On Mon, 13 Feb 2017 20:05:58 +0100
Daniel Bristot de Oliveira <bris...@redhat.com> wrote:
> 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 load system, this can cause the domino
> effect, making other tasks to 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 <bris...@redhat.com>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Juri Lelli <juri.le...@arm.com>
> Cc: Tommaso Cucinotta <tommaso.cucino...@sssup.it>
> Cc: Luca Abeni <luca.ab...@santannapisa.it>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Mike Galbraith <efa...@gmx.de>
> Cc: Romulo Silva de Oliveira <romulo.deolive...@ufsc.br>
> Cc: linux-kernel@vger.kernel.org
>
Reviewed-by: Steven Rostedt (VMware) <rost...@goodmis.org>
-- Steve
