Re: [PATCH v3 03/14] sched/core: uclamp: add CPU's clamp groups accounting

[Dietmar Eggemann]

On 08/16/2018 04:21 PM, Quentin Perret wrote:
> On Thursday 16 Aug 2018 at 15:45:45 (+0200), Dietmar Eggemann wrote:
> > On 08/16/2018 03:37 PM, Quentin Perret wrote:
> > > > > IMHO, if this is something which should not happen at all, a BUG_ON() is the
> > > > > right thing to do here.
> > > >
> > > > I don't agree on that. I agree it should not happen but since it's a
> > > > recoverable error it think we should not panic.
> > >
> > > FWIW, if this is a recoverable error, I think Linus will agree with
> > > Patrick on this one :-)
> > >
> > > https://lkml.org/lkml/2016/10/4/1
> >
> > Yeah, not really agreeing here that this is a recoverable error.
>
> A non-recoverable scenario could be, for example, if you corrupt your
> stack and there is absolutely _nothing_ you can do to keep the system up
> and running, because it's just too broken. I don't feel like we're
> talking about such an extreme case here ...

Yeah, that's the extreme. But what about this lovely BUG_ON(busiest == 
env.dst_rq) in fair.c's load_balance()?

We could recover by just bailing out ;-)

I guess we know by now that there are different opinions here.

> > Besides, we
> > only consider under-run here, what about over-run?

Important thing is to also detect the over-run, i.e. add the first task 
and the task counter is already > 0.

> > Currently this warning doesn't hit and if the code will be changed and it
> > hits, I still find a BUG_ON more appealing here ...
> >
> > So this error scenario can happen over and over again and we always recover
> > from ? The important thing is that we find the culprit for this behaviour as
> > fast as possible ...
>
> Agreed, we want to debug that ASAP, but WARN should let us do that just
> fine, I think.

+1.