linux-kernel - Re: [PATCH] preempt: Allow CONFIG_DEBUG_PREEMPT for CONFIG_PREEMPT

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Date:   Thu, 8 Jul 2021 13:37:59 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] preempt: Allow CONFIG_DEBUG_PREEMPT for
 CONFIG_PREEMPT_VOLUNTARY

On Wed, Jul 07, 2021 at 08:23:06PM +0100, Valentin Schneider wrote:
> While recently staring at some preempt_count dumpster fire for a
> CONFIG_PREEMPT_VOLUNTARY kernel, I wished I could turn on
> CONFIG_DEBUG_PREEMPT on top of CONFIG_DEBUG_ATOMIC_SLEEP to see *where*
> preemption had been disabled. It didn't take much to get that, and although
> it didn't help my particular case, I think it can still be useful.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/core.c | 3 +--
>  lib/Kconfig.debug   | 2 +-
>  2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0c22cd026440..5673b85fa22d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5109,8 +5109,7 @@ static inline void sched_tick_start(int cpu) { }
>  static inline void sched_tick_stop(int cpu) { }
>  #endif
>  
> -#if defined(CONFIG_PREEMPTION) && (defined(CONFIG_DEBUG_PREEMPT) || \
> -				defined(CONFIG_TRACE_PREEMPT_TOGGLE))
> +#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
>  /*
>   * If the value passed in is equal to the current preempt count
>   * then we just disabled preemption. Start timing the latency.
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 678c13967580..7fdd45eee343 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1204,7 +1204,7 @@ config DEBUG_TIMEKEEPING
>  
>  config DEBUG_PREEMPT
>  	bool "Debug preemptible kernel"
> -	depends on DEBUG_KERNEL && PREEMPTION && TRACE_IRQFLAGS_SUPPORT
> +	depends on DEBUG_KERNEL && PREEMPT_COUNT && TRACE_IRQFLAGS_SUPPORT

IIRC it used to be that way before. That was the reason for the split
of CONFIG_PREEMPT_COUNT from CONFIG_PREEMPT in the first place. Not
sure what happened. But anyway, your patch looks welcome.

Thanks.

>  	default y
>  	help
>  	  If you say Y here then the kernel will use a debug variant of the
> -- 
> 2.25.1
>