Re: [PATCH tip/core/rcu 4/7] rcu: Unify boost and kthread priorities

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 31, 2014 at 09:42:30AM -0700, Paul E. McKenney wrote:

> Well, you are supposed to determine the highest RT priority at which
> your workload might run CPU-bound tasks, and set the boost priority
> at some level above that.  My model of RCU priority boosting is that
> it should be used to make inadvertent high-priority infinite loops
> easier to debug, but others might have different approaches.

Ah, so DL will never be CPU-bound -- and RR/FIFO _should_ never be, but
I digress ;-)

> > We should be able to detect the case where more and work piles on and
> > the actual running does not appear to catch up, but I'm not sure what to
> > do about it, seeing how system stability is at risk.
> 
> I could imagine having a backup SCHED_FIFO task that handled the
> case where callbacks were piling up, but synchronizing it with the
> SCHED_DEADLINE task while avoiding callback misordering could be a bit
> "interesting".  (Recall that callback misordering messes up rcu_barrier().)

Ah, so there is talk of 'soft' CBS modes, which instead of hard throttle
either reclaim 'unused' DL bandwidth, or continue running in lower scheduling
classes.