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Date: Fri, 21 Sep 2012 23:15:40 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Rik van Riel <riel@redhat.com>
CC: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Avi Kivity <avi@redhat.com>, Srikar <srikar@linux.vnet.ibm.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
        KVM <kvm@vger.kernel.org>, Jiannan Ouyang <ouyang@cs.pitt.edu>,
        chegu vinod <chegu_vinod@hp.com>,
        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC 2/2] kvm: Be courteous to other VMs in overcommitted
 scenario in PLE handler
References: <20120921115942.27611.67488.sendpatchset@codeblue> <20120921120019.27611.66093.sendpatchset@codeblue> <20120921224636.18cccbd6c33b161975f2a8cb@gmail.com> <505C712F.4060800@redhat.com>
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On 09/21/2012 07:22 PM, Rik van Riel wrote:
> On 09/21/2012 09:46 AM, Takuya Yoshikawa wrote:
>> On Fri, 21 Sep 2012 17:30:20 +0530
>> Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
>>
>>> From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>>>
>>> When PLE handler fails to find a better candidate to yield_to, it
>>> goes back and does spin again. This is acceptable when we do not
>>> have overcommit.
>>> But in overcommitted scenarios (especially when we have large
>>> number of small guests), it is better to yield.
>>>
>>> Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
>>> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>>> ---
>>> virt/kvm/kvm_main.c | 4 ++++
>>> 1 files changed, 4 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index 8323685..713b677 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -1660,6 +1660,10 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>>> }
>>> }
>>> }
>>> + /* In overcommitted cases, yield instead of spinning */
>>> + if (!yielded && rq_nr_running() > 1)
>>> + schedule();
>>
>> How about doing cond_resched() instead?
>
> Actually, an actual call to yield() may be better.
>
> That will set scheduler hints to make the scheduler pick
> another task for one round, while preserving this task's
> top position in the runqueue.

I am not a scheduler expert, but I am also inclined towards
Rik's suggestion here since we set skip buddy here. Takuya?