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

Michael Mueller
2021-10-20T09:48:34Z

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@linux.ibm.com>
> 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.

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);
>