Re: [PATCH 13/15] sched: add bandwidth management for sched_dl.

[Peter Zijlstra <peterz@infradead.org>]

On Sat, 2012-05-26 at 13:07 +0200, Juri Lelli wrote:
> Hi,
> 
> On 05/25/2012 12:38 PM, Peter Zijlstra wrote:
> > On Wed, 2012-05-23 at 23:42 +0200, Juri Lelli wrote:
> >> +/*
> >> + * Coupling of -dl and -rt bandwidth.
> >> + *
> >> + * Here we check, while setting the system wide bandwidth available
> >> + * for -dl tasks and groups, if the new values are consistent with
> >> + * the system settings for the bandwidth available to -rt entities.
> >> + *
> >> + * IOW, we want to enforce that
> >> + *
> >> + *   rt_bandwidth + dl_bandwidth<= 100%
> >> + *
> >> + * is always true.
> >> + */
> >
> > I was thinking we could do it the other way around, have have
> > dl_bandwidth included in rt_bandwidth.
> >
> 
> If I understand correctly, you are proposing to treat -dl tasks as a
> special case of "real-time" tasks. Then we could reserve some bw to
> "real-time" (rt_bandwidth cap) activities and give a piece of this
> bw to -dl tasks (what remains is for -rt tasks). This is in principle
> nice and useful, but I'm not quite sure that this is the right point
> to achieve this logical behavior.
> I mean, -dl and -rt tasks are separately treated, so it is probably
> cleaner to manage their knobs separately. They have to coexist rather
> than be considered one a sub-case of the other. A better way to go
> for a common cap for them is probably the (long-term) hierarchical
> scheduling mechanism.
> 
> So, I would prefer to keep the interface as is for now, but I can also
> completely misunderstood your thoughts :-P.

The thing is, keeping it separate makes for an impossible configuration
scenario. Esp. once we enable !root usage. The proposed 5% is very
limiting and regular users won't have sufficient privilege to change it.

Also lowering FIFO/RR by default isn't a real option since people expect
that to get all time already (however silly that expectation is).