linux-kernel - Re: [PATCH] kvm: handle last_boosted

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Date:	Wed, 20 Jun 2012 22:11:01 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm: handle last_boosted_vcpu = 0 case

On 06/20/2012 04:12 PM, Raghavendra K T wrote:
> On 06/20/2012 02:21 AM, Rik van Riel wrote:

>> Please let me know how it goes.
>
> Yes, have got result today, too tired to summarize. got better
> performance result too. will come back again tomorrow morning.
> have to post, randomized start point patch also, which I discussed to
> know the opinion.

The other person's problem has also gone away with this
patch.

Avi, could I convince you to apply this obvious bugfix
to kvm.git? :)

>> ====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@...hat.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)
>>
>>
>


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