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

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Date:	Thu, 27 Sep 2012 11:33:56 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	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>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE
 handler

On 09/27/2012 11:11 AM, Gleb Natapov wrote:
>> 
>> User return notifier is per-cpu, not per-task.  There is a new task_work
>> (<linux/task_work.h>) that does what you want.  With these
>> technicalities out of the way, I think it's the wrong idea.  If a vcpu
>> thread is in userspace, that doesn't mean it's preempted, there's no
>> point in boosting it if it's already running.
>> 
> Ah, so you want to set bit in kvm->preempted_vcpus if task is _not_
> TASK_RUNNING in sched_out (you wrote opposite in your email)? If a task 
> is in userspace it is definitely not preempted.

No, as I originally wrote.  If it's TASK_RUNNING when it saw sched_out,
then it is preempted (i.e. runnable), not sleeping on some waitqueue,
voluntarily (HLT) or involuntarily (page fault).

>  
>> btw, we can have secondary effects.  A vcpu can be waiting for a lock in
>> the host kernel, or for a host page fault.  There's no point in boosting
>> anything for that.  Or a vcpu in userspace can be waiting for a lock
>> that is held by another thread, which has been preempted. 
> Do you mean userspace spinlock? Because otherwise task that's waits on
> a kernel lock will sleep in the kernel.

I meant a kernel mutex.

vcpu 0: take guest spinlock
vcpu 0: vmexit
vcpu 0: spin_lock(some_lock)
vcpu 1: take same guest spinlock
vcpu 1: PLE vmexit
vcpu 1: wtf?

Waiting on a host kernel spinlock is not too bad because we expect to be
out shortly.  Waiting on a host kernel mutex can be a lot worse.

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