linux-kernel - Re: [PATCH 7/9] mm/compaction: Get rid of RT ifdeffery

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Message-ID: <54a68cad-f15d-bef2-8627-3d3bfb6f030d@prevas.dk>
Date:   Thu, 18 Aug 2022 10:55:28 +0200
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Terrell <terrelln@...com>, linux-mm@...ck.org
Subject: Re: [PATCH 7/9] mm/compaction: Get rid of RT ifdeffery

On 17/08/2022 18.27, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Move the RT dependency for the initial value of
> sysctl_compact_unevictable_allowed into Kconfig.
> 
>  
> +config COMPACT_UNEVICTABLE_DEFAULT
> +	int
> +	default 0 if PREEMPT_RT
> +	default 1
> +
>  #
>  # support for free page reporting
>  config PAGE_REPORTING
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 640fa76228dd9..10561cb1aaad9 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1727,11 +1727,7 @@ typedef enum {
>   * Allow userspace to control policy on scanning the unevictable LRU for
>   * compactable pages.
>   */
> -#ifdef CONFIG_PREEMPT_RT
> -int sysctl_compact_unevictable_allowed __read_mostly = 0;
> -#else
> -int sysctl_compact_unevictable_allowed __read_mostly = 1;
> -#endif
> +int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNEVICTABLE_DEFAULT;

Why introduce a Kconfig symbol for this, and not just spell the
initializer "IS_ENABLED(CONFIG_PREEMPT_RT) ? 0 : 1" or simply
"!IS_ENABLED(CONFIG_PREEMPT_RT)"?

And if you do keep it in Kconfig, shouldn't the symbol be "depends on
COMPACTION" so it doesn't needlessly appear in .config when
!CONFIG_COMPACTION.

Rasmus