Re: SCHED_DEADLINE as user

[luca abeni <luca.abeni@santannapisa.it>]

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 <juri.lelli@gmail.com> wrote:

> On 18/08/18 15:26, Tim Blechmann wrote:
> > >> 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.
> > >   
> > >>  
> > >>> 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.
> 
> > [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