FROMLIST: softirq: defer softirq processing to ksoftirqd if CPU is busy with RT

Defer the softirq processing to ksoftirqd if a RT task is
running or queued on the current CPU. This complements the RT
task placement algorithm which tries to find a CPU that is not
currently busy with softirqs.

Currently NET_TX, NET_RX, BLOCK and IRQ_POLL softirqs are only
deferred as they can potentially run for long time.

Additionally, this patch stubs out ksoftirqd_running() logic,
in the CONFIG_RT_SOFTIRQ_AWARE_SCHED case, as deferring
potentially long-running softirqs will cause the logic to not
process shorter-running softirqs immediately. By stubbing it out
the potentially long running softirqs are deferred, but the
shorter running ones can still run immediately.

This patch includes folded-in fixes by:
  Lingutla Chandrasekhar <clingutla@codeaurora.org>
  Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>
  J. Avila <elavila@google.com>

Cc: John Dias <joaodias@google.com>
Cc: Connor O'Brien <connoro@google.com>
Cc: Rick Yiu <rickyiu@google.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Qais Yousef <qyousef@google.com>
Cc: Chris Redpath <chris.redpath@arm.com>
Cc: Abhijeet Dharmapurikar <adharmap@quicinc.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: kernel-team@android.com
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[satyap@codeaurora.org: trivial merge conflict resolution.]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>
[elavila: Port to mainline, squash with bugfix]
Signed-off-by: J. Avila <elavila@google.com>
[jstultz: Rebase to linus/HEAD, minor rearranging of code,
 included bug fix Reported-by: Qais Yousef <qais.yousef@arm.com> ]
Signed-off-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/lkml/20221116075929.453876-4-jstultz@google.com/
Change-Id: Iedb590216e373788950243165ff83f4c43015f31
Bug: 168521633
---
v4:
* Fix commit message to accurately note long-running softirqs
  (suggested by Qais)
* Switch to using rt_task(current) (suggested by Qais)
v5:
* Switch to using CONFIG_RT_SOFTIRQ_AWARE_SCHED (suggested by
  Joel Fernandes <joel@joelfernandes.org>)