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

[Gleb Natapov <gleb@redhat.com>]

On Tue, Sep 25, 2012 at 10:54:21AM +0200, Avi Kivity wrote:
> On 09/25/2012 10:09 AM, Raghavendra K T wrote:
> > On 09/24/2012 09:36 PM, Avi Kivity wrote:
> >> On 09/24/2012 05:41 PM, Avi Kivity wrote:
> >>>
> >>>>
> >>>> case 2)
> >>>> rq1 : vcpu1->wait(lockA) (spinning)
> >>>> rq2 : vcpu3 (running) ,  vcpu2->holding(lockA) [scheduled out]
> >>>>
> >>>> I agree that checking rq1 length is not proper in this case, and as
> >>>> you
> >>>> rightly pointed out, we are in trouble here.
> >>>> nr_running()/num_online_cpus() would give more accurate picture here,
> >>>> but it seemed costly. May be load balancer save us a bit here in not
> >>>> running to such sort of cases. ( I agree load balancer is far too
> >>>> complex).
> >>>
> >>> In theory preempt notifier can tell us whether a vcpu is preempted or
> >>> not (except for exits to userspace), so we can keep track of whether
> >>> it's we're overcommitted in kvm itself.  It also avoids false positives
> >>> from other guests and/or processes being overcommitted while our vm
> >>> is fine.
> >>
> >> It also allows us to cheaply skip running vcpus.
> >
> > Hi Avi,
> >
> > Could you please elaborate on how preempt notifiers can be used
> > here to keep track of overcommit or skip running vcpus?
> >
> > Are we planning set some flag in sched_out() handler etc?
> >
> 
> Keep a bitmap kvm->preempted_vcpus.
> 
> In sched_out, test whether we're TASK_RUNNING, and if so, set a vcpu
> flag and our bit in kvm->preempted_vcpus.  On sched_in, if the flag is
> set, clear our bit in kvm->preempted_vcpus.  We can also keep a counter
> of preempted vcpus.
> 
> We can use the bitmap and the counter to quickly see if spinning is
> worthwhile (if the counter is zero, better to spin).  If not, we can use
> the bitmap to select target vcpus quickly.
> 
> The only problem is that in order to keep this accurate we need to keep
> the preempt notifiers active during exits to userspace.  But we can
> prototype this without this change, and add it later if it works.
> 
Can user return notifier can be used instead? Set bit in
kvm->preempted_vcpus on return to userspace.

--
			Gleb.