[RFC PATCH 1/6] sched/fair: check_preempt_wakeup: Fix assumption on the default policy

From: Julien Desfossez
Date: Fri Sep 16 2016 - 17:17:24 EST

Tasks with RT or deadline scheduling class may inherit from a task with
a "fair" scheduling class. This priority inheritance changes the
scheduling class, but not the task "policy" field.

Therefore, the fair scheduler should not assume that policy !=
SCHED_NORMAL is the same as (policy == SCHED_BATCH || policy ==
SCHED_IDLE), because the policy could also be SCHED_RR, SCHED_FIFO, or

The incorrect comparison in check_preempt_wakeup makes RR, FIFO and
DEADLINE tasks which inherit from a fair task behave as if they were
IDLE or BATCH tasks, thus awaiting the following tick before preempting
the current task.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt (Red Hat) <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Signed-off-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxx>
kernel/sched/fair.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 07aaa7f..f3aef21 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5669,7 +5669,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
* Batch and idle tasks do not preempt non-idle tasks (their preemption
* is driven by the tick):
- if (unlikely(p->policy != SCHED_NORMAL) || !sched_feat(WAKEUP_PREEMPTION))
+ if (unlikely(p->policy == SCHED_BATCH || p->policy == SCHED_IDLE) ||
+ !sched_feat(WAKEUP_PREEMPTION))

find_matching_se(&se, &pse);