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    Subject[PATCH 4.4 14/14] sched/rt: Add a missing rescheduling point
    Date
    4.4-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
    @@ -1771,12 +1771,11 @@ static void switched_to_dl(struct rq *rq
    #ifdef CONFIG_SMP
    if (p->nr_cpus_allowed > 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
    @@ -2136,10 +2136,9 @@ static void switched_to_rt(struct rq *rq
    #ifdef CONFIG_SMP
    if (p->nr_cpus_allowed > 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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