Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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.