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From: Sean Christopherson <seanjc@google.com>
To: Wanpeng Li <kernellwp@gmail.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 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios
Date: Wed, 12 May 2021 16:59:15 +0000	[thread overview]
Message-ID: <YJwJYxM3BBuQEXw8@google.com> (raw)
In-Reply-To: <CANRm+Czbc9AX3=Qj7dDCENyWj27drWniimZLnyKd9=--Ag8F+g@mail.gmail.com>

On Wed, May 12, 2021, Wanpeng Li wrote:
> On Wed, 12 May 2021 at 05:44, Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Sat, May 08, 2021, Wanpeng Li wrote:
> > > From: Wanpeng Li <wanpengli@tencent.com>
> > >
> > > In case of undercomitted scenarios, vCPU can get scheduling easily,
> > > kvm_vcpu_yield_to adds extra overhead, we can observe a lot of race
> > > between vcpu->ready is true and yield fails due to p->state is
> > > TASK_RUNNING. Let's bail out is such scenarios by checking the length
> > > of current cpu runqueue.
> > >
> > > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > > ---
> > >  arch/x86/kvm/x86.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 5bd550e..c0244a6 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -8358,6 +8358,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
> > >       struct kvm_vcpu *target = NULL;
> > >       struct kvm_apic_map *map;
> > >
> > > +     if (single_task_running())
> > > +             goto no_yield;
> > > +
> >
> > Hmm, could we push the result of kvm_sched_yield() down into the guest?
> > Currently the guest bails after the first attempt, which is perfect for this
> > scenario, but it seems like it would make sense to keep trying to yield if there
> > are multiple preempted vCPUs and
> 
> It can have a race in case of sustain yield if there are multiple
> preempted vCPUs , the vCPU which you intend to yield may have already
> completed to handle IPI and be preempted now when the yielded sender
> is scheduled again and checks the next preempted candidate.

Ah, right, don't want to penalize the happy case.

> > Unrelated to this patch, but it's the first time I've really looked at the guest
> > side of directed yield...
> >
> > Wouldn't it also make sense for the guest side to hook .send_call_func_single_ipi?
> 
> reschedule ipi is called by .smp_send_reschedule hook, there are a lot
> of researches intend to accelerate idle vCPU reactivation, my original
> attemption is to boost synchronization primitive, I believe we need a
> lot of benchmarkings to consider inter-VM fairness and performance
> benefit for  hooks .send_call_func_single_ipi and
> .smp_send_reschedule.

I was thinking of the 2 vCPU case.  If the VM has 2 vCPUs, then this

	/*
	 * Choose the most efficient way to send an IPI. Note that the
	 * number of CPUs might be zero due to concurrent changes to the
	 * provided mask.
	 */
	if (nr_cpus == 1)
		send_call_function_single_ipi(last_cpu);
	else if (likely(nr_cpus > 1))
		arch_send_call_function_ipi_mask(cfd->cpumask_ipi);

means .send_call_func_single_ipi() will always be used to send an IPI to the
other vCPU, and thus 2 vCPU VMs will never utilize PV yield.

  reply	other threads:[~2021-05-12 17:55 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-08  9:31 [PATCH 1/3] KVM: PPC: Book3S HV: exit halt polling on need_resched() as well Wanpeng Li
2021-05-08  9:31 ` [PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios Wanpeng Li
2021-05-11 21:44   ` Sean Christopherson
2021-05-12  2:43     ` Wanpeng Li
2021-05-12 16:59       ` Sean Christopherson [this message]
2021-05-08  9:31 ` [PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point of view Wanpeng Li
2021-05-11  0:18   ` Sean Christopherson
2021-05-11 10:28     ` Wanpeng Li
2021-05-12  0:02 ` [PATCH 1/3] KVM: PPC: Book3S HV: exit halt polling on need_resched() as well Wanpeng Li

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