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

Peter Zijlstra
2019-01-21T12:51:20Z

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.