On Tue, Jun 19, 2012 at 04:51:04PM -0400, Rik van Riel wrote:
> On Wed, 20 Jun 2012 01:50:50 +0530
> Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx> wrote:
>
> >
> > In ple handler code, last_boosted_vcpu (lbv) variable is
> > serving as reference point to start when we enter.
>
> > Also statistical analysis (below) is showing lbv is not very well
> > distributed with current approach.
>
> You are the second person to spot this bug today (yes, today).
>
> Due to time zones, the first person has not had a chance yet to
> test the patch below, which might fix the issue...
>
> Please let me know how it goes.
>
> ====8<====
>
> If last_boosted_vcpu == 0, then we fall through all test cases and
> may end up with all VCPUs pouncing on vcpu 0. With a large enough
> guest, this can result in enormous runqueue lock contention, which
> can prevent vcpu0 from running, leading to a livelock.
>
> Changing < to <= makes sure we properly handle that case.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 7e14068..1da542b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1586,7 +1586,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> */
> for (pass = 0; pass < 2 && !yielded; pass++) {
> kvm_for_each_vcpu(i, vcpu, kvm) {
> - if (!pass && i < last_boosted_vcpu) {
> + if (!pass && i <= last_boosted_vcpu) {
> i = last_boosted_vcpu;
> continue;
> } else if (pass && i > last_boosted_vcpu)
>
Looks correct. We can simplify this by introducing something like:
#define kvm_for_each_vcpu_from(idx, n, vcpup, kvm) \
for (n = atomic_read(&kvm->online_vcpus); \
n && (vcpup = kvm_get_vcpu(kvm, idx)) != NULL; \
n--, idx = (idx+1) % atomic_read(&kvm->online_vcpus))
--
Gleb.
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