Hi Sean,
On Thu, Aug 19, 2021 at 3:37 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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));
> }
>
