答复: [PATCH] KVM: X86: set vcpu preempted only if it is preempted

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: Peter Zijlstra <peterz@infradead.org>
> 发送时间: 2022年1月13日 5:31
> 收件人: Sean Christopherson <seanjc@google.com>
> 抄送: Li,Rongqing <lirongqing@baidu.com>; pbonzini@redhat.com;
> vkuznets@redhat.com; wanpengli@tencent.com; jmattson@google.com;
> tglx@linutronix.de; bp@alien8.de; x86@kernel.org; kvm@vger.kernel.org;
> joro@8bytes.org
> 主题: Re: [PATCH] KVM: X86: set vcpu preempted only if it is preempted
> 
> On Wed, Jan 12, 2022 at 05:30:47PM +0000, Sean Christopherson wrote:
> > On Wed, Jan 12, 2022, Peter Zijlstra wrote:
> > > On Wed, Jan 12, 2022 at 08:02:01PM +0800, Li RongQing wrote:
> > > > vcpu can schedule out when run halt instruction, and set itself to
> > > > INTERRUPTIBLE and switch to idle thread, vcpu should not be set
> > > > preempted for this condition
> > >
> > > Uhhmm, why not? Who says the vcpu will run the moment it becomes
> > > runnable again? Another task could be woken up meanwhile occupying
> > > the real cpu.
> >
> > Hrm, but when emulating HLT, e.g. for an idling vCPU, KVM will
> > voluntarily schedule out the vCPU and mark it as preempted from the
> > guest's perspective.  The vast majority, probably all, usage of
> > steal_time.preempted expects it to truly mean "preempted" as opposed to
> "not running".
> 
> No, the original use-case was locking and that really cares about running.
> 
> If the vCPU isn't running, we must not busy-wait for it etc..
> 
> Similar to the scheduler use of it, if the vCPU isn't running, we should not
> consider it so. Getting the vCPU task scheduled back on the CPU can take a 'long'
> time.
> 
> If you have pinned vCPU threads and no overcommit, we have other knobs to
> indicate this I tihnk.


If vcpu is idle, and be marked as preempted, is it right in kvm_smp_send_call_func_ipi?

static void kvm_smp_send_call_func_ipi(const struct cpumask *mask)
{
    int cpu;

    native_send_call_func_ipi(mask);

    /* Make sure other vCPUs get a chance to run if they need to. */
    for_each_cpu(cpu, mask) {
        if (vcpu_is_preempted(cpu)) {
            kvm_hypercall1(KVM_HC_SCHED_YIELD, per_cpu(x86_cpu_to_apicid, cpu));
            break;
        }
    }
}


-Li