linux-kernel - [tip:sched/core] sched/rt: Add a missing rescheduling point

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Message-ID: <tip-619bd4a71874a8fd78eb6ccf9f272c5e98bcc7b7@git.kernel.org>
Date:   Mon, 30 Jan 2017 03:57:01 -0800
From:   tip-bot for Sebastian Andrzej Siewior <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     bigeasy@...utronix.de, hpa@...or.com, mingo@...nel.org,
        efault@....de, torvalds@...ux-foundation.org, peterz@...radead.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: [tip:sched/core] sched/rt: Add a missing rescheduling point

Commit-ID:  619bd4a71874a8fd78eb6ccf9f272c5e98bcc7b7
Gitweb:     http://git.kernel.org/tip/619bd4a71874a8fd78eb6ccf9f272c5e98bcc7b7
Author:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Tue, 24 Jan 2017 15:40:06 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 30 Jan 2017 11:46:37 +0100

sched/rt: Add a missing rescheduling point

Since the change in commit:

  fd7a4bed1835 ("sched, rt: Convert switched_{from, to}_rt() / prio_changed_rt() to balance callbacks")

... we don't reschedule a task under certain circumstances:

Lets say task-A, SCHED_OTHER, is running on CPU0 (and it may run only on
CPU0) and holds a PI lock. This task is removed from the CPU because it
used up its time slice and another SCHED_OTHER task is running. Task-B on
CPU1 runs at RT priority and asks for the lock owned by task-A. This
results in a priority boost for task-A. Task-B goes to sleep until the
lock has been made available. Task-A is already runnable (but not active),
so it receives no wake up.

The reality now is that task-A gets on the CPU once the scheduler decides
to remove the current task despite the fact that a high priority task is
enqueued and waiting. This may take a long time.

The desired behaviour is that CPU0 immediately reschedules after the
priority boost which made task-A the task with the lowest priority.

Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: fd7a4bed1835 ("sched, rt: Convert switched_{from, to}_rt() prio_changed_rt() to balance callbacks")
Link: http://lkml.kernel.org/r/20170124144006.29821-1-bigeasy@linutronix.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/deadline.c | 3 +--
 kernel/sched/rt.c       | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 491ff66..27737f3 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1729,12 +1729,11 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
 #ifdef CONFIG_SMP
 		if (tsk_nr_cpus_allowed(p) > 1 && rq->dl.overloaded)
 			queue_push_tasks(rq);
-#else
+#endif
 		if (dl_task(rq->curr))
 			check_preempt_curr_dl(rq, p, 0);
 		else
 			resched_curr(rq);
-#endif
 	}
 }
 
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 88254be..704f2b8 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2198,10 +2198,9 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
 #ifdef CONFIG_SMP
 		if (tsk_nr_cpus_allowed(p) > 1 && rq->rt.overloaded)
 			queue_push_tasks(rq);
-#else
+#endif /* CONFIG_SMP */
 		if (p->prio < rq->curr->prio)
 			resched_curr(rq);
-#endif /* CONFIG_SMP */
 	}
 }