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SubjectRe: [PATCH] KVM: Boost vCPU candidiate in user mode which is delivering interrupt
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On 20/04/21 10:48, Wanpeng Li wrote:
>> I was thinking of something simpler:
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 9b8e30dd5b9b..455c648f9adc 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -3198,10 +3198,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>> {
>> struct kvm *kvm = me->kvm;
>> struct kvm_vcpu *vcpu;
>> - int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
>> int yielded = 0;
>> int try = 3;
>> - int pass;
>> + int pass, num_passes = 1;
>> int i;
>>
>> kvm_vcpu_set_in_spin_loop(me, true);
>> @@ -3212,13 +3211,14 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>> * VCPU is holding the lock that we need and will release it.
>> * We approximate round-robin by starting at the last boosted VCPU.
>> */
>> - for (pass = 0; pass < 2 && !yielded && try; pass++) {
>> - kvm_for_each_vcpu(i, vcpu, kvm) {
>> - if (!pass && i <= last_boosted_vcpu) {
>> - i = last_boosted_vcpu;
>> - continue;
>> - } else if (pass && i > last_boosted_vcpu)
>> - break;
>> + for (pass = 0; pass < num_passes; pass++) {
>> + int idx = me->kvm->last_boosted_vcpu;
>> + int n = atomic_read(&kvm->online_vcpus);
>> + for (i = 0; i < n; i++, idx++) {
>> + if (idx == n)
>> + idx = 0;
>> +
>> + vcpu = kvm_get_vcpu(kvm, idx);
>> if (!READ_ONCE(vcpu->ready))
>> continue;
>> if (vcpu == me)
>> @@ -3226,23 +3226,36 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>> if (rcuwait_active(&vcpu->wait) &&
>> !vcpu_dy_runnable(vcpu))
>> continue;
>> - if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
>> - !kvm_arch_vcpu_in_kernel(vcpu))
>> - continue;
>> if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
>> continue;
>>
>> + if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
>> + !kvm_arch_vcpu_in_kernel(vcpu)) {
>> + /*
>> + * A vCPU running in userspace can get to kernel mode via
>> + * an interrupt. That's a worse choice than a CPU already
>> + * in kernel mode so only do it on a second pass.
>> + */
>> + if (!vcpu_dy_runnable(vcpu))
>> + continue;
>> + if (pass == 0) {
>> + num_passes = 2;
>> + continue;
>> + }
>> + }
>> +
>> yielded = kvm_vcpu_yield_to(vcpu);
>> if (yielded > 0) {
>> kvm->last_boosted_vcpu = i;
>> - break;
>> + goto done;
>> } else if (yielded < 0) {
>> try--;
>> if (!try)
>> - break;
>> + goto done;
>> }
>> }
>> }
>> +done:
>
> We just tested the above post against 96 vCPUs VM in an over-subscribe
> scenario, the score of pbzip2 fluctuated drastically. Sometimes it is
> worse than vanilla, but the average improvement is around 2.2%. The
> new version of my post is around 9.3%,the origial posted patch is
> around 10% which is totally as expected since now both IPI receivers
> in user-mode and lock-waiters are second class citizens.

Fair enough. Of the two patches you posted I prefer the original, so
I'll go with that one.

Paolo

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