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

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Date:   Wed, 20 Oct 2021 12:51:49 +0200
From:   Michael Mueller <mimu@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Halil Pasic <pasic@...ux.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 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@...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.
>
> 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);
>>>