linux-kernel - Re: [PATCH 3/3] KVM: s390: clear kicked

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Message-ID: <8cb919e7-e7ab-5ec1-591e-43f95f140d7b@linux.ibm.com>
Date:   Wed, 20 Oct 2021 11:48:22 +0200
From:   Michael Mueller <mimu@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>, farman@...ux.ibm.com,
        kvm@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: s390: clear kicked_mask if not idle after set



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@...ux.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);
>