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Date: Thu, 27 Sep 2012 12:08:06 +0200
From: Gleb Natapov <gleb@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>, Rik van Riel <riel@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
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        Srikar <srikar@linux.vnet.ibm.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
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        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE
 handler
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On Thu, Sep 27, 2012 at 12:04:58PM +0200, Avi Kivity wrote:
> 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).
> 
Without PV lock we will boost random preempted vcpu instead of going to
sleep in the situation you described.

--
			Gleb.