sched/cache: Honor migrate_llc_task semantics in active load balance, to fix LLC mis-scheduling bug

Cache aware scheduling introduced the migrate_llc_task migration type to direct
tasks toward their preferred LLC, but its semantics can be lost when passive
load balance falls back to active load balance (ALB). This may allow ALB to
select a candidate whose preferred LLC does not match the destination, moving
it away from its preferred LLC.

Example scenario:

src_rq has two runnable tasks, p1 and p2. p1 prefers dst_rq (dst_llc), while p2
prefers src_rq (src_llc). In this case, migrate_llc_task is set because src_rq
has at least one task, p1, that wants to migrate to dst_rq. In ALB,
can_migrate_task() finds p2 and returns true for it, thus moving p2 out of its
preferred LLC.

Solution:

The CPU stopper in ALB constructs a fresh lb_env that does not inherit
migration_type from the passive load-balance pass. Two approaches are
possible:

(a) Add a new member to struct rq so ALB can inherit migrate_llc_task
    from the passive LB that triggered it.

(b) Define a new flag LBF_ACTIVE_LB_LLC and select the stopper callback
    at kick time to preserve the migration semantics across the
    asynchronous boundary.

We choose (b) because it avoids passing migration_type through the
stopper, which would affect the meaning of migration_type for
delayed-dequeue tasks.

Fixes: e4c9a4cb244a ("sched/cache: Add migrate_llc_task migration type for cache-aware balancing")
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Lu Wang <wanglu.priv@gmail.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/cb39f64a17fc2b76097264aaec74a2d6dfff4315.1790035273.git.tim.c.chen@linux.intel.com

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