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    Subject[PATCH] sched_rt_rq_enqueue() resched idle
    Hi Peter,

    When sysctl_sched_rt_runtime is set to something other than -1 and the
    CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get into a state
    where we see one or more CPUs idling forvever even though there are
    tasks in their rt runqueue that are able to run (no longer throttled).

    The sequence is:

    - A real-time task is running when the timer sets the rt runqueue
    to throttled, and the rt task is resched_task()ed and switched
    out, and idle is switched in since there are no non-rt tasks to
    run on that cpu.

    - Eventually the do_sched_rt_period_timer() runs and un-throttles
    the rt runqueue, but we just exit the timer interrupt and go back
    to executing the idle task in the idle loop forever.

    If we change the sched_rt_rq_enqueue() routine to use some of the code
    from the CONFIG_RT_GROUP_SCHED enabled version of this same routine and
    resched_task() the currently executing task (idle in our case) if it is
    a lower priority task than the higher rt task in the now un-throttled
    runqueue, the problem is no longer observed.

    Thank you for your time and consideration.

    Signed-off-by: John Blackwood <>

    Index: a/kernel/sched_rt.c
    --- a.orig/kernel/sched_rt.c 2008-08-22 15:11:31.000000000 -0400
    +++ a/kernel/sched_rt.c 2008-08-22 15:12:36.000000000 -0400
    @@ -193,6 +193,12 @@

    static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
    + if (rt_rq->rt_nr_running) {
    + struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr;
    + if (rt_rq->highest_prio < curr->prio)
    + resched_task(curr);
    + }

    static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)

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      Last update: 2008-08-26 21:17    [W:0.023 / U:117.652 seconds]
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