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[PATCH 4.10 16/17] sched/rt: Add a missing rescheduling point

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4.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

commit 619bd4a71874a8fd78eb6ccf9f272c5e98bcc7b7 upstream.

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@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Thomas Gleixner <tglx@linutronix.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@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/sched/deadline.c |    3 +--
 kernel/sched/rt.c       |    3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1729,12 +1729,11 @@ static void switched_to_dl(struct rq *rq
 #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
 	}
 }
 
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2198,10 +2198,9 @@ static void switched_to_rt(struct rq *rq
 #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 */
 	}
 }
 

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[PATCH 4.10 00/17] 4.10.8-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:10 +0200
  [PATCH 4.10 08/17] c6x/ptrace: Remove useless PTRACE_SETREGSET implementation Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:10 +0200
  [PATCH 4.10 05/17] KVM: x86: cleanup the page tracking SRCU instance Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:10 +0200
  [PATCH 4.10 10/17] mips/ptrace: Preserve previous registers for short regset write Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 09/17] h8300/ptrace: Fix incorrect register transfer count Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 07/17] pinctrl: qcom: Dont clear status bit on irq_unmask Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 16/17] sched/rt: Add a missing rescheduling point Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 12/17] metag/ptrace: Preserve previous registers for short regset write Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 13/17] metag/ptrace: Provide default TXSTATUS for short NT_PRSTATUS Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 14/17] metag/ptrace: Reject partial NT_METAG_RPIPE writes Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  [PATCH 4.10 15/17] qla2xxx: Allow vref count to timeout on vport delete. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
    Re: [PATCH 4.10 15/17] qla2xxx: Allow vref count to timeout on  vport delete. "Nicholas A. Bellinger" <nab@linux-iscsi.org> - 2017-03-31 10:00 +0200
  [PATCH 4.10 11/17] sparc/ptrace: Preserve previous registers for short regset write Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-30 12:20 +0200
  Re: [PATCH 4.10 00/17] 4.10.8-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2017-03-30 21:00 +0200
    Re: [PATCH 4.10 00/17] 4.10.8-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-31 09:50 +0200
  Re: [PATCH 4.10 00/17] 4.10.8-stable review Guenter Roeck <linux@roeck-us.net> - 2017-03-31 05:50 +0200
    Re: [PATCH 4.10 00/17] 4.10.8-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-03-31 09:50 +0200

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