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

[Rasmus Villemoes]

On 20/12/2019 13.19, Peter Zijlstra wrote:
> 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;

extern const struct sched_class __begin_sched_classes[];

but yes, I get the idea.

> #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?

Yes, I think so - I was only thinking of the case where all the symbols
would be defined in the linker script, and for this to work you need the
C compiler to subtract the sizeof().

I'd probably not include the -1 in the definition of sched_class_lowest,
but instead put it in the for_each_class definition (i.e. use
sched_class_lowest-1 as _to parameter).

A whole other option is of course to make the whole thing a bona fide C
array defined in sched/core.c, with fair_sched_class being defined as
&sched_classes[1] etc. But that requires giving all the methods extern
linkage. The advantage might be that the compiler can see how much we
iterate over, though I wouldn't expect it to actually unroll the
for_each_class loops five times. So yes, the above is probably the best
way to go.

Rasmus