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

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Date:	Tue, 25 Sep 2012 19:19:03 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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 09/25/2012 02:24 PM, Avi Kivity wrote:
> On 09/25/2012 10:09 AM, Raghavendra K T wrote:
>> On 09/24/2012 09:36 PM, Avi Kivity wrote:
>>> 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.
>>
>> Hi Avi,
>>
>> Could you please elaborate on how preempt notifiers can be used
>> here to keep track of overcommit or skip running vcpus?
>>
>> Are we planning set some flag in sched_out() handler etc?
>>
>
> Keep a bitmap kvm->preempted_vcpus.
>
> In sched_out, test whether we're TASK_RUNNING, and if so, set a vcpu
> flag and our bit in kvm->preempted_vcpus.  On sched_in, if the flag is
> set, clear our bit in kvm->preempted_vcpus.  We can also keep a counter
> of preempted vcpus.
>
> We can use the bitmap and the counter to quickly see if spinning is
> worthwhile (if the counter is zero, better to spin).  If not, we can use
> the bitmap to select target vcpus quickly.
>
> The only problem is that in order to keep this accurate we need to keep
> the preempt notifiers active during exits to userspace.  But we can
> prototype this without this change, and add it later if it works.
>

Avi, Thanks for the idea.. I want to try this some time soon.

So ideally it means if we are under-committed the counter/ bitmap
effective value is zero.

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