Re: [PATCH 1/3] Added runqueue clock normalized with cpufreq

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-12-17 at 14:02 +0100, Harald Gustafsson wrote:

> This is a request for comments on additions to sched deadline v3 patches.
> Deadline scheduler is the first scheduler (I think) we introduce in Linux that
> specifies the runtime in time and not only as a weight or a relation.
> I have introduced a normalized runtime clock dependent on the CPU frequency.
> This is used, in [PATCH 2/3], to calculate the deadline thread's runtime
> so that approximately the same number of cycles are giving to the thread
> independent of the CPU frequency. 
> 
> I suggest that this is important for users of hard reservation based schedulers
> that the intended amount of work can be accomplished independent of the CPU frequency.
> The usage of CPU frequency scaling is important on mobile devices and hence 
> the combination of deadline scheduler and cpufreq should be solved.

> So before I do this for the linux tip I would welcome a discussion about if this
> is a good idea and also suggestions on how to improve this. 

I'm thinking this is going about it totally wrong..

Solving the CPUfreq problem involves writing a SCHED_DEADLINE aware
CPUfreq governor. The governor must know about the constraints placed on
the system by the task-set. You simply cannot lower the frequency when
your system is at u=1.

Once you have a governor that keeps the freq such that: freq/max_freq >=
utilization (which is only sufficient for deadline == period systems),
then you need to frob the SCHED_DEADLINE runtime accounting.

Adding a complete normalized clock to the system like you've done is a
total no-go, it adds overhead even for the !SCHED_DEADLINE case.

The simple solution would be to slow down the runtime accounting of
SCHED_DEADLINE tasks by freq/max_freq. So instead of having:

  dl_se->runtime -= delta;

you do something like:

  dl_se->runtime -= (freq * delta) / max_freq;

Which auto-magically grows the actual bandwidth, and since the deadlines
are wall-time already it all works out nicely. It also keeps the
overhead inside SCHED_DEADLINE.