• Dario Faggioli's avatar
    sched_rt.c: resch needed in rt_rq_enqueue() for the root rt_rq · f6121f4f
    Dario Faggioli authored
    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.
    Signed-off-by: default avatarDario Faggioli <raistlin@linux.it>
    Signed-off-by: default avatarMichael Trimarchi <trimarchimichael@yahoo.it>
    Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: <stable@kernel.org>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    f6121f4f
sched_rt.c 35.6 KB