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

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Message-ID: <20191220121947.GH2844@hirez.programming.kicks-ass.net>
Date:   Fri, 20 Dec 2019 13:19:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Kirill Tkhai <tkhai@...dex.ru>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 1/4] sched: Force the address order of each sched
 class descriptor

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?