linux-kernel - Re: [PATCH v6 03/16] sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets

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Date:   Mon, 21 Jan 2019 13:51:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...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 v6 03/16] sched/core: uclamp: Map TASK's clamp values
 into CPU's clamp buckets

On Mon, Jan 21, 2019 at 12:27:10PM +0000, Patrick Bellasi wrote:
> On 21-Jan 11:15, Peter Zijlstra wrote:
> > On Tue, Jan 15, 2019 at 10:15:00AM +0000, Patrick Bellasi wrote:
> > > +/*
> > > + * Number of utilization clamp buckets.
> > > + *
> > > + * The first clamp bucket (bucket_id=0) is used to track non clamped tasks, i.e.
> > > + * util_{min,max} (0,SCHED_CAPACITY_SCALE). Thus we allocate one more bucket in
> > > + * addition to the compile time configured number.
> > > + */
> > > +#define UCLAMP_BUCKETS (CONFIG_UCLAMP_BUCKETS_COUNT + 1)
> > > +
> > > +/*
> > > + * Utilization clamp bucket
> > > + * @value:		clamp value tracked by a clamp bucket
> > > + * @bucket_id:		the bucket index used by the fast-path
> > > + * @mapped:		the bucket index is valid
> > > + *
> > > + * A utilization clamp bucket maps a:
> > > + *   clamp value (value), i.e.
> > > + *   util_{min,max} value requested from userspace
> > > + * to a:
> > > + *   clamp bucket index (bucket_id), i.e.
> > > + *   index of the per-cpu RUNNABLE tasks refcounting array
> > > + *
> > > + * The mapped bit is set whenever a task has been mapped on a clamp bucket for
> > > + * the first time. When this bit is set, any:
> > > + *   uclamp_bucket_inc() - for a new clamp value
> > > + * is matched by a:
> > > + *   uclamp_bucket_dec() - for the old clamp value
> > > + */
> > > +struct uclamp_se {
> > > +	unsigned int value		: bits_per(SCHED_CAPACITY_SCALE);
> > > +	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
> > > +	unsigned int mapped		: 1;
> > > +};
> > 
> > Do we want something like:
> > 
> > 	BUILD_BUG_ON(sizeof(struct uclamp_se) == sizeof(unsigned int));
> 
> Mmm... isn't "!=" what you mean ?

Quite.

> We cannot use less then an unsigned int for the fields above... am I
> missing something?

I wanted to ensure we don't accidentally use more.

> > And/or put a limit on CONFIG_UCLAMP_BUCKETS_COUNT that guarantees that ?
> 
> The number of buckets is currently KConfig limited to a max of 20, which gives:
> 
>    UCLAMP_BUCKETS: 21 => 5bits
> 
> Thus, even on 32 bit targets and assuming 21bits for an "extended"
> SCHED_CAPACITY_SCALE range we should always fit into an unsigned int
> and have at least 6 bits for flags.
> 
> Are you afraid of some compiler magic related to bitfields packing ?

Nah, I missed the Kconfig limit and was afraid that some weird configs
would end up with massively huge structures.