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Date:   Fri, 9 Oct 2020 12:48:08 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Peter Zijlstra <peterz@infradead.org>
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: <20201009104808.GK4967@dhcp22.suse.cz>
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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.
 
> 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.

Thanks!
-- 
Michal Hocko
SUSE Labs