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Date:   Fri, 9 Oct 2020 12:14:05 +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: <20201009101405.GI4967@dhcp22.suse.cz>
References: <20201007120401.11200-1-mhocko@kernel.org>
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 <20201007123553.GK29020@dhcp22.suse.cz>
 <20201009094741.GH2628@hirez.programming.kicks-ass.net>
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On Fri 09-10-20 11:47:41, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 02:35:53PM +0200, Michal Hocko wrote:
> > On Wed 07-10-20 14:21:44, Peter Zijlstra wrote:
> > > On Wed, Oct 07, 2020 at 02:04:01PM +0200, Michal Hocko wrote:
> > > > I wanted to make sure that the idea is sound for maintainers first. The
> > > > next step would be extending the command line to support full preemption
> > > > as well but there is much more work in that area. Frederic has promissed
> > > > to look into that.
> > > 
> > > The sanest way there is to static_call() __preempt_schedule() I think.
> > 
> > Yes, I have checked the code and identified few other places like
> > irqentry_exit_cond_resched. We also need unconditional
> > CONFIG_PREEMPT_COUNT IIUC and there are quite some places guarded by
> > CONFIG_PREEMPTION that would need to be examined. Some of them are
> > likely pretending to be more clever than they really are/should be -
> > e.g. mm/slub.c. So there is likely a lot of leg work.
> 
> The easiest way might be to introduce PREEMPT_DYNAMIC that
> depends/selects PREEMPT. That way you're basically running a PREEMPT=y
> kernel.
> 
> Then have PREEMPT_DYNAMIC allow disabling the __preempt_schedule /
> preempt_schedule_irq() callsites using static_call/static_branch
> respectively.
> 
> 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.

Does that sound reasonable?

-- 
Michal Hocko
SUSE Labs