While reading sched deadline code, I find out that a constrained
deadline task could be replenished before the next period if
activated after the deadline, opening the window to run for more
than Q/P. The patch [2] explains and fixes this problem.
Furthermore, while fixing this issue, I found that the replenishment
timer was being fired at the deadline of the task. This works fine
for implicit deadline tasks (deadline == period) because the deadline
is at the same point in time of the next period. But that is not true
for constrained deadline tasks (deadline < period). This problem is
not as visible as the first because the runtime leakage takes
place only in the second activation. Next activations receive the
correct bandwidth. However, after the 2nd activation, tasks are
activated in the (period - dl_deadline) instant, which is before
the expected activation. This problem is explained in the fix
description as well.
Changes from V1:
- Fix a broken comment style.
- Fixes dl_is_constrained():
A constrained deadline task has dl_deadline < dl_period; so
"dl_runtime < dl_period"; s/runtime/deadline/
Daniel Bristot de Oliveira (2):
sched/deadline: Replenishment timer should fire in the next period
sched/deadline: Throttle a constrained deadline task activated after
the deadline
kernel/sched/deadline.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 52 insertions(+), 2 deletions(-)
--
2.9.3
