linux-kernel - Re: [PATCH v3] sched/core: Address classes via __begin_sched

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Date:   Tue, 17 May 2022 08:42:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christophe de Dinechin <dinechin@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] sched/core: Address classes via __begin_sched_classes

On Mon, May 16, 2022 at 08:00:23PM -0700, Kees Cook wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d58c0389eb23..f2bcc7f15381 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5665,7 +5665,8 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
>  	 * We can terminate the balance pass as soon as we know there is
>  	 * a runnable task of @class priority or higher.
>  	 */
> -	for_class_range(class, prev->sched_class, &idle_sched_class) {
> +	for_class_range(class, prev->sched_class,
> +			sched_class_higher(&idle_sched_class)) {
>  		if (class->balance(rq, prev, rf))
>  			break;
>  	}
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8dccb34eb190..c757bd26b01a 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2193,11 +2193,16 @@ const struct sched_class name##_sched_class \
>  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 sched_class_higher(class) ((class) + 1)
>  
> +#define sched_class_highest (&__begin_sched_classes[__end_sched_classes     \
> +						    - __begin_sched_classes \
> +						    - 1])
> +#define sched_class_lowest  (&__begin_sched_classes[0])
> +
> +/* For each class, inclusive from _from down to _to. */
>  #define for_class_range(class, _from, _to) \
> -	for (class = (_from); class != (_to); class--)
> +	for (class = (_from); class >= (_to); class--)
>  
>  #define for_each_class(class) \
>  	for_class_range(class, sched_class_highest, sched_class_lowest)


Urgh, and we're back to unreadable garbage just because GCC is insane :/