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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Sep 27, 2012 at 11:33:56AM +0200, Avi Kivity wrote:
> 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).
> 
Of course, I got it all backwards. Need more coffee.

> >  
> >> 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?

--
			Gleb.