FROMLIST: sched: Fix proxy/current (push,pull)ability

Proxy execution forms atomic pairs of tasks: The waiting donor
task (scheduling context) and a proxy (execution context). The
donor task, along with the rest of the blocked chain, follows
the proxy wrt CPU placement.

They can be the same task, in which case push/pull doesn't need any
modification. When they are different, however,
FIFO1 & FIFO42:

	      ,->  RT42
	      |     | blocked-on
	      |     v
blocked_donor |   mutex
	      |     | owner
	      |     v
	      `--  RT1

   RT1
   RT42

  CPU0            CPU1
   ^                ^
   |                |
  overloaded    !overloaded
  rq prio = 42  rq prio = 0

RT1 is eligible to be pushed to CPU1, but should that happen it will
"carry" RT42 along. Clearly here neither RT1 nor RT42 must be seen as
push/pullable.

Unfortunately, only the donor task is usually dequeued from the rq,
and the proxy'ed execution context (rq->curr) remains on the rq.
This can cause RT1 to be selected for migration from logic like the
rt pushable_list.

Thus, adda a dequeue/enqueue cycle on the proxy task before __schedule
returns, which allows the sched class logic to avoid adding the now
current task to the pushable_list.

Furthermore, tasks becoming blocked on a mutex don't need an explicit
dequeue/enqueue cycle to be made (push/pull)able: they have to be running
to block on a mutex, thus they will eventually hit put_prev_task().

Cc: Joel Fernandes <joelaf@google.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: kernel-team@android.com
Change-Id: I7a71321999c457089ccdff65c40fd16887346c74
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/lkml/20241125195204.2374458-7-jstultz@google.com/
Bug: 306081722
---
v3:
* Tweaked comments & commit message
v5:
* Minor simplifications to utilize the fix earlier
  in the patch series.
* Rework the wording of the commit message to match selected/
  proxy terminology and expand a bit to make it more clear how
  it works.
v6:
* Dropped now-unused proxied value, to be re-added later in the
  series when it is used, as caught by Dietmar
v7:
* Unused function argument fixup
* Commit message nit pointed out by Metin Kaya
* Dropped unproven unlikely() and use sched_proxy_exec()
  in proxy_tag_curr, suggested by Metin Kaya
v8:
* More cleanups and typo fixes suggested by Metin Kaya
v11:
* Cleanup of comimt message suggested by Metin
v12:
* Rework for rq_selected -> rq->donor renaming