linux-kernel - [PATCH v4 13/13] sched: Fixups to find_exec

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Date:   Thu,  1 Jun 2023 05:58:16 +0000
From:   John Stultz <jstultz@...gle.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     John Stultz <jstultz@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Qais Yousef <qyousef@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>,
        Youssef Esmat <youssefesmat@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>, kernel-team@...roid.com
Subject: [PATCH v4 13/13] sched: Fixups to find_exec_ctx

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@...gle.com>
Cc: Qais Yousef <qyousef@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Valentin Schneider <vschneid@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
Cc: Youssef Esmat <youssefesmat@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: "Paul E . McKenney" <paulmck@...nel.org>
Cc: kernel-team@...roid.com
Signed-off-by: John Stultz <jstultz@...gle.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