_func_single_ipi() will always be used to send an IPI to the
other vCPU, and thus 2 vCPU VMs will never utilize PV yield.

[PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point of view

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

Commit 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's 
CPUID) avoids to access pv tlb shootdown host side logic when this pv feature 
is not exposed to guest, however, kvm_steal_time.preempted not only leveraged 
by pv tlb shootdown logic but also mitigate the lock holder preemption issue. 
From guest point of view, vCPU is always preempted since we lose the reset of
kvm_steal_time.preempted before vmentry if pv tlb shootdown feature is not 
exposed. This patch fixes it by clearing kvm_steal_time.preempted before 
vmentry.

Fixes: 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's CPUID)
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/x86.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c0244a6..c38e990 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3105,7 +3105,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 				       st->preempted & KVM_VCPU_FLUSH_TLB);
 		if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
 			kvm_vcpu_flush_tlb_guest(vcpu);
-	}
+	} else
+		st->preempted = 0;
 
 	vcpu->arch.st.preempted = 0;
 
-- 
2.7.4

Re: [PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point of view

[Sean Christopherson]

On Sat, May 08, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Commit 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's 
> CPUID) avoids to access pv tlb shootdown host side logic when this pv feature 
> is not exposed to guest, however, kvm_steal_time.preempted not only leveraged 
> by pv tlb shootdown logic but also mitigate the lock holder preemption issue. 
> From guest point of view, vCPU is always preempted since we lose the reset of
> kvm_steal_time.preempted before vmentry if pv tlb shootdown feature is not 
> exposed. This patch fixes it by clearing kvm_steal_time.preempted before 
> vmentry.
> 
> Fixes: 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's CPUID)
> Cc: stable@vger.kernel.org
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c0244a6..c38e990 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3105,7 +3105,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>  				       st->preempted & KVM_VCPU_FLUSH_TLB);
>  		if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
>  			kvm_vcpu_flush_tlb_guest(vcpu);
> -	}
> +	} else
> +		st->preempted = 0;

Curly braces needed since the if-statment needs 'em.  Other than that,

Reviewed-by: Sean Christopherson <seanjc@google.com>

>  
>  	vcpu->arch.st.preempted = 0;
>  
> -- 
> 2.7.4
> 

Re: [PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point of view

[Wanpeng Li]

On Tue, 11 May 2021 at 08:18, Sean Christopherson <seanjc@google.com> wrote:
>
> On Sat, May 08, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > Commit 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's
> > CPUID) avoids to access pv tlb shootdown host side logic when this pv feature
> > is not exposed to guest, however, kvm_steal_time.preempted not only leveraged
> > by pv tlb shootdown logic but also mitigate the lock holder preemption issue.
> > From guest point of view, vCPU is always preempted since we lose the reset of
> > kvm_steal_time.preempted before vmentry if pv tlb shootdown feature is not
> > exposed. This patch fixes it by clearing kvm_steal_time.preempted before
> > vmentry.
> >
> > Fixes: 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's CPUID)
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> >  arch/x86/kvm/x86.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index c0244a6..c38e990 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3105,7 +3105,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> >                                      st->preempted & KVM_VCPU_FLUSH_TLB);
> >               if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
> >                       kvm_vcpu_flush_tlb_guest(vcpu);
> > -     }
> > +     } else
> > +             st->preempted = 0;
>
> Curly braces needed since the if-statment needs 'em.  Other than that,

Will send out a new version after 1-2 get reviewed. :)

>
> Reviewed-by: Sean Christopherson <seanjc@google.com>
>
> >
> >       vcpu->arch.st.preempted = 0;
> >
> > --
> > 2.7.4
> >

Re: [PATCH 1/3] KVM: PPC: Book3S HV: exit halt polling on need_resched() as well

[Wanpeng Li]

Cc more guys,
On Sat, 8 May 2021 at 17:32, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> From: Wanpeng Li <wanpengli@tencent.com>
>
> Inspired by commit 262de4102c7bb8 (kvm: exit halt polling on need_resched()
> as well), due to PPC implements an arch specific halt polling logic, we should
> add the need_resched() checking there as well.
>

Update the patch description:

Inspired by commit 262de4102c7bb8 (kvm: exit halt polling on need_resched()
as well), CFS_BANDWIDTH throttling will use resched_task() when there is just
one task to get the task to block. It was likely allowing VMs to overrun their
quota when halt polling. Due to PPC implements an arch specific halt polling
logic, we should add the need_resched() checking there as well.

> Cc: Paul Mackerras <paulus@ozlabs.org>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 28a80d2..6199397 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3936,7 +3936,8 @@ static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
>                                 break;
>                         }
>                         cur = ktime_get();
> -               } while (single_task_running() && ktime_before(cur, stop));
> +               } while (single_task_running() && !need_resched() &&
> +                        ktime_before(cur, stop));
>
>                 spin_lock(&vc->lock);
>                 vc->vcore_state = VCORE_INACTIVE;
> --
> 2.7.4
>