Re: SCHED_DEADLINE as user

Hi Juri,

sorry for the late reply (I am just back from vacations), and thanks
for cc-ing me.

I see you opened a bug on github about this, so I am going to add more
details there, but basically I think that in order to use
SCHED_DEADLINE as non-root we need to:
1) Disable the boosting mechanism (not the inheritance, just the "soft
   enforcement behaviour" of tasks holding mutexes)
2) Implement some mechanism to limit the amount of dl bandwidth a user
   can allocate to itself (I think the cgroup-based approach we
   discussed some time ago should be OK... If I remember well, you even
   had a patch implementing it :)



				Luca

On Mon, 20 Aug 2018 10:54:20 +0200
Juri Lelli [off-list ref] wrote:

On 18/08/18 15:26, Tim Blechmann wrote:

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audio applications typically use SCHED_FIFO. however they are
rate-monotonic by design, so i wanted to evaluate how
SCHED_DEADLINE behaves here.  

Right. Exactly what I started to think about lately as well.
  

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Asking because we might want to have a look at how to relax the
current privileges requirement.  

please :)  

Anything to try attracting new users. :-P

More seriously, could you please describe in a bit more details
your specific use case? Based on JACK I guess? Is it the
application you are interested into public available somewhere?  

it could be based on JACK or natively on ALSA. furthermore the fun
part starts when trying to distribute work to multiple threads:
during the time slice of one audio callback (could be as low as
1.3ms), we need to wake helper threads and (busy?)wait to collect
all the results. (while one of the applications is open source,
it's not entirely straight-forward to use)

for me it's more an investigation if SCHED_DEADLINE is actually
feasible in real-world audio application these days. tommaso used
to have a research prototype [1], built directly into JACK. however
currently i'd be more interested in using it in ALSA. but yeah,
it's more an investigation as SCHED_FIFO does a decent job for my
use case ...  

I (almost) see. Thanks for the details.

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[1] http://retis.sssup.it/~tommaso/publications/LAC-2011.pdf  

So, this was IIRC using AQuoSA hierarchical RT on EDF scheduler
implementation; which it was handy to easy the task of assigning CBS
server parameters to JACK clients (graph).

In your case - assuming you could already use DEADLINE for !root - do
you think you would need to use the same interface for the multiple
threads you are distributing work among? Or would you be fine with
assigning runtime/deadline for each one of those?

BTW, cc-ing Tommaso and Luca. Discussion started from
https://www.spinics.net/lists/linux-rt-users/msg19296.html