[PATCH] kvm: handle last_boosted_vcpu = 0 case

[Rik van Riel <riel@redhat.com>]

On Wed, 20 Jun 2012 01:50:50 +0530
Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> 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@redhat.com>
---
 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)