Re: [PATCH] KVM: stats: add stats to detect if vcpu is currently halted

[Jing Zhang <jingzhangos@google.com>]

Hi Sean,

On Thu, Aug 19, 2021 at 3:37 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Wed, Aug 18, 2021, Cannon Matthews wrote:
> > Since a guest has explictly asked for a vcpu to HLT, this is "useful work on
> > behalf of the guest" even though the thread is "blocked" from running.
> >
> > This allows answering questions like, are we spending too much time waiting
> > on mutexes, or long running kernel routines rather than running the vcpu in
> > guest mode, or did the guest explictly tell us to not doing anything.
> >
> > So I would suggest keeping the "halt" part of the counters' name, and remove
> > the "blocked" part rather than the other way around. We explicitly do not
> > want to include non-halt blockages in this.
>
> But this patch does include non-halt blockages, which is why I brought up the
> technically-wrong naming.  Specifically, x86 reaches this path for any !RUNNABLE
> vCPU state, e.g. if the vCPU is in WFS.  Non-x86 usage appears to mostly call
> this for halt-like behavior, but PPC looks like it has at least one path that's
> not halt-like.
>
> I doubt anyone actually cares if the stat is a misnomer in some cases, but at the
> same time I think there's opportunity for clean up here.  E.g. halt polling if a
> vCPU is in WFS or UNINITIALIZED is a waste of cycles.  Ditto for the calls to
> kvm_arch_vcpu_blocking() and kvm_arch_vcpu_unblocking() when halt polling is
> successful, e.g. arm64 puts and reloads the vgic, which I assume is a complete
> waste of cycles if the vCPU doesn't actually block.  And kvm_arch_vcpu_block_finish()
> can be dropped by moving the one line of code into s390, which can add its own
> wrapper if necessary.
>
> So with a bit of massaging and a slight change in tracing behavior, I believe we
> can isolate the actual wait/halt and avoid "halted" being technically-wrong, and
> fix some inefficiencies at the same time.
>
> Jing, can you do a v2 of this patch and send it to me off-list?  With luck, my
> idea will work and I can fold your patch in, and if not we can always post v2
> standalone in a few weeks.
Of course, will do.
Thanks,
Jing
>
> E.g. I'm thinking something like...
>
> void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
> {
>         vcpu->stat.generic.halted = 1;
>
>         if (<halt polling failed>)
>                 kvm_vcpu_block();
>
>         vcpu->stat.generic.halted = 0;
>
>         <update halt polling stuff>
> }
>
> void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> {
>         bool waited = false;
>         ktime_t start, cur;
>         u64 block_ns;
>
>         start = ktime_get();
>
>
>         prepare_to_rcuwait(&vcpu->wait);
>         for (;;) {
>                 set_current_state(TASK_INTERRUPTIBLE);
>
>                 if (kvm_vcpu_check_block(vcpu) < 0)
>                         break;
>
>                 waited = true;
>                 schedule();
>         }
>         finish_rcuwait(&vcpu->wait);
>
>         block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>         trace_kvm_vcpu_wakeup(block_ns, waited, vcpu_valid_wakeup(vcpu));
> }
>