Re: [PATCH] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt

Wanpeng Li
2021-11-23T06:24:37Z

On Tue, 23 Nov 2021 at 12:11, yaoaili [么爱利] <yaoaili@kingsoft.com> wrote:
>
> > On Tue, 23 Nov 2021 at 03:14, Sean Christopherson <seanjc@google.com>
> > wrote:
> > >
> > > On Mon, Nov 22, 2021, Aili Yao wrote:
> > > > From: Aili Yao <yaoaili@kingsoft.com>
> > > >
> > > > When we isolate some pyhiscal cores, We may not use them for kvm
> > > > guests, We may use them for other purposes like DPDK, or we can make
> > > > some kvm guests isolated and some not, the global judgement
> > > > pi_inject_timer is not enough; We may make wrong decisions:
> > > >
> > > > In such a scenario, the guests without isolated cores will not be
> > > > permitted to use vmx preemption timer, and tscdeadline fastpath also
> > > > be disabled, both will lead to performance penalty.
> > > >
> > > > So check whether the vcpu->cpu is isolated, if not, don't post timer
> > > > interrupt.
> > > >
> > > > Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
> > > > ---
> > > > arch/x86/kvm/lapic.c | 4 +++-
> > > > 1 file changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index
> > > > 759952dd1222..72dde5532101 100644
> > > > --- a/arch/x86/kvm/lapic.c
> > > > +++ b/arch/x86/kvm/lapic.c
> > > > @@ -34,6 +34,7 @@
> > > > #include <asm/delay.h>
> > > > #include <linux/atomic.h>
> > > > #include <linux/jump_label.h>
> > > > +#include <linux/sched/isolation.h>
> > > > #include "kvm_cache_regs.h"
> > > > #include "irq.h"
> > > > #include "ioapic.h"
> > > > @@ -113,7 +114,8 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic
> > > > *apic)
> > > >
> > > > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu) {
> > > > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > > > + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> > > > + !housekeeping_cpu(vcpu->cpu, HK_FLAG_TIMER);
> > >
> > > I don't think this is safe, vcpu->cpu will be -1 if the vCPU isn't scheduled in.
>
> Yes, vcpu->cpu is -1 before vcpu create, but in my environments, it didn't
> trigger this issue. I need to dig more, Thanks!
> Maybe I need one valid check here.
>
> > > This also doesn't play nice with the admin forcing pi_inject_timer=1.
> > > Not saying there's a reasonable use case for doing that, but it's
> > > supported today and this would break that behavior. It would also
> > > lead to weird behavior if a vCPU were migrated on/off a housekeeping
> > > vCPU. Again, probably not a reasonable use case, but I don't see anything
> > that would outright prevent that behavior.
>
> Yes, this is not one common operation, But I did do test some scenarios:
> 1. isolated cpu --> housekeeping cpu;
> isolated guest timer is in housekeeping CPU, for migration, kvm_can_post_timer_interrupt
> will return false, so the timer may be migrated to vcpu->cpu;
> This seems works in my test;
> 2. isolated --> isolated
> Isolated guest timer is in housekeeping cpu, for migration,kvm_can_post_timer_interrupt return
> true, timer is not migrated
> 3. housekeeping CPU --> isolated CPU
> non-isolated CPU timer is usually in vcpu->cpu, for migration to isolated, kvm_can_post_timer_interrupt
> will be true, the timer remain on the same CPU;
> This seems works in my test;
> 4. housekeeping CPU --> housekeeping CPU
> timer migrated;
> It seems this is not an affecting problem;
>
> > >
> > > The existing behavior also feels a bit unsafe as pi_inject_timer is
> > > writable while KVM is running, though I supposed that's orthogonal to this
> > discussion.
> > >
> > > Rather than check vcpu->cpu, is there an existing vCPU flag that can
> > > be queried, e.g. KVM_HINTS_REALTIME?
> >
> > How about something like below:
> >
> > From 67f605120e212384cb3d5788ba8c83f15659503b Mon Sep 17 00:00:00
> > 2001
> > From: Wanpeng Li <wanpengli@tencent.com>
> > Date: Tue, 23 Nov 2021 10:36:10 +0800
> > Subject: [PATCH] KVM: LAPIC: To keep the vCPUs in non-root mode for timer-
> > pi
> >
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > As commit 0c5f81dad46 (KVM: LAPIC: Inject timer interrupt via posted
> > interrupt) mentioned that the host admin should well tune the guest setup,
> > so that vCPUs are placed on isolated pCPUs, and with several pCPUs surplus
> > for
> > *busy* housekeeping.
> > It is better to disable mwait/hlt/pause vmexits to keep the vCPUs in non-root
> > mode. However, we may isolate pCPUs for other purpose like DPDK or we
> > can make some guests isolated and others not, Let's add the checking
> > kvm_mwait_in_guest() to kvm_can_post_timer_interrupt() since we can't
> > benefit from timer posted-interrupt w/o keeping the vCPUs in non-root
> > mode.
> >
> > Reported-by: Aili Yao <yaoaili@kingsoft.com>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> > arch/x86/kvm/lapic.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index
> > 759952dd1222..8257566d44c7 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -113,14 +113,13 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic
> > *apic)
> >
> > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu) {
> > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > + return pi_inject_timer && kvm_mwait_in_guest(vcpu->kvm) &&
> > kvm_vcpu_apicv_active(vcpu);
> > }
> >
> > bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu) {
> > return kvm_x86_ops.set_hv_timer
> > - && !(kvm_mwait_in_guest(vcpu->kvm) ||
> > - kvm_can_post_timer_interrupt(vcpu));
> > + && !kvm_mwait_in_guest(vcpu->kvm);
> > }
> > EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);
>
> This method seems more quick and safe, but I have one question: Does this kvm_mwait_in_guest
> can guarantee the CPU isolated, in some production environments and usually, MWAIT feature is disabled in host
> and even guests with isolated CPUs. And also we can set guests kvm_mwait_in_guest true with CPUs just pinned, not isolated.

You won't benefit from timer posted-interrupt if mwait is not exposed
to the guest since you can't keep CPU in non-root mode.
kvm_mwait_in_guest() will not guarantee the CPU is isolated, but
what's still bothering?

Wanpeng