[lkml]   [2008]   [Oct]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    Patch in this message
    Subject[PATCH] sched_rt.c: resch needed in rt_rq_enqueue() for the root rt_rq
    Hi all,

    While working on the new version of the code for SCHED_SPORADIC I
    noticed something strange in the present throttling mechanism. More
    specifically in the throttling timer handler in sched_rt.c
    (do_sched_rt_period_timer()) and in rt_rq_enqueue().

    The problem is that, when unthrottling a runqueue, rt_rq_enqueue() only
    asks for rescheduling if the runqueue has a sched_entity associated to
    it (i.e., rt_rq->rt_se != NULL).
    Now, if the runqueue is the root rq (which has a rt_se = NULL)
    rescheduling does not take place, and it is delayed to some undefined
    instant in the future.

    This imply some random bandwidth usage by the RT tasks under throttling.
    For instance, setting rt_runtime_us/rt_period_us = 950ms/1000ms an RT
    task will get less than 95%. In our tests we got something varying
    between 70% to 95%.
    Using smaller time values, e.g., 95ms/100ms, things are even worse, and
    I can see values also going down to 20-25%!!

    The tests we performed are simply running 'yes' as a SCHED_FIFO task,
    and checking the CPU usage with top, but we can investigate thoroughly
    if you think it is needed.

    Things go much better, for us, with the attached patch... Don't know if
    it is the best approach, but it solved the issue for us.

    Best Regards,
    Dario Faggioli

    Signed-off-by: Dario Faggioli <>
    Signed-off-by: Michael Trimarchi <>
    diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
    index 1113157..37f0721 100644
    --- a/kernel/sched_rt.c
    +++ b/kernel/sched_rt.c
    @@ -102,12 +102,12 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se);

    static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
    + struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr;
    struct sched_rt_entity *rt_se = rt_rq->rt_se;

    - if (rt_se && !on_rt_rq(rt_se) && rt_rq->rt_nr_running) {
    - struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr;
    - enqueue_rt_entity(rt_se);
    + if (rt_rq->rt_nr_running) {
    + if (rt_se && !on_rt_rq(rt_se))
    + enqueue_rt_entity(rt_se);
    if (rt_rq->highest_prio < curr->prio)
    <<This happens because I choose it to happen!>>
    (Raistlin Majere, DragonLance Chronicles -Dragons of Spring Drawning-)
    Dario Faggioli
    GNU/Linux Registered User: #340657
    SIP Account: or
    Jabber Account:
    GnuPG Key ID: 4DC83AC4
    GnuPG Key Fingerprint: 2A78 AD5D B9CF A082 0836 08AD 9385 DA04 4DC8 3AC4[unhandled content-type:application/pgp-signature]
     \ /
      Last update: 2008-10-03 19:51    [W:0.021 / U:5.012 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site