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

[Avi Kivity <avi@redhat.com>]

On 09/27/2012 12:08 PM, Gleb Natapov wrote:
> 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.

True.  In theory boosting a random vcpu shouldn't have any negative
effects though.  Right now the problem is that the boosting itself is
expensive.


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