Re: [RFC][PATCH 1/4] sched: Force the address order of each sched class descriptor

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 20, 2019 at 11:12:37AM +0100, Rasmus Villemoes wrote:
> On 20/12/2019 11.00, Peter Zijlstra wrote:

> >>> +/*
> >>> + * The order of the sched class addresses are important, as they are
> >>> + * used to determine the order of the priority of each sched class in
> >>> + * relation to each other.
> >>> + */
> >>> +#define SCHED_DATA				\
> >>> +	*(__idle_sched_class)			\
> >>> +	*(__fair_sched_class)			\
> >>> +	*(__rt_sched_class)			\
> >>> +	*(__dl_sched_class)			\
> >>> +	STOP_SCHED_CLASS
> > 
> > I'm confused, why does that STOP_SCHED_CLASS need magic here at all?
> > Doesn't the linker deal with empty sections already by making them 0
> > sized?
> 
> Yes, but dropping the STOP_SCHED_CLASS define doesn't prevent one from
> needing some ifdeffery to define highest_sched_class if they are laid
> out in (higher sched class <-> higher address) order.

Would not something like:

	__begin_sched_classes = .;
	*(__idle_sched_class)
	*(__fair_sched_class)
	*(__rt_sched_class)
	*(__dl_sched_class)
	*(__stop_sched_class)
	__end_sched_classes = .;

combined with something like:

extern struct sched_class *__begin_sched_classes;
extern struct sched_class *__end_sched_classes;

#define sched_class_highest (__end_sched_classes - 1)
#define sched_class_lowest  (__begin_sched_classes - 1)

#define for_class_range(class, _from, _to) \
	for (class = (_from); class != (_to), class--)

#define for_each_class(class) \
	for_class_range(class, sched_class_highest, sched_class_lowest)

just work?

When no __stop_sched_class is present, that section is 0 sized, and
__end_sched_classes points to one past __dl_sched_class, no?