linux-kernel - Re: [PATCH] sched: Tweak default dynamic preempt mode selection

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Message-ID: <8735o6uca5.mognet@arm.com>
Date:   Mon, 08 Nov 2021 11:17:38 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH] sched: Tweak default dynamic preempt mode selection

On 06/11/21 05:40, Mike Galbraith wrote:
> On Fri, 2021-11-05 at 10:40 +0000, Valentin Schneider wrote:
>> Commit c597bfddc9e9 ("sched: Provide Kconfig support for default dynamic
>> preempt mode") changed the selectable config names for the preemption
>> model. This means a config file must now select
>>
>>   CONFIG_PREEMPT_BEHAVIOUR=y
>>
>> rather than
>>
>>   CONFIG_PREEMPT=y
>>
>> to get a preemptible kernel. This means all arch config files need to be
>> updated - right now arm64 defconfig selects CONFIG_PREEMPT=y but ends up
>> with CONFIG_PREEMPT_NONE_BEHAVIOUR=y.
>>
>> Instead, have CONFIG_*PREEMPT be the selectable configs again, and make
>> them select their _BEHAVIOUR equivalent if CONFIG_PREEMPT_DYNAMIC is set.
>
>
> Is there any way to get to PREEMPT_RT in the first selection again as
> well?  I had created a behavior entry for RT (below) and inverted the
> dependency to make it appear in the initial selection again, but that's
> clearly not gonna fly.
>
> Starting with a 5.15 config, to select RT you currently must first
> select a model you don't want, then reject PREEMPT_DYNAMIC and you'll
> be offered the full menu of models immediately. With your patch added,
> that became worse.  After rejecting PREEMPT_DYNAMIC, I had to go
> through new 5.15+ options before finally being offered the full menu.
>

Do you mean at the syncconfig step? I've only really played with upstream
arm64 / x86 defconfigs and didn't have to fight with any prompts, though
yes for x86 the default-y PREEMPT_DYNAMIC makes it a bit annoying to select
PREEMPT_RT.