Re: [PATCH] KVM: x86: Include host suspended time in steal time.

[Suleiman Souhlal]

Hello,

On Wed, Jul 10, 2024 at 4:44 PM Suleiman Souhlal <suleiman@xxxxxxxxxx> 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 can subtract the
> duration from the tasks' runtime.
>
> Signed-off-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c       | 23 ++++++++++++++++++++++-
>  include/linux/kvm_host.h |  4 ++++
>  2 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0763a0f72a067f..94bbdeef843863 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3669,7 +3669,7 @@ 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_duration;
>         u32 version;
>
>         if (kvm_xen_msr_enabled(vcpu->kvm)) {
> @@ -3696,6 +3696,12 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>                         return;
>         }
>
> +       suspend_duration = 0;
> +       if (READ_ONCE(vcpu->suspended)) {
> +               suspend_duration = vcpu->kvm->last_suspend_duration;
> +               vcpu->suspended = 0;
> +       }
> +
>         st = (struct kvm_steal_time __user *)ghc->hva;
>         /*
>          * Doing a TLB flush here, on the guest's behalf, can avoid
> @@ -3749,6 +3755,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>         unsafe_get_user(steal, &st->steal, out);
>         steal += current->sched_info.run_delay -
>                 vcpu->arch.st.last_steal;
> +       steal += suspend_duration;
>         vcpu->arch.st.last_steal = current->sched_info.run_delay;
>         unsafe_put_user(steal, &st->steal, out);
>
> @@ -6920,6 +6927,7 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm)
>
>         mutex_lock(&kvm->lock);
>         kvm_for_each_vcpu(i, vcpu, kvm) {
> +               WRITE_ONCE(vcpu->suspended, 1);
>                 if (!vcpu->arch.pv_time.active)
>                         continue;
>
> @@ -6932,15 +6940,28 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm)
>         }
>         mutex_unlock(&kvm->lock);
>
> +       kvm->suspended_time = ktime_get_boottime_ns();
> +
>         return ret ? NOTIFY_BAD : NOTIFY_DONE;
>  }
>
> +static int
> +kvm_arch_resume_notifier(struct kvm *kvm)
> +{
> +       kvm->last_suspend_duration = ktime_get_boottime_ns() -
> +           kvm->suspended_time;
> +       return NOTIFY_DONE;
> +}
> +
>  int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
>  {
>         switch (state) {
>         case PM_HIBERNATION_PREPARE:
>         case PM_SUSPEND_PREPARE:
>                 return kvm_arch_suspend_notifier(kvm);
> +       case PM_POST_HIBERNATION:
> +       case PM_POST_SUSPEND:
> +               return kvm_arch_resume_notifier(kvm);
>         }
>
>         return NOTIFY_DONE;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 692c01e41a18ef..2d37af9a348648 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -366,6 +366,8 @@ struct kvm_vcpu {
>         } async_pf;
>  #endif
>
> +       bool suspended;
> +
>  #ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT
>         /*
>          * Cpu relax intercept or pause loop exit optimization
> @@ -840,6 +842,8 @@ struct kvm {
>         struct xarray mem_attr_array;
>  #endif
>         char stats_id[KVM_STATS_NAME_SIZE];
> +       u64 last_suspend_duration;
> +       u64 suspended_time;
>  };
>
>  #define kvm_err(fmt, ...) \
> --
> 2.45.2.993.g49e7a77208-goog
>

Gentle ping.

Thanks,
-- Suleiman