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

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Message-ID: <20181107144448.GH9761@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Nov 2018 15:44:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v5 03/15] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups

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.