FROMLIST: sched: Add an initial sketch of the find_proxy_task() function

Add a find_proxy_task() function which doesn't do much.

When we select a blocked task to run, we will just deactivate it
and pick again. The exception being if it has become unblocked
after find_proxy_task() was called.

Greatly simplified from patch by:
  Peter Zijlstra (Intel) <peterz@infradead.org>
  Juri Lelli <juri.lelli@redhat.com>
  Valentin Schneider <valentin.schneider@arm.com>
  Connor O'Brien <connoro@google.com>

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
[jstultz: Split out from larger proxy patch and simplified
 for review and testing.]
Change-Id: I836ca179b5c6f9a5c61bc4cab79825f0ddca5cce
Signed-off-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/lkml/20241125195204.2374458-6-jstultz@google.com/


Bug: 306081722
---
v5:
* Split out from larger proxy patch
v7:
* Fixed unused function arguments, spelling nits, and tweaks for
  clarity, pointed out by Metin Kaya
* Fix build warning Reported-by: kernel test robot <lkp@intel.com>
  Closes: https://lore.kernel.org/oe-kbuild-all/202311081028.yDLmCWgr-lkp@intel.com/
v8:
* Fixed case where we might return a blocked task from find_proxy_task()
* Continued tweaks to handle avoiding returning blocked tasks
v9:
* Add zap_balance_callbacks helper to unwind balance_callbacks
  when we will re-call pick_next_task() again.
* Add extra comment suggested by Metin
* Typo fixes from Metin
* Moved adding proxy_resched_idle earlier in the series, as suggested
  by Metin
* Fix to call proxy_resched_idle() *prior* to deactivating next, to avoid
  crashes caused by stale references to next
* s/PROXY/SCHED_PROXY_EXEC/ as suggested by Metin
* Number of tweaks and cleanups suggested by Metin
* Simplify proxy_deactivate as suggested by Metin
v11:
* Tweaks for earlier simplification in try_to_deactivate_task
v13:
* Rename "next" to "donor" in find_proxy_task() for clarity
* Similarly use "donor" instead of next in proxy_deactivate
* Refactor/simplify proxy_resched_idle
* Moved up a needed fix from later in the series