Re: [PATCH v3 2/3] KVM: x86: Include host suspended time in steal time.

Sean Christopherson
2025-01-07T15:37:17Z

On Tue, Jan 07, 2025, 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

No "we".

> suspended in steal time, which lets the guest can subtract the
> duration from the tasks' runtime.
>
> Note that the case of a suspend happening during a VM migration
> might not be accounted.

And this isn't considered a bug because? I asked for documentation, not a
statement of fact.

> Change-Id: I18d1d17d4d0d6f4c89b312e427036e052c47e1fa

gerrit.

> Signed-off-by: Suleiman Souhlal <suleiman@google.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 e159e44a6a1b61..01d44d527a7f88 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -897,6 +897,7 @@ 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 c8160baf383851..12439edc36f83a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3650,7 +3650,7 @@ 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)) {
> @@ -3677,6 +3677,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> return;
> }
>
> + suspend_ns = kvm_total_suspend_ns();
> st = (struct kvm_steal_time __user *)ghc->hva;
> /*
> * Doing a TLB flush here, on the guest's behalf, can avoid
> @@ -3731,6 +3732,13 @@ 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.
> + */

This is not a useful comment. It's quite clear what that suspend time is being
accumulated into steal_time, and restating the migration caveat does more harm
than good, as that flaw is an issue with the overall design, i.e. has nothing to
do with this specific snippet of code.

> + steal += suspend_ns - vcpu->arch.st.last_suspend_ns;
> + vcpu->arch.st.last_suspend_ns = suspend_ns;
> unsafe_put_user(steal, &st->steal, out);
>
> version += 1;
> @@ -12299,6 +12307,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> if (r)
> goto free_guest_fpu;
>
> + vcpu->arch.st.last_suspend_ns = kvm_total_suspend_ns();
> kvm_xen_init_vcpu(vcpu);
> vcpu_load(vcpu);
> kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
> --
> 2.47.1.613.gc27f4b7a9f-goog
>