Re: [PATCH v2 10/32] perf/x86/intel/cqm: introduce (I)state and limbo prmids

Thomas Gleixner
2016-05-25T08:53:23Z

On Tue, 24 May 2016, David Carrillo-Cisneros wrote:
> >> +static inline bool __pmonr__in_instate(struct pmonr *pmonr)
> >> +{
> >> + lockdep_assert_held(&__pkg_data(pmonr, pkg_data_lock));
> >> + return __pmonr__in_istate(pmonr) && !__pmonr__in_ilstate(pmonr);
> >> }
> >
> > This state tracking sucks. It's completely non obvious which combinations of
> > members are denoting a certain state.
> >
> > What's wrong with having:
> >
> > pmonr->state
> >
> > and a enum
> >
> > enum pmonr_state {
> > PMONR_UNUSED,
> > PMONR_ACTIVE,
> > PMONR_LIMBO,
> > PMONR_INHERITED,
> > };
> >
> > That would make all this horror readable and understandable. I bet you can't
> > remember the meaning of all this state stuff 3 month from now. That's going to
> > be the hell of a ride to track down a problem in this code.
>
> In the pmonr, the state can be inferred by the values of:
> - pmonr->ancestor_pmonr
> - pmonr->prmid
> - pmonr->limbo_prmid

And exaclty that stuff drives me nuts. You update stuff here and there and
then you infer the state from this.

> Redundantly storing the state in an extra variable opens the door to
> bugs that updates pmonr::state inconsistently with the member above.

Well, your 'infer' state from three other variables is error prone as well and
you can simply add a debug feature which makes sure that the variables are
consistent.

validate_state(p)
{
switch (p->state) {
case PMONR_UNUSED:
WARN_ON(p->xxxx .....);

case PMONR_ACTIVE:
WARN_ON(p->xxxx .....);
}
}

That's way better than relying on three variables which are updated here and
there to reflect the proper state.

It's not only the 3 variables which are involved there. You also have lists
and whatever which depend on this. So having a proper 'state' variable as the
central anchor gives you the ability to verify the dependent contents of your
other variables, lists etc.

Thanks,

tglx