[Qemu-devel] Re: [PATCH] qemu-kvm: response to SIGUSR1 to start/stop a VCPU (v2)

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

On Thu, Dec 02, 2010 at 03:49:44PM +0200, Avi Kivity wrote:
> On 12/02/2010 03:13 PM, Srivatsa Vaddagiri wrote:
> >On Thu, Dec 02, 2010 at 02:41:35PM +0200, Avi Kivity wrote:
> >>  >>   What I'd like to see in directed yield is donating exactly the
> >>  >>   amount of vruntime that's needed to make the target thread run.
> >>  >
> >>  >I presume this requires the target vcpu to move left in rb-tree to run
> >>  >earlier than scheduled currently and that it doesn't involve any
> >>  >change to the sched_period() of target vcpu?
> >>  >
> >>  >Just was wondering how this would work in case of buggy guests. Lets say that a
> >>  >guest ran into a AB<->BA deadlock. VCPU0 spins on lock B (held by VCPU1
> >>  >currently), while VCPU spins on lock A (held by VCPU0 currently). Both keep
> >>  >boosting each other's vruntime, potentially affecting fairtime for other guests
> >>  >(to the point of starving them perhaps)?
> >>
> >>  We preserve vruntime overall.  If you give vruntime to someone, it
> >>  comes at your own expense.  Overall vruntime is preserved.
> >
> >Hmm ..so I presume that this means we don't affect target thread's position in
> >rb-tree upon donation, rather we influence its sched_period() to include
> >donated time? IOW donation has no effect on causing the target thread to run
> >"immediately", rather it will have the effect of causing it run "longer"
> >whenever it runs next?
> 
> No.  The intent (at least mine, maybe Rik has other ideas) is to

CCing Rik now ..

> move some vruntime from current to target such that target would be
> placed before current in the timeline.

Well ok, then this is what I had presumed earlier (about shifting target towards
left in rb-tree).

> >Even that would require some precaution in directed yield to ensure that it
> >doesn't unduly inflate vruntime of target, hurting fairness for other guests on
> >same cpu as target (example guest code that can lead to this situation
> >below):
> >
> >vcpu0:				vcpu1:
> >
> >				spinlock(A);
> >
> >spinlock(A);
> >
> >                         	while(1)
> >				;
> >
> >			 	spin_unlock(A);
> 
> directed yield should preserve the invariant that sum(vruntime) does
> not change.

Hmm don't think I understand this invariant sum() part. Lets take a simple
example as below:


p0	-> A0 B0 A1

p1	-> B1 C0 C1

A/B/C are VMs and A0 etc are virtual cpus. p0/1 are physical cpus

Let's say A0/A1 hit AB-BA spin-deadlock (which one can write in userspace
delibrately). When A0 spins and exits (due to PLE) what does its directed yield
do? Going by your statement, it can put target before current, leading to
perhaps this arrangement in runqueue:

p0	-> A1 B0 A0

Now A1 spins and wants to do a directed yield back to A0, leading to :

p0	-> A0 B0 A1

This can go back and forth, starving B0 (iow leading to some sort of DoS
attack).

Where does the "invariant sum" part of directed yield kick in to avoid such 
nastiness?

- vatsa