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

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Date:   Wed, 24 Aug 2022 15:50:10 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     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 Thu, Aug 18 2022 at 10:55, Rasmus Villemoes wrote:
> On 17/08/2022 18.27, Sebastian Andrzej Siewior wrote:
>> -#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)"?

The reason for the config symbol is that Linus requested to have
semantically obvious constructs which can be utilized even without RT
and clearly spell out what the construct does. When RT selects this then
it's a documented requirement/dependency.

> 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.

Sure.

Thanks,

        tglx