Messages in this thread | | | Date | Thu, 05 Apr 2012 16:10:52 +0530 | From | Raghavendra K T <> | Subject | Re: [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.
| |