Re: [PATCH] KVM: Don't shrink/grow vCPU halt_poll_ns if host side polling is disabled

Wanpeng Li
2019-09-29T00:58:14Z

On Fri, 27 Sep 2019 at 22:42, Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Fri, Sep 27, 2019 at 04:27:02PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > Don't waste cycles to shrink/grow vCPU halt_poll_ns if host
> > side polling is disabled.
> >
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> > virt/kvm/kvm_main.c | 28 +++++++++++++++-------------
> > 1 file changed, 15 insertions(+), 13 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index e6de315..b368be4 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2359,20 +2359,22 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> > kvm_arch_vcpu_unblocking(vcpu);
> > block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
> >
> > - if (!vcpu_valid_wakeup(vcpu))
> > - shrink_halt_poll_ns(vcpu);
> > - else if (halt_poll_ns) {
> > - if (block_ns <= vcpu->halt_poll_ns)
> > - ;
> > - /* we had a long block, shrink polling */
> > - else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> > + if (!kvm_arch_no_poll(vcpu)) {
>
> Can vcpu->halt_poll_ns be cached and used both here and in the similar
> check above? E.g.:
>
> unsigned int vcpu_halt_poll_ns;
>
> vcpu_halt_poll_ns = kvm_arch_no_poll(vcpu) ? 0 : vcpu->halt_poll_ns;
>
> if (vcpu_halt_poll_ns) {
> ...
> }

This is not correct, !kvm_arch_no_poll(vcpu) && vcpu->halt_poll_ns ==
0, you will stop grow.

>
> > + if (!vcpu_valid_wakeup(vcpu))
> > shrink_halt_poll_ns(vcpu);
> > - /* we had a short halt and our poll time is too small */
> > - else if (vcpu->halt_poll_ns < halt_poll_ns &&
> > - block_ns < halt_poll_ns)
> > - grow_halt_poll_ns(vcpu);
> > - } else
> > - vcpu->halt_poll_ns = 0;
> > + else if (halt_poll_ns) {
> > + if (block_ns <= vcpu->halt_poll_ns)
> > + ;
> > + /* we had a long block, shrink polling */
> > + else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> > + shrink_halt_poll_ns(vcpu);
> > + /* we had a short halt and our poll time is too small */
> > + else if (vcpu->halt_poll_ns < halt_poll_ns &&
> > + block_ns < halt_poll_ns)
> > + grow_halt_poll_ns(vcpu);
> > + } else
> > + vcpu->halt_poll_ns = 0;
>
>
> Not your code,

Not the truth. :)

>but it'd be a good time to add braces to the 'if' and
> 'else'. Per Documentation/process/coding-style.rst:
>
> Do not unnecessarily use braces where a single statement will do.
>
> ...
>
> This does not apply if only one branch of a conditional statement is a single
> statement; in the latter case use braces in both branches:
>
> if (condition) {
> do_this();
> do_that();
> } else {
> otherwise();
> }

Will do in v2.

Wanpeng