[LITMUS^RT] prop/sched-domains

[Björn Brandenburg]

On 25 Feb 2014, at 23:37, Glenn Elliott <gelliott at cs.unc.edu> wrote:

> Arg!  I used GFP_ATOMIC in all of my code, not realizing Linux’s proc_create_data() would call plain kmalloc().  Here are two possible solutions:
> 1) Use kworker to set up the proc files after successful activation.
> 2) Use kworker to perform plugin activation.
> 
> I have run into a similar problem in my GPUSync branch.  Here, I need plugin deactivation to wait/join on interrupt handling threads that are signaled to exit/shutdown.  In GPUSync, an echo to /proc/litmus/active_plugin triggers an event on kworker, which invokes the real activation routine.  Problem solved.  However, plugin activation becomes asynchronous.  You have to wait for some epsilon after activating a plugin to know if it was successful.  That’s not very nice.  I don’t know how else to approach the problem though.
> 
> Back to my sched-domains patch, normally I would advocate solution #1, since plugin activation remains synchronous.  However, since I’ve run into this problem before, I am willing to entertain solution #2.  I think we can defend against race conditions by serializing all plugin activation/deactivations on the same worker thread (perhaps we set up a reserved kworker just for litmus).
> 
> Questions of the Litmus community:  Does anyone have an opinion one way or the other?  Has atomic plugin activation caused you problems in the past?

Hi Glenn,

is there a way to avoid introducing kthreads for this? How about we call a “prepare_plugin()” callback from switch_sched_plugin() (https://github.com/LITMUS-RT/litmus-rt/blob/staging/litmus/litmus.c#L447) before executing in atomic mode?

Basically: 

1) acquire switching mutex
2) check preconditions (plugin known, no RT tasks, etc.)
3) call prepare_plugin() on new plugin while sleeping is ok, abort if non-zero return value
4) call stop_machine(), recheck preconditions, etc., do the actual switch
5) call cleanup_plugin() on old plugin
6) release switching mutex

This would keep the switch synchronous, allow sleeping to take place as part of prepare_plugin(), and does not require kthreads. Anything I’ve overlooked?

Thanks,
Björn

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