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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-12-17 at 15:29 +0100, Peter Zijlstra wrote:
> 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.

This is all assuming lowering the frequency is sensible to begin with in
the first place... but that's all part of the CPUfreq governor, it needs
to find a way to lower energy usage while conforming to the system
constraints.