[PATCH v2 2/7] sched: rework of sched_domain topology definition

[Peter Zijlstra]

The keyboard deity gave us delete, please apply graciously when replying
to large emails.

On Wed, Mar 19, 2014 at 11:27:12AM +0000, Dietmar Eggemann wrote:
> On 18/03/14 17:56, Vincent Guittot wrote:
> > +       if (sd->flags & SD_SHARE_CPUPOWER) {
> > +               sd->imbalance_pct = 110;
> > +               sd->smt_gain = 1178; /* ~15% */
> > +               sd->flags |= arch_sd_sibling_asym_packing();
> > +
> > +       } else if (sd->flags & SD_SHARE_PKG_RESOURCES) {
> > +               sd->imbalance_pct = 117;
> > +               sd->cache_nice_tries = 1;
> > +               sd->busy_idx = 2;
> > +
> > +#ifdef CONFIG_NUMA
> > +       } else if (sd->flags & SD_NUMA) {
> > +               sd->cache_nice_tries = 2;
> > +               sd->busy_idx = 3;
> > +               sd->idle_idx = 2;
> > +
> > +               sd->flags |= SD_SERIALIZE;
> > +               if (sched_domains_numa_distance[tl->numa_level] > RECLAIM_DISTANCE) {
> > +                       sd->flags &= ~(SD_BALANCE_EXEC |
> > +                                      SD_BALANCE_FORK |
> > +                                      SD_WAKE_AFFINE);
> > +               }
> > +
> > +#endif
> > +       } else {
> > +               sd->flags |= SD_PREFER_SIBLING;
> > +               sd->cache_nice_tries = 1;
> > +               sd->busy_idx = 2;
> > +               sd->idle_idx = 1;
> > +       }
> 
> This 'if ... else statement' is still a weak point from the perspective
> of making the code robust:

<snip>

> Is there a way to check that MC and GMC have to have
> SD_SHARE_PKG_RESOURCES set so that this can't happen unnoticed?

So from the core codes perspective those names mean less than nothing.
Its just a string to carry along for us meat-bags. The string isn't even
there when !SCHED_DEBUG.

So from this codes POV you told it it had a domain without PKGSHARE,
that's fine.

That said; yeah the thing isn't the prettiest piece of code. But it has
the big advantage of being the one place where we convert topology into
behaviour.