linux-kernel - Re: [PATCH] sched: Relax a restriction in sched_rt_can

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Date:	Tue, 5 May 2015 17:19:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Zefan Li <lizefan@...wei.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH] sched: Relax a restriction in sched_rt_can_attach()

On Tue, May 05, 2015 at 10:18:38AM -0400, Tejun Heo wrote:
> > Now you can kludge around some of this, for example you can make the
> > default depend on the parent setting etc.. But that's horribly
> > inconsistent.
> 
> I don't think we can kludge this.  For all other resources, we're
> defining the limits that can't be crossed so nesting them w/ -1 by
> default is fine.  RR slices are different it that we're really slicing
> up and guaranteeing a portion of something finite, so unlimited by
> default thing doesn't really work here.

Note that you _could_ do the same thing with IO bandwidth; esp. with
these modern no-seek-penalty devices this could make sense.


> > So I really prefer not to go that way; if people use RR/FIFO they had
> > better bloody know what they're doing; which includes setting up the
> > system.
> 
> The problem is that this is tied to the normal cpu controller.  Users
> who don't have any intention of mucking with RT scheduling end up
> being dragged into it.  Given the strict nature of RR slicing, I'm
> don't even think it's actually useful to make the slicing
> hierarchical.  From cgroup's POV, it'd be best if RR slicing can be
> detached.

Like in the other mail; hierarchy still makes perfect sense for the
container case.

> > The whole RR/FIFO thing is so enormously broken (by definition; this
> > truly is unfixable) that you simply _cannot_ automate it.
> 
> Yeah, exactly.

I don't think you're quite agreeing to the same reasons I am. My main
objection to the whole SCHED_RR/FIFO thing as defined by POSIX is that
it does not in fact allow the OS to do what an OS _should_ do, namely
resource arbitration and control.

The whole rt-cgroup controller tries to somewhat contain that, but
fundamentally once you use RR/FIFO you've given up your system to
userspace control -- which btw is why its usually limited to root.

SCHED_DEADLINE avoids all these problems, at the cost of a more complex
setup.

But the fact that both need fixed portions of a limited total does not
in fact mean they're broken.
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