Re: [PATCH 0/3] sched: use EDF to throttle RT task groups v2

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-02-23 at 19:56 +0100, Fabio Checconi wrote:
> This patchset introduces a group level EDF scheduler extending the
> throttling mechanism, in order to make it support generic period
> assignments.  With this patch, the runtime and period parameters
> can be used to specify arbitrary CPU reservations for RT tasks.
> 
> From the previous post [1] I've integrated Peter's suggestions, using
> a multi-level hierarchy to do admission control, but a one-level only
> equivalent hierarchy for scheduling, and I've not removed the bandwidth
> migration mechanism, trying to adapt it to EDF scheduling.  In this
> version tasks are still inserted into priority arrays and only groups
> are kept in a per-rq edf tree.
> 
> The main design issues involved:
> 
>   - Since it is not easy to mix tasks and groups on the same scheduler
>     queue (tasks have no deadlines), the bandwidth reserved to the tasks
>     in a group is controlled with two additional cgroup attributes:
>     rt_task_runtime_us and rt_task_period_us.  These attributes control,
>     within a cgroup, how much bandwidth is reserved to the tasks it
>     contains.  The old attributes, rt_runtime_us and rt_period_us, are
>     still there, and control the bandwidth assigned to the cgroup.  They
>     are used only for admission control.
> 
>   - Shared resources are still handled using boosting.  When a group
>     contains a task inside a critical section it is scheduled according
>     the highest priority among the ones of the tasks it contains.
>     In this way, the same group has two modes: when it is not boosted
>     it is scheduled according to its deadline; when it is boosted, it
>     is scheduled according its priority.  Boosted groups are always
>     favored over non-boosted ones.
> 
>   - Given that the various rt_rq's belonging to the same task group
>     are activated independently, there is the need of a timer per
>     each rt_rq.
> 
>   - While balancing the bandwidth assigned to a cgroup on various cpus
>     we have to make sure that utilization for the rt_rq's on each cpu
>     does not exceed the global utilization limit for RT tasks.  
> 

> As usual, feedback welcome.

Looks very nice, good work! comments in individual replies.