Re: 1 RT task blocks 4-core machine ?

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-10-05 at 00:26 +0200, Tommaso Cucinotta wrote:
> A possible explanation might be that the CFS load balancing logic sees
> my only active task (e.g., the ssh server or shell etc.) as running
> alone on its core, and does not detect that it is inhibited to actually
> run due to RT tasks on the same core. Therefore, it will not migrate
> the task to the free cores. Does this explanation make sense
> or is it completely wrong ?

Possibly, its got some logic to detect this but maybe it gets confused
still, in particular look at the adaptive cpu_power in
update_cpu_power() and calling functions.

> Also, I'd like to hear whether this is considered the "normal/desired"
> behavior of intermixing RT and non-RT tasks. 

Pegging a cpu using sched_fifo/rr pretty much means you get to keep the
pieces, if it works nice, if you can make it work better kudos, but no
polling from sched_fifo/rr is not something that is considered sane for
the general health of your system.