Re: [RFC] sched/core: Don't schedule threads on pre-empted vcpus

[Rohit Jain]

Hi Steve,


On 05/04/2018 10:32 AM, Steven Sistare wrote:
> On 5/4/2018 1:22 PM, Rohit Jain wrote:
> > Hi Peter,
> >
> > On 05/04/2018 02:47 AM, Peter Zijlstra wrote:
> > > On Wed, May 02, 2018 at 01:52:10PM -0700, Rohit Jain wrote:
> > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > index 5e10aae..75d1ecf 100644
> > > > --- a/kernel/sched/core.c
> > > > +++ b/kernel/sched/core.c
> > > > @@ -4033,6 +4033,9 @@ int idle_cpu(int cpu)
> > > >  ÂÂÂÂÂÂÂÂÂ return 0;
> > > >  Â #endif
> > > >  Â +ÂÂÂ if (vcpu_is_preempted(cpu))
> > > > +ÂÂÂÂÂÂÂ return 0;
> > > > +
> > > >  ÂÂÂÂÂ return 1;
> > > >  Â }
> > > Basically OK with this, but did you consider idle_cpu() usage outside of
> > > select_idle_sibling()?
> > >
> > > For instance, I think got_nohz_idle_kick() isn't quite right with this
> > > on. Similarly for scheduler_tick(), that wants the actual idle state.
> > As far as intent is concerned, yes I agree you might be right. I left
> > the VM running for a couple of days, didn't see anything weird however.
> >
> > We could add a check at each of those places or something to that effect
> > if this is an issue. Please let me know how you want to proceed.
> The point is that some idle_cpu() call sites should consider preemption state
> and some should not, and they must be considered on a case by case basis.  You
> could define a new accessor to abstract the difference, and call it from
> select_idle_sibling and anywhere else it makes sense.
>
> available_idle_cpu()
> {
>    return idle_cpu() && !vcpu_is_preempted()
> }

Great! That's what I was thinking as "something to that effect" :)

Thanks,
Rohit