linux-kernel - Re: [PATCH 1/2] KVM: Boost vCPUs that are delivering interrupts

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Date:   Thu, 18 Jul 2019 15:50:04 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     wanpengli@...cent.com, rkrcmar@...hat.com, paulus@...abs.org,
        maz@...nel.org
Subject: Re: [PATCH 1/2] KVM: Boost vCPUs that are delivering interrupts

On 18/07/19 15:45, Christian Borntraeger wrote:
> 
> 
> On 18.07.19 15:37, Paolo Bonzini wrote:
>> From: Wanpeng Li <wanpengli@...cent.com>
>>
>> Inspired by commit 9cac38dd5d (KVM/s390: Set preempted flag during
>> vcpu wakeup and interrupt delivery), we want to also boost not just
>> lock holders but also vCPUs that are delivering interrupts. Most
>> smp_call_function_many calls are synchronous, so the IPI target vCPUs
>> are also good yield candidates.  This patch introduces vcpu->ready to
>> boost vCPUs during wakeup and interrupt delivery time; unlike s390 we do
>> not reuse vcpu->preempted so that voluntarily preempted vCPUs are taken
>> into account by kvm_vcpu_on_spin, but vmx_vcpu_pi_put is not affected
>> (VT-d PI handles voluntary preemption separately, in pi_pre_block).
>>
>> Testing on 80 HT 2 socket Xeon Skylake server, with 80 vCPUs VM 80GB RAM:
>> ebizzy -M
>>
>>             vanilla     boosting    improved
>> 1VM          21443       23520         9%
>> 2VM           2800        8000       180%
>> 3VM           1800        3100        72%
>>
>> Testing on my Haswell desktop 8 HT, with 8 vCPUs VM 8GB RAM, two VMs,
>> one running ebizzy -M, the other running 'stress --cpu 2':
>>
>> w/ boosting + w/o pv sched yield(vanilla)
>>
>>             vanilla     boosting   improved
>>               1570         4000      155%
>>
>> w/ boosting + w/ pv sched yield(vanilla)
>>
>>             vanilla     boosting   improved
>>               1844         5157      179%
>>
>> w/o boosting, perf top in VM:
>>
>>  72.33%  [kernel]       [k] smp_call_function_many
>>   4.22%  [kernel]       [k] call_function_i
>>   3.71%  [kernel]       [k] async_page_fault
>>
>> w/ boosting, perf top in VM:
>>
>>  38.43%  [kernel]       [k] smp_call_function_many
>>   6.31%  [kernel]       [k] async_page_fault
>>   6.13%  libc-2.23.so   [.] __memcpy_avx_unaligned
>>   4.88%  [kernel]       [k] call_function_interrupt
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Cc: Christian Borntraeger <borntraeger@...ibm.com>
>> Cc: Paul Mackerras <paulus@...abs.org>
>> Cc: Marc Zyngier <maz@...nel.org>
>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>> 	v2->v3: put it in kvm_vcpu_wake_up, use WRITE_ONCE
> 
> 
> Looks good. Some more comments
> 
>>
>>  arch/s390/kvm/interrupt.c | 2 +-
>>  include/linux/kvm_host.h  | 1 +
>>  virt/kvm/kvm_main.c       | 9 +++++++--
> [...]
> 
>> @@ -4205,6 +4206,8 @@ static void kvm_sched_in(struct preempt_notifier *pn, int cpu)
>>  
>>  	if (vcpu->preempted)
>>  		vcpu->preempted = false;
>> +	if (vcpu->ready)
>> +		WRITE_ONCE(vcpu->ready, false);
> 
> What is the rationale of checking before writing. Avoiding writable cache line ping pong?

I think it can be removed.  The only case where you'd have ping pong is
when vcpu->ready is true due to kvm_vcpu_wake_up, so it's not saving
anything.

>>  	kvm_arch_sched_in(vcpu, cpu);
>>  
>> @@ -4216,8 +4219,10 @@ static void kvm_sched_out(struct preempt_notifier *pn,
>>  {
>>  	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
>>  
>> -	if (current->state == TASK_RUNNING)
>> +	if (current->state == TASK_RUNNING) {
>>  		vcpu->preempted = true;
> 
> WOuld it make sense to also use WRITE_ONCE for vcpu->preempted ?

vcpu->preempted is not read/written anymore by other threads after this
patch.
> 
>> +		WRITE_ONCE(vcpu->ready, true);
>> +	}
>>  	kvm_arch_vcpu_put(vcpu);
>>  }
>>  
>>
>