linux-kernel - Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler

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Message-ID: <5060851E.1030404@redhat.com>
Date:	Mon, 24 Sep 2012 18:06:54 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Rik van Riel <riel@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...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>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE
 handler

On 09/24/2012 05:41 PM, Avi Kivity wrote:
> 
>> 
>> case 2)
>> rq1 : vcpu1->wait(lockA) (spinning)
>> rq2 : vcpu3 (running) ,  vcpu2->holding(lockA) [scheduled out]
>> 
>> I agree that checking rq1 length is not proper in this case, and as you
>> rightly pointed out, we are in trouble here.
>> nr_running()/num_online_cpus() would give more accurate picture here,
>> but it seemed costly. May be load balancer save us a bit here in not
>> running to such sort of cases. ( I agree load balancer is far too
>> complex).
> 
> In theory preempt notifier can tell us whether a vcpu is preempted or
> not (except for exits to userspace), so we can keep track of whether
> it's we're overcommitted in kvm itself.  It also avoids false positives
> from other guests and/or processes being overcommitted while our vm is fine.

It also allows us to cheaply skip running vcpus.

We would probably need a ->sched_exit() preempt notifier to make this
work.  Peter, I know how much you love those, would it be acceptable?
We'd still need yield_to() but the pressure on it might be reduced.

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