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Date:   Fri, 9 Oct 2020 12:20:09 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Frederic Weisbecker <fweisbecker@suse.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@suse.de>, Ingo Molnar <mingo@redhat.com>
Subject: Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE,
 PREEMPT_VOLUNTARY on kernel command line
Message-ID: <20201009102009.GK2628@hirez.programming.kicks-ass.net>
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 <20201009094741.GH2628@hirez.programming.kicks-ass.net>
 <20201009101405.GI4967@dhcp22.suse.cz>
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On Fri, Oct 09, 2020 at 12:14:05PM +0200, Michal Hocko wrote:
> On Fri 09-10-20 11:47:41, Peter Zijlstra wrote:

> > That is, work backwards (from PREEMPT back to VOLUNTARY) instead of the
> > other way around.
> 
> My original idea was that the config would only define the default
> preemption mode. preempt_none parameter would then just act as an
> override. That would mean that CONFIG_PREEMPTION would be effectively
> gone from the kernel. The reason being that any code outside of the
> scheduler shouldn't really care about the preemption mode. I suspect
> this will prevent from dubious hacks and provide a more robust code in
> the end.

Sure; but the way of arriving at that destination might be easier if
you work backwards from PREEMPT=y, because while there _should_ not be
dependencies outside of the scheduler, we both know there are.

This also makes your patches independent of the series that makes
CONFIG_PREEMPTION unconditional.

It also gives Kconfig space to limit the dynamic thing to archs that
have sufficient support (we'll be relying on static_call/static_branch,
and not everybody has that implemented in a way that makes it the
dynamic change worth-while).