On Thu, Aug 30, 2012 at 12:51:01AM +0530, Raghavendra K T wrote:
> The idea of starting from next vcpu (source of yield_to + 1) seem to work
> well for overcomitted guest rather than using last boosted vcpu. We can also
> remove per VM variable with this approach.
>
> Iteration for eligible candidate after this patch starts from vcpu source+1
> and ends at source-1 (after wrapping)
>
> Thanks Nikunj for his quick verification of the patch.
>
> Please let me know if this patch is interesting and makes sense.
>
This last_boosted_vcpu thing caused us trouble during attempt to
implement vcpu destruction. It is good to see it removed from this POV.
> ====8<====
> From: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
>
> Currently we use next vcpu to last boosted vcpu as starting point
> while deciding eligible vcpu for directed yield.
>
> In overcomitted scenarios, if more vcpu try to do directed yield,
> they start from same vcpu, resulting in wastage of cpu time (because of
> failing yields and double runqueue lock).
>
> Since probability of same vcpu trying to do directed yield is already
> prevented by improved PLE handler, we can start from next vcpu from source
> of yield_to.
>
> Suggested-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> ---
>
> include/linux/kvm_host.h | 1 -
> virt/kvm/kvm_main.c | 12 ++++--------
> 2 files changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index b70b48b..64a090d 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -275,7 +275,6 @@ struct kvm {
> #endif
> struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
> atomic_t online_vcpus;
> - int last_boosted_vcpu;
> struct list_head vm_list;
> struct mutex lock;
> struct kvm_io_bus *buses[KVM_NR_BUSES];
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2468523..65a6c83 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1584,7 +1584,6 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> {
> struct kvm *kvm = me->kvm;
> struct kvm_vcpu *vcpu;
> - int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
> int yielded = 0;
> int pass;
> int i;
> @@ -1594,21 +1593,18 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> * currently running, because it got preempted by something
> * 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.
> + * We approximate round-robin by starting at the next VCPU.
> */
> for (pass = 0; pass < 2 && !yielded; pass++) {
> kvm_for_each_vcpu(i, vcpu, kvm) {
> - if (!pass && i <= last_boosted_vcpu) {
> - i = last_boosted_vcpu;
> + if (!pass && i <= me->vcpu_id) {
> + i = me->vcpu_id;
> continue;
> - } else if (pass && i > last_boosted_vcpu)
> + } else if (pass && i >= me->vcpu_id)
> break;
> - if (vcpu == me)
> - continue;
> if (waitqueue_active(&vcpu->wq))
> continue;
> if (kvm_vcpu_yield_to(vcpu)) {
> - kvm->last_boosted_vcpu = i;
> yielded = 1;
> break;
> }
--
Gleb.
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