sched/eevdf: Move to a single runqueue

Change fair/cgroup to a single runqueue.

Infamously fair/cgroup isn't working for a number of people; typically
the complaint is latencies and/or overhead. The latency issue is due
to the intermediate entries that represent a combination of tasks and
thereby obfuscate the runnability of tasks.

The approach here is to leave the cgroup hierarchy as is; including
the intermediate enqueue/dequeue but move the actual EEVDF runqueue
outside. This means things like the shares_weight approximation are
fully preserved.

That is, given a hierarchy like:

          R
          |
          se--G1
              / \
        G2--se   se--G3
       / \           |
  T1--se se--T2      se--T3

This is fully maintained for load tracking, however the EEVDF parts of
cfs_rq/se go unused for the intermediates and are instead connected
like:

     _R_
    / | \
   T1 T2 T3

Since the effective weight of the entities is determined by the
hierarchy, this gets recomputed on enqueue,set_next_task and tick.

Notably, the effective weight (se->h_load) is computed from the
hierarchical fraction: se->load / cfs_rq->load.

Since EEVDF is now exclusively operating on rq->cfs, it needs to
consider cfs_rq->h_nr_queued rather than cfs_rq->nr_queued. Similarly,
only tasks can get delayed, simplifying some of the cgroup cleanup.

One place where additional information was required was
set_next_task() / put_prev_task(), where we need to track 'current'
both in the hierarchical sense (cfs_rq->h_curr) and in the flat sense
(cfs_rq->curr).

As a result of only having a single level to pick from, much of the
complications in pick_next_task() and preemption go away.

Since many of the hierarchical operations are still there, this won't
immediately fix the performance issues, but hopefully it will fix some
of the latency issues.

TODO: split struct cfs_rq / struct sched_entity
TODO: try and get rid of h_curr

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260605124052.227463677%40infradead.org

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