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

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Date:	Wed, 25 May 2016 10:51:14 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Carrillo-Cisneros <davidcc@...gle.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Tony Luck <tony.luck@...el.com>,
	Stephane Eranian <eranian@...gle.com>,
	Paul Turner <pjt@...gle.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 10/32] perf/x86/intel/cqm: introduce (I)state and
 limbo prmids

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