From 37e117c07b89194aae7062bc63bde1104c03db02 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 14 Feb 2014 12:25:08 +0100 Subject: sched: Guarantee task priority in pick_next_task() Michael spotted that the idle_balance() push down created a task priority problem. Previously, when we called idle_balance() before pick_next_task() it wasn't a problem when -- because of the rq->lock droppage -- an rt/dl task slipped in. Similarly for pre_schedule(), rt pre-schedule could have a dl task slip in. But by pulling it into the pick_next_task() loop, we'll not try a higher task priority again. Cure this by creating a re-start condition in pick_next_task(); and triggering this from pick_next_task_{rt,fair}(). It also fixes a live-lock where we get stuck in pick_next_task_fair() due to idle_balance() seeing !0 nr_running but there not actually being any fair tasks about. Reported-by: Michael Wang Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()") Tested-by: Sasha Levin Signed-off-by: Peter Zijlstra Cc: Juri Lelli Cc: Steven Rostedt Link: http://lkml.kernel.org/r/20140224121218.GR15586@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) (limited to 'kernel/sched/core.c') diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a8a73b8897bf..cde573d3f12e 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2586,24 +2586,28 @@ static inline void schedule_debug(struct task_struct *prev) static inline struct task_struct * pick_next_task(struct rq *rq, struct task_struct *prev) { - const struct sched_class *class; + const struct sched_class *class = &fair_sched_class; struct task_struct *p; /* * Optimization: we know that if all tasks are in * the fair class we can call that function directly: */ - if (likely(prev->sched_class == &fair_sched_class && + if (likely(prev->sched_class == class && rq->nr_running == rq->cfs.h_nr_running)) { p = fair_sched_class.pick_next_task(rq, prev); - if (likely(p)) + if (likely(p && p != RETRY_TASK)) return p; } +again: for_each_class(class) { p = class->pick_next_task(rq, prev); - if (p) + if (p) { + if (unlikely(p == RETRY_TASK)) + goto again; return p; + } } BUG(); /* the idle class will always have a runnable task */ -- cgit v1.2.3