Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 20 Dec 2019 13:19:47 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        Kirill Tkhai <tkhai@yandex.ru>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "juri.lelli@redhat.com" <juri.lelli@redhat.com>,
        "vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
        "dietmar.eggemann@arm.com" <dietmar.eggemann@arm.com>,
        "bsegall@google.com" <bsegall@google.com>,
        "mgorman@suse.de" <mgorman@suse.de>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC][PATCH 1/4] sched: Force the address order of each sched
 class descriptor
Message-ID: <20191220121947.GH2844@hirez.programming.kicks-ass.net>
References: <20191219214451.340746474@goodmis.org>
 <20191219214558.510271353@goodmis.org>
 <0a957e8d-7af8-613c-11ae-f51b9b241eb7@rasmusvillemoes.dk>
 <20191220100033.GE2844@hirez.programming.kicks-ass.net>
 <1ac359e8-fa7f-7ded-e2f2-9f7d0b23a4e1@rasmusvillemoes.dk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1ac359e8-fa7f-7ded-e2f2-9f7d0b23a4e1@rasmusvillemoes.dk>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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?