Re: [PATCH 3/3] KVM: s390: clear kicked_mask if not idle after set

[Michael Mueller]

On 20.10.21 12:31, Christian Borntraeger wrote:
> Am 20.10.21 um 11:48 schrieb Michael Mueller:
> > On 19.10.21 19:54, Halil Pasic wrote:
> > > The idea behind kicked mask is that we should not re-kick a vcpu
> > > from __airqs_kick_single_vcpu() that is already in the middle of
> > > being kicked by the same function.
> > >
> > > If however the vcpu that was idle before when the idle_mask was
> > > examined, is not idle any more after the kicked_mask is set, that
> > > means that we don't need to kick, and that we need to clear the
> > > bit we just set because we may be beyond the point where it would
> > > get cleared in the wake-up process. Since the time window is short,
> > > this is probably more a theoretical than a practical thing: the race
> > > window is small.
> > >
> > > To get things harmonized let us also move the clear from vcpu_pre_run()
> > > to __unset_cpu_idle().
> > >
> > > Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > > Fixes: 9f30f6216378 ("KVM: s390: add gib_alert_irq_handler()")
> >
> > Before releasing something like this, where none of us is sure if
> > it really saves cpu cost, I'd prefer to run some measurement with
> > the whole kicked_mask logic removed and to compare the number of
> > vcpu wake ups with the number of interrupts to be processed by
> > the gib alert mechanism in a slightly over committed host while
> > driving with Matthews test load.
>
> But I think patch 1 and 2 can go immediately as they measurably or
> testable fix things. Correct?

Yes

> > A similar run can be done with this code.
> >
> > > ---
> > >   arch/s390/kvm/interrupt.c | 7 ++++++-
> > >   arch/s390/kvm/kvm-s390.c  | 2 --
> > >   2 files changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> > > index 2245f4b8d362..3c80a2237ef5 100644
> > > --- a/arch/s390/kvm/interrupt.c
> > > +++ b/arch/s390/kvm/interrupt.c
> > > @@ -426,6 +426,7 @@ static void __unset_cpu_idle(struct kvm_vcpu *vcpu)
> > >   {
> > >       kvm_s390_clear_cpuflags(vcpu, CPUSTAT_WAIT);
> > >       clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.idle_mask);
> > > +    clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask);
> > >   }
> > >   static void __reset_intercept_indicators(struct kvm_vcpu *vcpu)
> > > @@ -3064,7 +3065,11 @@ static void __airqs_kick_single_vcpu(struct 
> > > kvm *kvm, u8 deliverable_mask)
> > >               /* lately kicked but not yet running */
> > >               if (test_and_set_bit(vcpu_idx, gi->kicked_mask))
> > >                   return;
> > > -            kvm_s390_vcpu_wakeup(vcpu);
> > > +            /* if meanwhile not idle: clear  and don't kick */
> > > +            if (test_bit(vcpu_idx, kvm->arch.idle_mask))
> > > +                kvm_s390_vcpu_wakeup(vcpu);
> > > +            else
> > > +                clear_bit(vcpu_idx, gi->kicked_mask);
> > >               return;
> > >           }
> > >       }
> > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > > index 1c97493d21e1..6b779ef9f5fb 100644
> > > --- a/arch/s390/kvm/kvm-s390.c
> > > +++ b/arch/s390/kvm/kvm-s390.c
> > > @@ -4067,8 +4067,6 @@ static int vcpu_pre_run(struct kvm_vcpu *vcpu)
> > >           kvm_s390_patch_guest_per_regs(vcpu);
> > >       }
> > > -    clear_bit(vcpu->vcpu_idx, vcpu->kvm->arch.gisa_int.kicked_mask);
> > > -
> > >       vcpu->arch.sie_block->icptcode = 0;
> > >       cpuflags = atomic_read(&vcpu->arch.sie_block->cpuflags);
> > >       VCPU_EVENT(vcpu, 6, "entering sie flags %x", cpuflags);