[PATCH v4 13/13] sched: Fixups to find_exec_ctx

[John Stultz <jstultz@google.com>]

find_exec_ctx() would sometimes cause the rt task pushing
to try to push tasks in the chain that ends in the rq->curr.

This caused lots of migration noise and effecively livelock
 where tasks would get pushed off to other cpus, then
proxy-migrated back to the lockowner's cpu, over and over.

This kept other cpus constantly proxy-migrating away and
never actually selecting a task to run  - effectively
hanging the system.

So this patch reworks some of the find_exec_ctx logic
so we stop when we hit rq->curr, and changes the logic
that was returning NULL when we came across
rq_selected(), as I'm not sure why we'd stop there.

Cc: Joel Fernandes <joelaf@google.com>
Cc: Qais Yousef <qyousef@google.com>
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: Youssef Esmat <youssefesmat@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
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: kernel-team@android.com
Signed-off-by: John Stultz <jstultz@google.com>
---
 kernel/sched/core.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e0e6c2feefd0..9cdabb79d450 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3881,7 +3881,15 @@ struct task_struct *find_exec_ctx(struct rq *rq, struct task_struct *p)
 		if (owner == exec_ctx)
 			break;
 
-		if (!task_queued_on_rq(rq, owner) || task_current_selected(rq, owner)) {
+		/* If we get to current, that's the exec ctx! */
+		if (task_current(rq, owner))
+			return owner;
+
+		/*
+		 * XXX This previously was checking task_current_selected()
+		 * but that doesnt' make much sense to me. -jstultz
+		 */
+		if (!task_queued_on_rq(rq, owner)) {
 			exec_ctx = NULL;
 			break;
 		}
-- 
2.41.0.rc0.172.g3f132b7071-goog