From: Sean Christopherson <seanjc@google.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch
Date: Mon, 17 May 2021 17:51:16 +0000 [thread overview]
Message-ID: <YKKtFOl3oklFp1lW@google.com> (raw)
In-Reply-To: <1621260028-6467-5-git-send-email-wanpengli@tencent.com>
On Mon, May 17, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>
> Let's treat lapic_timer_advance_ns automatically tune logic as hypervisor
> overhead, move it before wait_lapic_expire instead of between wait_lapic_expire
> and the world switch, the wait duration should be calculated by the
> up-to-date guest_tsc after the overhead of automatically tune logic. This
> patch reduces ~30+ cycles for kvm-unit-tests/tscdeadline-latency when testing
> busy waits.
>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> arch/x86/kvm/lapic.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index c0ebef560bd1..552d2acf89ab 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1598,11 +1598,12 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
>
> - if (guest_tsc < tsc_deadline)
> - __wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
> -
> if (lapic_timer_advance_dynamic)
> adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
> +
> + guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
This is redundant and unnecessary if automatic tuning is disabled, or if the
timer did not arrive early. A comment would also be helpful. E.g. I think this
would micro-optimize all paths:
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index c0ebef560bd1..5d91f2367c31 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1598,11 +1598,19 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
+ if (lapic_timer_advance_dynamic) {
+ adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
+ /*
+ * If the timer fired early, reread the TSC to account for the
+ * overhead of the above adjustment to avoid waiting longer
+ * than is necessary.
+ */
+ if (guest_tsc < tsc_deadline)
+ guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
+ }
+
if (guest_tsc < tsc_deadline)
__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
-
- if (lapic_timer_advance_dynamic)
- adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
}
void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> + if (guest_tsc < tsc_deadline)
> + __wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
> }
>
> void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> --
> 2.25.1
>
next prev parent reply other threads:[~2021-05-17 17:51 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 14:00 [PATCH v3 1/5] KVM: exit halt polling on need_resched() for both book3s and generic halt-polling Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 2/5] KVM: X86: Bail out of direct yield in case of under-committed scenarios Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 3/5] KVM: X86: Fix vCPU preempted state from guest's point of view Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 4/5] KVM: x86: hyper-v: Task srcu lock when accessing kvm_memslots() Wanpeng Li
2021-05-17 14:00 ` [PATCH v3 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch Wanpeng Li
2021-05-17 17:51 ` Sean Christopherson [this message]
2021-05-18 12:03 ` Wanpeng Li
2021-05-17 16:34 ` [PATCH v3 1/5] KVM: exit halt polling on need_resched() for both book3s and generic halt-polling David Matlack
2021-05-18 12:03 ` Wanpeng Li
2021-05-17 18:13 ` Venkatesh Srinivas
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