linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

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Message-ID: <506801D3.6040305@redhat.com>
Date:	Sun, 30 Sep 2012 10:24:51 +0200
From:	Avi Kivity <avi@...hat.com>
To:	habanero@...ux.vnet.ibm.com
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	chegu vinod <chegu_vinod@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios
 in PLE handler

On 09/28/2012 01:40 PM, Andrew Theurer wrote:
>> 
>> >>
>> >> IIRC, with defer preemption :
>> >> we will have hook in spinlock/unlock path to measure depth of lock held,
>> >> and shared with host scheduler (may be via MSRs now).
>> >> Host scheduler 'prefers' not to preempt lock holding vcpu. (or rather
>> >> give say one chance.
>> >
>> > A downside is that we have to do that even when undercommitted.
> 
> Hopefully vcpu preemption is very rare when undercommitted, so it should
> not happen much at all.

As soon as you're preempted, you're effectively overcommitted (even if
the system as a whole is undercommitted).  What I meant was that you
need to communicate your lock state to the host, and with fine-grained
locking this can happen a lot.  It may be as simple as an
increment/decrement instruction though.



-- 
error compiling committee.c: too many arguments to function
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