From: Wanpeng Li <kernellwp@gmail.com>
To: Sean Christopherson <seanjc@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <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: Tue, 18 May 2021 20:03:52 +0800 [thread overview]
Message-ID: <CANRm+CyHSQBXa6D2VYgysNaVwnRbK8xQ02_zoHkvxrKXAh6+CQ@mail.gmail.com> (raw)
In-Reply-To: <YKKtFOl3oklFp1lW@google.com>
On Tue, 18 May 2021 at 01:51, Sean Christopherson <seanjc@google.com> wrote:
>
> 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:
Do it in v4, thanks.
Wanpeng
>
> 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:[~2021-05-18 12:04 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
2021-05-18 12:03 ` Wanpeng Li [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CANRm+CyHSQBXa6D2VYgysNaVwnRbK8xQ02_zoHkvxrKXAh6+CQ@mail.gmail.com \
--to=kernellwp@gmail.com \
--cc=jmattson@google.com \
--cc=joro@8bytes.org \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=pbonzini@redhat.com \
--cc=seanjc@google.com \
--cc=vkuznets@redhat.com \
--cc=wanpengli@tencent.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).