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

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Message-ID: <b38c31be32d779800edcc49aa93dc45eb28d089c.camel@gmx.de>
Date:   Wed, 10 Nov 2021 06:44:42 +0100
From:   Mike Galbraith <efault@....de>
To:     Valentin Schneider <valentin.schneider@....com>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: Tweak default dynamic preempt mode selection

On Wed, 2021-11-10 at 00:03 +0000, Valentin Schneider wrote:
> On 09/11/21 11:25, Frederic Weisbecker wrote:
> > On Fri, Nov 05, 2021 at 10:40:35AM +0000, Valentin Schneider wrote:
> > > @@ -75,17 +86,6 @@ config PREEMPT_RT
> > >
> > >  endchoice
> > >
> > > -config PREEMPT_NONE
> > > -       bool
> > > -
> > > -config PREEMPT_VOLUNTARY
> > > -       bool
> > > -
> > > -config PREEMPT
> > > -       bool
> > > -       select PREEMPTION
> > > -       select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
> > > -
> > >  config PREEMPT_COUNT
> > >         bool
> >
> > This must be breaking cond_resched() and might_resched() definitions.
> >
> > Since CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY and CONFIG_PREEMPT aren't too widely
> > spread around within ifdefferies, you can:
> >
> > 1) Rename CONFIG_PREEMPT_NONE to CONFIG_PREEMPT_NONE_STATIC
> >    Rename CONFIG_PREEMPT_VOLUNTARY to CONFIG_PREEMPT_VOLUNTARY_STATIC
> >    Rename CONFIG_PREEMPT to CONFIG_PREEMPT_STATIC
> >
> > 2) Keep the old CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY,
> >    CONFIG_PREEMPT around for compatibility and make them select their
> >    corresponding BEHAVIOUR entries.
> >
>
> Damn, I was too zealous on my inbox clearing and skipped that one, sorry!
> (and ofc in the meantime Mike and I went down separate rabbit holes).
>
> The old CONFIG_PREEMPT* would have to remain somewhere in the menu entry
> (e.g. "legacy" preemption mode selection?), but other than that it seems to
> work with this kind of structure:
>
> choice
>         prompt "Preemption Model"
>         default PREEMPT_BEHAVIOUR if PREEMPT
>         default PREEMPT_VOLUNTARY_BEHAVIOUR if PREEMPT_VOLUNTARY
>         default PREEMPT_NONE_BEHAVIOUR
>
> This however doesn't solve Mike's concern wrt deselecting PREEMPT_RT to
> flip PREEMPT_DYNAMIC and vice versa...

OTOH it's not quite here/now relevant, whereas your config woes are...

	-Mike