> -----邮件原件-----
> 发件人: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> 发送时间: 2021年7月28日 14:12
> 收件人: Li,Rongqing <lirongqing@xxxxxxxxx>
> 抄送: kvm@xxxxxxxxxxxxxxx; Paolo Bonzini <pbonzini@xxxxxxxxxx>;
> seanjc@xxxxxxxxxx
> 主题: Re: [PATCH][v2] KVM: use cpu_relax when halt polling
>
> '
> > > > "Rather than disallowing halt-polling entirely, on x86 it should
> > > > be sufficient to simply have the hardware thread yield to its
> > > > sibling(s) via PAUSE. It probably won't get back all performance,
> > > > but I would expect it to be close.
> > > > This compiles on all KVM architectures, and AFAICT the intended
> > > > usage of cpu_relax() is identical for all architectures."
> > >
> > > For sure change to cpu_relax() is better.
> > > Was just curious to know if you got descent performance improvement
> > > compared to previously reported with Unixbench.
> > >
> > > Thanks,
> > > Pankaj
> >
> > The test as below:
> >
> > 1. run unixbench dhry2reg: ./Run -c 1 dhry2reg -i 1 without SMT
> > disturbance, the score is 3172 with a {while(1)i++} SMT disturbance,
> > the score is 1583 with a {while(1)(rep nop/pause)} SMT disturbance,
> > the score is 1729.4
> >
> > seems cpu_relax can not get back all performance , what wrong?
>
> Maybe because of pause intercept filtering, comparatively Mayless VM Exits?
>
In vm;
I retest it in bare metal, pause instruction works as expect, the score with "pause loop" disturbance is 2886; about 90% of no disturbance
-Li
> >
> >
> > 2. back to haltpoll
> > run unixbench dhry2reg ./Run -c 1 dhry2reg -i 1 without SMT
> > disturbance, the score is 3172
> >
> > with redis-benchmark SMT disturbance, redis-benchmark takes 90%cpu:
> > without patch, the score is 1776.9
> > with my first patch, the score is 1782.3 with cpu_relax patch, the
> > score is 1778
> >
> > with redis-benchmark SMT disturbance, redis-benchmark takes 33%cpu:
> > without patch, the score is 1929.9
> > with my first patch, the score is 2294.6 with cpu_relax patch, the
> > score is 2005.3
> >
> >
> > cpu_relax give less than stop halt polling, but it should have little
> > effect for redis-benchmark which get benefit from halt polling
>
> We are seeing improvement with cpu_relax() though not to the level of stopping
> the halt polling when sibling CPU running redis workload. For 90% case I think its
> expected to have similar performance.
>
> For 33% stopping halt poll gives better result because of the workload. Overall I
> think this patch helps and not impact performance in normal cases.
>
> Reviewed-by: Pankaj Gupta <pankaj.gupta@xxxxxxxxx>
>
> Best regards,
> Pankaj
>
>
> >
> >
> > -Li
> >
> > > >
> > > > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > > Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> > > > ---
> > > > diff v1: using cpu_relax, rather that stop halt-polling
> > > >
> > > > virt/kvm/kvm_main.c | 1 +
> > > > 1 file changed, 1 insertion(+)
> > > >
> > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index
> > > > 7d95126..1679728 100644
> > > > --- a/virt/kvm/kvm_main.c
> > > > +++ b/virt/kvm/kvm_main.c
> > > > @@ -3110,6 +3110,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> > > >
> > > ++vcpu->stat.generic.halt_poll_invalid;
> > > > goto out;
> > > > }
> > > > + cpu_relax();
> > > > poll_end = cur = ktime_get();
> > > > } while (kvm_vcpu_can_poll(cur, stop));
> > > > }
> > > > --
> > > > 2.9.4
> > > >
