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Date: Fri, 28 Sep 2012 19:41:22 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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CC: Avi Kivity <avi@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
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        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
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Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios
 in PLE handler
References: <20120921115942.27611.67488.sendpatchset@codeblue> <1348486479.11847.46.camel@twins> <50604988.2030506@linux.vnet.ibm.com> <1348490165.11847.58.camel@twins> <50606050.309@linux.vnet.ibm.com> <1348494895.11847.64.camel@twins> <50606B33.1040102@linux.vnet.ibm.com> <5061B437.8070300@linux.vnet.ibm.com> <5064101A.5070902@redhat.com> <50643745.6010202@linux.vnet.ibm.com> <506440AF.9080202@redhat.com> <506537C7.9070909@linux.vnet.ibm.com> <1348832438.5551.5.camel@oc6622382223.ibm.com>
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On 09/28/2012 05:10 PM, Andrew Theurer wrote:
> On Fri, 2012-09-28 at 11:08 +0530, Raghavendra K T wrote:
>> On 09/27/2012 05:33 PM, Avi Kivity wrote:
>>> On 09/27/2012 01:23 PM, Raghavendra K T wrote:
>>>>>
[...]
>>>
>>> Also there may be a lot of false positives (deferred preemptions even
>>> when there is no contention).
>
> It will be interesting to see how this behaves with a very high lock
> activity in a guest.  Once the scheduler defers preemption, is it for a
> fixed amount of time, or does it know to cut the deferral short as soon
> as the lock depth is reduced [by x]?

Design/protocol that Vatsa, had in mind was something like this:

- scheduler does not give a vcpu holding lock forever, it may give one
chance that would give only few ticks. In addition to giving chance,
scheduler also sets some indication that he has been given chance.

- vcpu once he release (all) the lock(s), if it had given chance,
it should clear that (ACK), and relinquish the cpu.