sched/cache: Introduce helper functions to enforce LLC migration policy

sched/cache: Introduce helper functions to enforce LLC migration policy

Cache-aware scheduling aggregates threads onto their preferred LLC,
mainly through load balancing. When the preferred LLC becomes
saturated, more threads are still placed there, increasing latency.
A mechanism is needed to limit aggregation so that the preferred LLC
does not become overloaded.

Introduce helper functions can_migrate_llc() and
can_migrate_llc_task() to enforce the LLC migration policy:

  1. Aggregate a task to its preferred LLC if both source and
     destination LLCs are not too busy, or if doing so will not
     leave the preferred LLC much more imbalanced than the
     non-preferred one (>20% utilization difference, a little
     higher than the default imbalance_pct(17%) of the LLC domain
     as hysteresis). Later this threshold will be turned into tunable
     debugfs.
  2. Allow moving a task from overloaded preferred LLC to a non
     preferred LLC if this will not cause the non preferred LLC
     to become too imbalanced to cause a later migration back.
  3. If both LLCs are too busy, let the generic load balance to
     spread the tasks.

Further (hysteresis)action could be taken in the future to prevent tasks
from being migrated into and out of the preferred LLC frequently (back and
forth): the threshold for migrating a task out of its preferred LLC should
be higher than that for migrating it into the LLC.

Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/d19b52589cdceaee5e625980959f4d1982d6d7c9.1775065312.git.tim.c.chen@linux.intel.com