sched/deadline: Fix deadline parameter modification handling

Commit 67dfa1b756f2 ("sched/deadline: Implement cancel_dl_timer() to
use in switched_from_dl()") removed the hrtimer_try_cancel() function
call out from init_dl_task_timer(), which gets called from
__setparam_dl().

The result is that we can now re-init the timer while its active --
this is bad and corrupts timer state.

Furthermore; changing the parameters of an active deadline task is
tricky in that you want to maintain guarantees, while immediately
effective change would allow one to circumvent the CBS guarantees --
this too is bad, as one (bad) task should not be able to affect the
others.

Rework things to avoid both problems. We only need to initialize the
timer once, so move that to __sched_fork() for new tasks.

Then make sure __setparam_dl() doesn't affect the current running
state but only updates the parameters used to calculate the next
scheduling period -- this guarantees the CBS functions as expected
(albeit slightly pessimistic).

This however means we need to make sure __dl_clear_params() needs to
reset the active state otherwise new (and tasks flipping between
classes) will not properly (re)compute their first instance.

Todo: close class flipping CBS hole.
Todo: implement delayed BW release.

Reported-by: Luca Abeni <luca.abeni@unitn.it>
Acked-by: Juri Lelli <juri.lelli@arm.com>
Tested-by: Luca Abeni <luca.abeni@unitn.it>
Fixes: 67dfa1b756f2 ("sched/deadline: Implement cancel_dl_timer() to use in switched_from_dl()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150128140803.GF23038@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index b52092f2636d..726470d47f87 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1094,6 +1094,7 @@ static void task_dead_dl(struct task_struct *p)
 	 * Since we are TASK_DEAD we won't slip out of the domain!
 	 */
 	raw_spin_lock_irq(&dl_b->lock);
+	/* XXX we should retain the bw until 0-lag */
 	dl_b->total_bw -= p->dl.dl_bw;
 	raw_spin_unlock_irq(&dl_b->lock);
 
@@ -1614,8 +1615,8 @@ static void cancel_dl_timer(struct rq *rq, struct task_struct *p)
 
 static void switched_from_dl(struct rq *rq, struct task_struct *p)
 {
+	/* XXX we should retain the bw until 0-lag */
 	cancel_dl_timer(rq, p);
-
 	__dl_clear_params(p);
 
 	/*