Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenariosin PLE handler

[Raghavendra K T]

On 10/04/2012 06:14 PM, Avi Kivity wrote:
> On 10/04/2012 12:56 PM, Raghavendra K T wrote:
> > On 10/03/2012 10:55 PM, Avi Kivity wrote:
> > > On 10/03/2012 04:29 PM, Raghavendra K T wrote:
> > > > * Avi Kivity <avi@xxxxxxxxxx> [2012-09-27 14:03:59]:
> > > >
> > > > > On 09/27/2012 01:23 PM, Raghavendra K T wrote:
> > > > > > >
> > > > [...]
> > > > > > 2) looking at the result (comparing A & C) , I do feel we have
> > > > > > significant in iterating over vcpus (when compared to even vmexit)
> > > > > > so We still would need undercommit fix sugested by PeterZ
> > > > > > (improving by
> > > > > > 140%). ?
> > > > >
> > > > > Looking only at the current runqueue?  My worry is that it misses a lot
> > > > > of cases.  Maybe try the current runqueue first and then others.
> > > >
> > > > Okay. Do you mean we can have something like
> > > >
> > > > +       if (rq->nr_running == 1 && p_rq->nr_running == 1) {
> > > > +               yielded = -ESRCH;
> > > > +               goto out_irq;
> > > > +       }
> > > >
> > > > in the Peter's patch ?
> > > >
> > > > ( I thought lot about && or || . Both seem to have their own cons ).
> > > > But that should be only when we have short term imbalance, as PeterZ
> > > > told.
> > >
> > > I'm missing the context.  What is p_rq?
> >
> > p_rq is the run queue of target vcpu.
> > What I was trying below was to address Rik concern. Suppose
> > rq of source vcpu has one task, but target probably has two task,
> > with a eligible vcpu waiting to be scheduled.
> >
> > > What I mean was:
> > >
> > >     if can_yield_to_process_in_current_rq
> > >        do that
> > >     else if can_yield_to_process_in_other_rq
> > >        do that
> > >     else
> > >        return -ESRCH
> >
> > I think you are saying we have to check the run queue of the
> > source vcpu, if we have a vcpu belonging to same VM and try yield to
> > that? ignoring whatever the target vcpu we received for yield_to.
> >
> > Or is it that kvm_vcpu_yield_to should now check the vcpus of same vm
> > belonging to same run queue first. If we don't succeed, go again for
> > a vcpu in different runqueue.
>
> Right.  Prioritize vcpus that are cheap to yield to.  But may return bad
> results if all vcpus on the current runqueue are spinners, so probably
> not a good idea.

Okay. 'll drop vcpu from same rq idea now.

> > Does it add more overhead especially in <= 1x scenario?
>
> The current runqueue should have just our vcpu in that case, so low
> overhead.  But it's a bad idea due to the above scenario.

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