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SubjectRe: [PATCH RFC V6 0/11] Paravirtualized ticketlocks
On 04/05/2012 02:31 PM, Avi Kivity wrote:
> On 04/02/2012 12:51 PM, Raghavendra K T wrote:
>> On 04/01/2012 07:23 PM, Avi Kivity wrote:
>>> On 04/01/2012 04:48 PM, Raghavendra K T wrote:
>>>>>> I have patch something like below in mind to try:
>>>>>>
>>>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>>>> index d3b98b1..5127668 100644
>>>>>> --- a/virt/kvm/kvm_main.c
>>>>>> +++ b/virt/kvm/kvm_main.c
>>>>>> @@ -1608,15 +1608,18 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>>>>>> * else and called schedule in __vcpu_run. Hopefully that
>>>>>> * VCPU is holding the lock that we need and will release it.
>>>>>> * We approximate round-robin by starting at the last boosted
>>>>>> VCPU.
>>>>>> + * Priority is given to vcpu that are unhalted.
>>>>>> */
>>>>>> - for (pass = 0; pass< 2&& !yielded; pass++) {
>>>>>> + for (pass = 0; pass< 3&& !yielded; pass++) {
>>>>>> kvm_for_each_vcpu(i, vcpu, kvm) {
>>>>>> struct task_struct *task = NULL;
>>>>>> struct pid *pid;
>>>>>> - if (!pass&& i< last_boosted_vcpu) {
>>>>>> + if (!pass&& !vcpu->pv_unhalted)
>>>>>> + continue;
>>>>>> + else if (pass == 1&& i< last_boosted_vcpu) {
>>>>>> i = last_boosted_vcpu;
>>>>>> continue;
>>>>>> - } else if (pass&& i> last_boosted_vcpu)
>>>>>> + } else if (pass == 2&& i> last_boosted_vcpu)
>>>>>> break;
>>>>>> if (vcpu == me)
>>>>>> continue;
>>>>>>
>>>>>
>>>>> Actually I think this is unneeded. The loops tries to find vcpus
>> that
>>>>> are runnable but not running (vcpu_active(vcpu->wq)), and halted
>> vcpus
>>>>> don't match this condition.
>>>>>
>>
>> Oh! I think I misinterpreted your statement. hmm I got it. you told to
>> remove if (vcpu == me) condition.
>
> No, the entire patch is unneeded. My original comment was:
>
>> from the PLE handler, don't wake up a vcpu that is sleeping because it
> is waiting for a kick
>
> But the PLE handler never wakes up sleeping vcpus anyway.

I agree with you. It is already doing that. But my approach here is
little different.

In 2 classes of vcpus we have (one is subset of another when we try to
do yield_to) viz,

1) runnable and kicked < (subset of) 2) just runnable

what we are trying to do here is targeting 1) first so that we get good
lock progress.

Here was the sequence I was talking.

vcpu1 releases lock->finds that vcpu2 is next candidate ->
kick hypercall -> kvm_pv_kick_cpu_op -> set kicked flag ->
vcpu->kick(vcpu2)

at this point we have vcpu2 waiting for getting scheduled. But
above yield call can wake *anybody*.

I agree this is not serious (rather it is overhead) when there are are
less number of vcpus, But when we have more number of vcpu/vm.. it may
not scale well. My attempt was to fix that.

Let me know if I am completely missing something..

>
> There's still a conflict with PLE in that it may trigger during the spin
> phase and send a random yield_to() somewhere. Maybe it's sufficient to
> tune the PLE timeout to be longer than the spinlock timeout.
>

Ok ... But we also should be cautious that, we may do more halt, though
we are about to get spinlock.
Need more study on this.



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