sched/deadline: Clear dl_entity params when setscheduling to different class

When a task is using SCHED_DEADLINE and the user setschedules it to a
different class its sched_dl_entity static parameters are not cleaned
up. This causes a bug if the user sets it back to SCHED_DEADLINE with
the same parameters again.  The problem resides in the check we
perform at the very beginning of dl_overflow():

	if (new_bw == p->dl.dl_bw)
		return 0;

This condition is met in the case depicted above, so the function
returns and dl_b->total_bw is not updated (the p->dl.dl_bw is not
added to it). After this, admission control is broken.

This patch fixes the thing, properly clearing static parameters for a
task that ceases to use SCHED_DEADLINE.

Reported-by: Daniele Alessandrelli <daniele.alessandrelli@gmail.com>
Reported-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Reported-by: Vincent Legout <vincent@legout.info>
Tested-by: Luca Abeni <luca.abeni@unitn.it>
Tested-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Tested-by: Vincent Legout <vincent@legout.info>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Fabio Checconi <fchecconi@gmail.com>
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1411118561-26323-2-git-send-email-juri.lelli@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -1808,6 +1808,20 @@ int wake_up_state(struct task_struct *p, unsigned int state)
 	return try_to_wake_up(p, state, 0);
 }
 
+/*
+ * This function clears the sched_dl_entity static params.
+ */
+void __dl_clear_params(struct task_struct *p)
+{
+	struct sched_dl_entity *dl_se = &p->dl;
+
+	dl_se->dl_runtime = 0;
+	dl_se->dl_deadline = 0;
+	dl_se->dl_period = 0;
+	dl_se->flags = 0;
+	dl_se->dl_bw = 0;
+}
+
 /*
  * Perform scheduler related setup for a newly forked process p.
  * p is forked by current.
@@ -1832,10 +1846,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 	RB_CLEAR_NODE(&p->dl.rb_node);
 	hrtimer_init(&p->dl.dl_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
-	p->dl.dl_runtime = p->dl.runtime = 0;
-	p->dl.dl_deadline = p->dl.deadline = 0;
-	p->dl.dl_period = 0;
-	p->dl.flags = 0;
+	__dl_clear_params(p);
 
 	INIT_LIST_HEAD(&p->rt.run_list);
 

kernel/sched/deadline.c

@@ -1565,6 +1565,8 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 	if (hrtimer_active(&p->dl.dl_timer) && !dl_policy(p->policy))
 		hrtimer_try_to_cancel(&p->dl.dl_timer);
 
+	__dl_clear_params(p);
+
 #ifdef CONFIG_SMP
 	/*
 	 * Since this might be the only -deadline task on the rq,

kernel/sched/sched.h

@@ -130,6 +130,9 @@ struct rt_bandwidth {
 	u64			rt_runtime;
 	struct hrtimer		rt_period_timer;
 };
+
+void __dl_clear_params(struct task_struct *p);
+
 /*
  * To keep the bandwidth of -deadline tasks and groups under control
  * we need some place where: