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

Christian Borntraeger
2021-10-20T10:31:33Z

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@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.

But I think patch 1 and 2 can go immediately as they measurably or
testable fix things. Correct?

> 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);
>>