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 <[email protected]> Cc: Steven Rostedt (Red Hat) <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Signed-off-by: Mathieu Desnoyers <[email protected]> Signed-off-by: Julien Desfossez <[email protected]> --- 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
