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

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Date:	Thu, 27 Sep 2012 12:04:58 +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 11:58 AM, Gleb Natapov wrote:
> 
>> >  
>> >> 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.
>> 
> We can't do much about it without PV spinlock since there is not
> information about what vcpu holds which guest spinlock, no?

It doesn't help.  If the lock holder is waiting for another lock in the
host kernel, boosting it doesn't help even if we know who it is.  We
need to boost the real lock holder, but we have no idea who it is (and
even if we did, we often can't do anything about it).


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