linux-kernel - Re: [PATCH v2 2/3] KVM: x86: Include host suspended time in steal time.

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Message-ID: <ZsWJsPkrhDReU4ez@intel.com>
Date: Wed, 21 Aug 2024 14:31:12 +0800
From: Chao Gao <chao.gao@...el.com>
To: Suleiman Souhlal <suleiman@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
	<seanjc@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
	<mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, "H. Peter Anvin"
	<hpa@...or.com>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<ssouhlal@...ebsd.org>
Subject: Re: [PATCH v2 2/3] KVM: x86: Include host suspended time in steal
 time.

On Tue, Aug 20, 2024 at 01:35:42PM +0900, Suleiman Souhlal wrote:
>When the host resumes from a suspend, the guest thinks any task
>that was running during the suspend ran for a long time, even though
>the effective run time was much shorter, which can end up having
>negative effects with scheduling. This can be particularly noticeable
>if the guest task was RT, as it can end up getting throttled for a
>long time.
>
>To mitigate this issue, we include the time that the host was
>suspended in steal time, which lets the guest subtract the duration from
>the tasks' runtime.
>
>Note that the case of a suspend happening during a VM migration
>might not be accounted.
>
>Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
>---
> arch/x86/include/asm/kvm_host.h |  1 +
> arch/x86/kvm/x86.c              | 11 ++++++++++-
> 2 files changed, 11 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>index 4a68cb3eba78f8..728798decb6d12 100644
>--- a/arch/x86/include/asm/kvm_host.h
>+++ b/arch/x86/include/asm/kvm_host.h
>@@ -898,6 +898,7 @@ struct kvm_vcpu_arch {
> 		u8 preempted;
> 		u64 msr_val;
> 		u64 last_steal;
>+		u64 last_suspend_ns;
> 		struct gfn_to_hva_cache cache;
> 	} st;
> 
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 70219e4069874a..104f3d318026fa 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -3654,7 +3654,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> 	struct kvm_steal_time __user *st;
> 	struct kvm_memslots *slots;
> 	gpa_t gpa = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;
>-	u64 steal;
>+	u64 steal, suspend_ns;
> 	u32 version;
> 
> 	if (kvm_xen_msr_enabled(vcpu->kvm)) {
>@@ -3735,6 +3735,14 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> 	steal += current->sched_info.run_delay -
> 		vcpu->arch.st.last_steal;
> 	vcpu->arch.st.last_steal = current->sched_info.run_delay;
>+	/*
>+	 * Include the time that the host was suspended in steal time.
>+	 * Note that the case of a suspend happening during a VM migration
>+	 * might not be accounted.
>+	 */
>+	suspend_ns = kvm_total_suspend_ns();
>+	steal += suspend_ns - vcpu->arch.st.last_suspend_ns;
>+	vcpu->arch.st.last_suspend_ns = suspend_ns;

The document in patch 3 states:

  Time during which the vcpu is idle, will not be reported as steal time

I'm wondering if all host suspend time should be reported as steal time,
or if the suspend time during a vCPU halt should be excluded.

> 	unsafe_put_user(steal, &st->steal, out);
> 
> 	version += 1;
>@@ -12280,6 +12288,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> 
> 	vcpu->arch.arch_capabilities = kvm_get_arch_capabilities();
> 	vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
>+	vcpu->arch.st.last_suspend_ns = kvm_total_suspend_ns();

is this necessary? I doubt this because KVM doesn't capture
current->sched_info.run_delay here.

> 	kvm_xen_init_vcpu(vcpu);
> 	vcpu_load(vcpu);
> 	kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
>-- 
>2.46.0.184.g6999bdac58-goog
>