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

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Message-Id: <1474060148-13171-1-git-send-email-jdesfossez@efficios.com>
Date:   Fri, 16 Sep 2016 17:09:03 -0400
From:   Julien Desfossez <jdesfossez@...icios.com>
To:     peterz@...radead.org, tglx@...utronix.de, rostedt@...dmis.org,
        mingo@...hat.com, daolivei@...hat.com
Cc:     mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        Julien Desfossez <jdesfossez@...icios.com>
Subject: [RFC PATCH 1/6] sched/fair: check_preempt_wakeup: Fix assumption on the default policy

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
SCHED_DEADLINE.

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@...radead.org>
Cc: Steven Rostedt (Red Hat) <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Julien Desfossez <jdesfossez@...icios.com>
---
 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))
 		return;
 
 	find_matching_se(&se, &pse);
-- 
1.9.1