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

[Raghavendra K T]

On 09/25/2012 02:24 PM, 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.

Avi, Thanks for the idea.. I want to try this some time soon.

So ideally it means if we are under-committed the counter/ bitmap
effective value is zero.

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