On Fri, Oct 03, 2014 at 11:37:31AM -0400, Rik van Riel wrote:

Some more brainstorming points...

1) We should probably (lazily/batched?) propagate load information
   up the sched_group tree.  This will be useful for wake_affine,
   load_balancing, find_idlest_cpu, and select_idle_sibling

2) With both find_idlest_cpu and select_idle_sibling walking down
   the tree from the LLC level, they could probably share code

3) Counting both blocked and runnable load may give better long
   term stability of loads, resulting in a reduction in work
   preserving behaviour, but an improvement in locality - this
   could be worthwhile, but it is hard to say in advance

4) We can be pretty sure that CPU makers are not going to stop
   at a mere 18 cores. We need to subdivide things below the LLC
   level, turning select_idle_sibling and find_idlest_cpu into
   a tree walk.

   This means whatever selection criteria are used by these need
   to be propagated up the sched_group tree. This, in turn, means
   we probably need to restrict ourselves to things that do not get
   changed/updated too often.

Am I overlooking anything?

Well, we can certainly try something like that; but your last point
seems like a contradition; seeing how _the_ important point for
select_idle_sibling() is the actual idle state, and that per definition
is something that can change/update often.

But yes, the only viable option is some artificial breakup of the
topology and we can indeed try and bridge the gap with some caching.