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Date:   Wed, 22 Apr 2020 15:26:48 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de,
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Subject: Re: [PATCH 01/23] sched: Provide sched_set_fifo()
Message-ID: <20200422132648.GJ20730@hirez.programming.kicks-ass.net>
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On Wed, Apr 22, 2020 at 06:11:38AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 22, 2020 at 01:27:20PM +0200, Peter Zijlstra wrote:
> > SCHED_FIFO (or any static priority scheduler) is a broken scheduler
> > model; it is fundamentally incapable of resource management, the one
> > thing an OS is actually supposed to do.
> > 
> > It is impossible to compose static priority workloads. One cannot take
> > two well designed and functional static priority workloads and mash
> > them together and still expect them to work.
> > 
> > Therefore it doesn't make sense to expose the priority field; the
> > kernel is fundamentally incapable of setting a sensible value, it
> > needs systems knowledge that it doesn't have.
> > 
> > Take away sched_setschedule() / sched_setattr() from modules and
> > replace them with:
> > 
> >   - sched_set_fifo(p); create a FIFO task (at prio 50)
> >   - sched_set_fifo_low(p); create a task higher than NORMAL,
> > 	which ends up being a FIFO task at prio 1.
> >   - sched_set_normal(p, nice); (re)set the task to normal
> > 
> > This stops the proliferation of randomly chosen, and irrelevant, FIFO
> > priorities that dont't really mean anything anyway.
> > 
> > The system administrator/integrator, whoever has insight into the
> > actual system design and requirements (userspace) can set-up
> > appropriate priorities if and when needed.
> 
> The sched_setscheduler_nocheck() calls in rcu_spawn_gp_kthread(),
> rcu_cpu_kthread_setup(), and rcu_spawn_one_boost_kthread() all stay as
> is because they all use the rcutree.kthread_prio boot parameter, which is
> set at boot time by the system administrator (or {who,what}ever, correct?

Correct, also they are not modular afaict, so they escaped the dance ;-)