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Date:   Fri, 9 Oct 2020 13:39:09 +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: <20201009113909.GL2628@hirez.programming.kicks-ass.net>
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 <20201009101405.GI4967@dhcp22.suse.cz>
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On Fri, Oct 09, 2020 at 12:48:08PM +0200, Michal Hocko wrote:
> On Fri 09-10-20 12:20:09, Peter Zijlstra wrote:
> > 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.
> 
> Wouldn't we need to examine each of the CONFIG_PREEMPTION code anyway?
> And wouldn't that be even more tricky? The boot time option would result
> in a more restrictive preemption mode while the code is actually
> assuming a less restrictive one.

Sure, in the end we'll have to look at all of that.

> > 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).
> 
> Hmm, this is actually a good argument. I can imagine that kernels
> without CONFIG_JUMP_LABEL might increase a runtime overhead for
> something that users of that kernel might be not really interested in.
> This would make CONFIG_PREEMPT_DYNAMIC be selected by CONFIG_JUMP_LABEL.
> 
> I will add the CONFIG_PREEMPT_DYNAMIC in the next version. I just have
> to think whether flipping the direction is really safe and easier in the
> end. For our particular usecase we are more interested in
> NONE<->VOLUNTARY at this moment and having full preemption in the mix
> later is just fine. If you insist on the other direction then we can
> work on that.

Ah, I was purely thinking of the FULL preempt case. For the
NONE/VOLATILE case you can probably keep it simpler.