linux-kernel - Re: [PATCH v5 2/2] KVM: x86: Include host suspended time in steal time

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Message-ID: <aAidb496s6ke8RoO@google.com>
Date: Wed, 23 Apr 2025 07:57:35 +0000
From: Tzung-Bi Shih <tzungbi@...nel.org>
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>, Chao Gao <chao.gao@...el.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, ssouhlal@...ebsd.org
Subject: Re: [PATCH v5 2/2] KVM: x86: Include host suspended time in steal
 time

On Tue, Mar 25, 2025 at 01:13:50PM +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.
> 
> [...]
> 
> Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>

Saw the corresponding host suspended time has been compensated in
update_rq_clock_task():
Tested-by: Tzung-Bi Shih <tzungbi@...nel.org>

With 1 minor comment:
Reviewed-by: Tzung-Bi Shih <tzungbi@...nel.org>

> @@ -917,8 +918,13 @@ struct kvm_vcpu_arch {
>  
>  	struct {
>  		u8 preempted;
> +		bool host_suspended;

Use it in bool manner.

> +static void wait_for_resume(struct kvm_vcpu *vcpu)
> +{
> +	wait_event_interruptible(vcpu->arch.st.resume_waitq,
> +	    vcpu->arch.st.host_suspended == 0);

E.g.: !vcpu->arch.st.host_suspended.

> @@ -6939,6 +6954,19 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm)
>  
>  	mutex_lock(&kvm->lock);
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		vcpu->arch.st.last_suspend = ktime_get_boottime_ns();
> +		/*
> +		 * Tasks get thawed before the resume notifier has been called
> +		 * so we need to block vCPUs until the resume notifier has run.
> +		 * Otherwise, suspend steal time might get applied too late,
> +		 * and get accounted to the wrong guest task.
> +		 * This also ensures that the guest paused bit set below
> +		 * doesn't get checked and cleared before the host actually
> +		 * suspends.
> +		 */
> +		vcpu->arch.st.host_suspended = 1;

E.g.: true.

> +static int kvm_arch_resume_notifier(struct kvm *kvm)
> +{
> +	struct kvm_vcpu *vcpu;
> +	unsigned long i;
> +
> +	mutex_lock(&kvm->lock);
> +	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		vcpu->arch.st.host_suspended = 0;

E.g.: false.