On Mon, 2008-11-03 at 15:07 -0600, Dimitri Sivanich wrote:
>   
> >> When load balancing gets switched off for a set of cpus via the
> >> sched_load_balance flag in cpusets, those cpus wind up with the
> >> globally defined def_root_domain attached.  The def_root_domain is
> >> attached when partition_sched_domains calls detach_destroy_domains().
> >> A new root_domain is never allocated or attached as a sched domain
> >> will never be attached by __build_sched_domains() for the non-load
> >> balanced processors.
> >>
> >> The problem with this scenario is that on systems with a large number
> >> of processors with load balancing switched off, we start to see the
> >> cpupri->pri_to_cpu->lock in the def_root_domain becoming contended.
> >> This starts to become much more apparent above 8 waking RT threads
> >> (with each RT thread running on it's own cpu, blocking and waking up
> >> continuously).
> >>