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Date: Fri, 21 Sep 2012 09:02:03 -0400
From: Rik van Riel <riel@redhat.com>
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To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
CC: Peter Zijlstra <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Avi Kivity <avi@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Marcelo Tosatti <mtosatti@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 1/2] kvm: Handle undercommitted guest case in PLE
 handler
References: <20120921115942.27611.67488.sendpatchset@codeblue> <20120921120000.27611.71321.sendpatchset@codeblue>
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On 09/21/2012 08:00 AM, Raghavendra K T wrote:
> From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>
> When total number of VCPUs of system is less than or equal to physical CPUs,
> PLE exits become costly since each VCPU can have dedicated PCPU, and
> trying to find a target VCPU to yield_to just burns time in PLE handler.
>
> This patch reduces overhead, by simply doing a return in such scenarios by
> checking the length of current cpu runqueue.

I am not convinced this is the way to go.

The VCPU that is holding the lock, and is not releasing it,
probably got scheduled out. That implies that VCPU is on a
runqueue with at least one other task.

> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1629,6 +1629,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>   	int pass;
>   	int i;
>
> +	if (unlikely(rq_nr_running() == 1))
> +		return;
> +
>   	kvm_vcpu_set_in_spin_loop(me, true);
>   	/*
>   	 * We boost the priority of a VCPU that is runnable but not
>


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