linux-kernel - Re: [PATCH 3/8] sched: rt-group: interface

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Date:	Sat, 23 Feb 2008 21:26:29 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Menage <menage@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	tong.n.li@...el.com
Subject: Re: [PATCH 3/8] sched: rt-group: interface

On Sat, 2008-02-23 at 12:02 -0800, Paul Menage wrote:
> On Sat, Feb 23, 2008 at 11:57 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >  > If so, could we avoid that problem by using 0 rather than -1 as the
> >  > "unlimited" value? It looks from what I've read in the Documentation
> >  > changes as though 0 isn't really a meaningful value.
> >
> >  0 means no time, quite useful and clearly distinct from inf. time.
> >
> 
> So a real-time task in a cgroup with a 0 rt_runtime can be in the R
> state but never actually get to run? OK, if people need to be able to
> do that then fair enough.

Yeah, its an awkward situation, and we refuse new rt tasks in such
groups. But the 0 value is needed so you can have groups that don't
participate in the realtime scheduling because we enforce a
schedulalbility constraint over the groups.

Each group has a runtime ratio, namely: rt_runtime / rt_period.
The sum of this ratio over all groups must be smaller or equal to the
global ratio which must be smaller or equal to 1.

> In that case I guess I'll have to add signed versions of the
> read_uint/write_uint methods.

Yes, I looked at that, I found the interface somewhat unfortunate, it
would mean growing the struct with two more function pointers. Perhaps a
read and write function with abstract data would be better suited. That
would allow for this and more. Sadly it looses type information.

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