On 18/07/19 15:45, Christian Borntraeger wrote:
>
>
> On 18.07.19 15:37, Paolo Bonzini wrote:
>> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
>> Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Cc: Paul Mackerras <paulus@xxxxxxxxxx>
>> Cc: Marc Zyngier <maz@xxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> ---
>> 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);
>> }
>>
>>
>
