Re: [PATCH v5 03/15] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

From: Peter Zijlstra
Date: Wed Nov 07 2018 - 09:44:56 EST


On Wed, Nov 07, 2018 at 02:24:28PM +0000, Patrick Bellasi wrote:
> On 07-Nov 14:44, Peter Zijlstra wrote:
> > On Mon, Oct 29, 2018 at 06:32:57PM +0000, Patrick Bellasi wrote:

> > > +static void uclamp_group_get(struct uclamp_se *uc_se, unsigned int clamp_id,
> > > + unsigned int clamp_value)
> > > +{
> > > + union uclamp_map *uc_maps = &uclamp_maps[clamp_id][0];
> > > + unsigned int prev_group_id = uc_se->group_id;
> > > + union uclamp_map uc_map_old, uc_map_new;
> > > + unsigned int free_group_id;
> > > + unsigned int group_id;
> > > + unsigned long res;
> > > +
> > > +retry:
> > > +
> > > + free_group_id = UCLAMP_GROUPS;
> > > + for (group_id = 0; group_id < UCLAMP_GROUPS; ++group_id) {
> > > + uc_map_old.data = atomic_long_read(&uc_maps[group_id].adata);
> > > + if (free_group_id == UCLAMP_GROUPS && !uc_map_old.se_count)
> > > + free_group_id = group_id;
> > > + if (uc_map_old.value == clamp_value)
> > > + break;
> > > + }
> > > + if (group_id >= UCLAMP_GROUPS) {
> > > +#ifdef CONFIG_SCHED_DEBUG
> > > +#define UCLAMP_MAPERR "clamp value [%u] mapping to clamp group failed\n"
> > > + if (unlikely(free_group_id == UCLAMP_GROUPS)) {
> > > + pr_err_ratelimited(UCLAMP_MAPERR, clamp_value);
> > > + return;
> > > + }
> > > +#endif
> >
> > Can you please put in a comment, either here or on top, on why this can
> > not in fact happen? And we're always guaranteed a free group.
>
> You right, that's confusing especially because up to this point we are
> not granted. We are always granted a free group once we add:
>
> sched/core: uclamp: add clamp group bucketing support
>
> I've kept it separated to better document how we introduce that
> support.
>
> Is it ok for for you if I better call out in the change log that the
> guarantee comes from a following patch... and add the comment in
> that later patch ?

Urgh.. that is mighty confusing and since this stuff actually 'works'
might result in bisection issues too, right?

I would really rather prefer a series that makes sense in the order you
read it.