Don't understand the reasoning behind why 3 is a good choice.
On Mon, Nov 26, 2012 at 05:38:04PM +0530, Raghavendra K T wrote:
> From: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
>
> yield_to returns -ESRCH, When source and target of yield_to
> run queue length is one. When we see three successive failures of
> yield_to we assume we are in potential undercommit case and abort
> from PLE handler.
> The assumption is backed by low probability of wrong decision
> for even worst case scenarios such as average runqueue length
> between 1 and 2.
>
> note that we do not update last boosted vcpu in failure cases.
> Thank Avi for raising question on aborting after first fail from yield_to.
>
> Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 26 ++++++++++++++++----------
> 1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index be70035..053f494 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1639,6 +1639,7 @@ bool kvm_vcpu_yield_to(struct kvm_vcpu *target)
> {
> struct pid *pid;
> struct task_struct *task = NULL;
> + bool ret = false;
>
> rcu_read_lock();
> pid = rcu_dereference(target->pid);
> @@ -1646,17 +1647,15 @@ bool kvm_vcpu_yield_to(struct kvm_vcpu *target)
> task = get_pid_task(target->pid, PIDTYPE_PID);
> rcu_read_unlock();
> if (!task)
> - return false;
> + return ret;
> if (task->flags & PF_VCPU) {
> put_task_struct(task);
> - return false;
> - }
> - if (yield_to(task, 1)) {
> - put_task_struct(task);
> - return true;
> + return ret;
> }
> + ret = yield_to(task, 1);
> put_task_struct(task);
> - return false;
> +
> + return ret;
> }
> EXPORT_SYMBOL_GPL(kvm_vcpu_yield_to);
>
> @@ -1697,12 +1696,14 @@ bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
> return eligible;
> }
> #endif
> +
> 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 try = 3;
> int pass;
> int i;
>
> @@ -1714,7 +1715,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> * 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; pass++) {
> + 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;
> @@ -1727,10 +1728,15 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> continue;
> if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
> continue;
> - if (kvm_vcpu_yield_to(vcpu)) {
> +
> + yielded = kvm_vcpu_yield_to(vcpu);
> + if (yielded > 0) {
> kvm->last_boosted_vcpu = i;
> - yielded = 1;
> break;
> + } else if (yielded < 0) {
> + try--;
> + if (!try)
> + break;
> }
> }
> }
>
> --
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