lkml.org 
[lkml]   [2017]   [Dec]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 4.4 045/115] sched/deadline: Throttle a constrained deadline task activated after the deadline
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

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

    From: Daniel Bristot de Oliveira <bristot@redhat.com>


    [ Upstream commit df8eac8cafce7d086be3bd5cf5a838fa37594dfb ]

    During the activation, CBS checks if it can reuse the current task's
    runtime and period. If the deadline of the task is in the past, CBS
    cannot use the runtime, and so it replenishes the task. This rule
    works fine for implicit deadline tasks (deadline == period), and the
    CBS was designed for implicit deadline tasks. However, a task with
    constrained deadline (deadine < period) might be awakened after the
    deadline, but before the next period. In this case, replenishing the
    task would allow it to run for runtime / deadline. As in this case
    deadline < period, CBS enables a task to run for more than the
    runtime / period. In a very loaded system, this can cause a domino
    effect, making other tasks miss their deadlines.

    To avoid this problem, in the activation of a constrained deadline
    task after the deadline but before the next period, throttle the
    task and set the replenishing timer to the begin of the next period,
    unless it is boosted.

    Reproducer:

    --------------- %< ---------------
    int main (int argc, char **argv)
    {
    int ret;
    int flags = 0;
    unsigned long l = 0;
    struct timespec ts;
    struct sched_attr attr;

    memset(&attr, 0, sizeof(attr));
    attr.size = sizeof(attr);

    attr.sched_policy = SCHED_DEADLINE;
    attr.sched_runtime = 2 * 1000 * 1000; /* 2 ms */
    attr.sched_deadline = 2 * 1000 * 1000; /* 2 ms */
    attr.sched_period = 2 * 1000 * 1000 * 1000; /* 2 s */

    ts.tv_sec = 0;
    ts.tv_nsec = 2000 * 1000; /* 2 ms */

    ret = sched_setattr(0, &attr, flags);

    if (ret < 0) {
    perror("sched_setattr");
    exit(-1);
    }

    for(;;) {
    /* XXX: you may need to adjust the loop */
    for (l = 0; l < 150000; l++);
    /*
    * The ideia is to go to sleep right before the deadline
    * and then wake up before the next period to receive
    * a new replenishment.
    */
    nanosleep(&ts, NULL);
    }

    exit(0);
    }
    --------------- >% ---------------

    On my box, this reproducer uses almost 50% of the CPU time, which is
    obviously wrong for a task with 2/2000 reservation.

    Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Juri Lelli <juri.lelli@arm.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Luca Abeni <luca.abeni@santannapisa.it>
    Cc: Mike Galbraith <efault@gmx.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Romulo Silva de Oliveira <romulo.deoliveira@ufsc.br>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Tommaso Cucinotta <tommaso.cucinotta@sssup.it>
    Link: http://lkml.kernel.org/r/edf58354e01db46bf42df8d2dd32418833f68c89.1488392936.git.bristot@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/sched/deadline.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 45 insertions(+)

    --- a/kernel/sched/deadline.c
    +++ b/kernel/sched/deadline.c
    @@ -704,6 +704,37 @@ void init_dl_task_timer(struct sched_dl_
    timer->function = dl_task_timer;
    }

    +/*
    + * During the activation, CBS checks if it can reuse the current task's
    + * runtime and period. If the deadline of the task is in the past, CBS
    + * cannot use the runtime, and so it replenishes the task. This rule
    + * works fine for implicit deadline tasks (deadline == period), and the
    + * CBS was designed for implicit deadline tasks. However, a task with
    + * constrained deadline (deadine < period) might be awakened after the
    + * deadline, but before the next period. In this case, replenishing the
    + * task would allow it to run for runtime / deadline. As in this case
    + * deadline < period, CBS enables a task to run for more than the
    + * runtime / period. In a very loaded system, this can cause a domino
    + * effect, making other tasks miss their deadlines.
    + *
    + * To avoid this problem, in the activation of a constrained deadline
    + * task after the deadline but before the next period, throttle the
    + * task and set the replenishing timer to the begin of the next period,
    + * unless it is boosted.
    + */
    +static inline void dl_check_constrained_dl(struct sched_dl_entity *dl_se)
    +{
    + struct task_struct *p = dl_task_of(dl_se);
    + struct rq *rq = rq_of_dl_rq(dl_rq_of_se(dl_se));
    +
    + if (dl_time_before(dl_se->deadline, rq_clock(rq)) &&
    + dl_time_before(rq_clock(rq), dl_next_period(dl_se))) {
    + if (unlikely(dl_se->dl_boosted || !start_dl_timer(p)))
    + return;
    + dl_se->dl_throttled = 1;
    + }
    +}
    +
    static
    int dl_runtime_exceeded(struct sched_dl_entity *dl_se)
    {
    @@ -958,6 +989,11 @@ static void dequeue_dl_entity(struct sch
    __dequeue_dl_entity(dl_se);
    }

    +static inline bool dl_is_constrained(struct sched_dl_entity *dl_se)
    +{
    + return dl_se->dl_deadline < dl_se->dl_period;
    +}
    +
    static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
    {
    struct task_struct *pi_task = rt_mutex_get_top_task(p);
    @@ -984,6 +1020,15 @@ static void enqueue_task_dl(struct rq *r
    }

    /*
    + * Check if a constrained deadline task was activated
    + * after the deadline but before the next period.
    + * If that is the case, the task will be throttled and
    + * the replenishment timer will be set to the next period.
    + */
    + if (!p->dl.dl_throttled && dl_is_constrained(&p->dl))
    + dl_check_constrained_dl(&p->dl);
    +
    + /*
    * If p is throttled, we do nothing. In fact, if it exhausted
    * its budget it needs a replenishment and, since it now is on
    * its rq, the bandwidth timer callback (which clearly has not

    \
     
     \ /
      Last update: 2017-12-18 19:07    [W:4.168 / U:0.308 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site