From 37e117c07b89194aae7062bc63bde1104c03db02 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 14 Feb 2014 12:25:08 +0100 Subject: [PATCH] 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 ++++++++---- kernel/sched/fair.c | 13 ++++++++++++- kernel/sched/rt.c | 10 +++++++++- kernel/sched/sched.h | 5 +++++ 4 files changed, 34 insertions(+), 6 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a8a73b8..cde573d 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 */ diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index be4f7d9..16042b5 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4686,6 +4686,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev) struct cfs_rq *cfs_rq = &rq->cfs; struct sched_entity *se; struct task_struct *p; + int new_tasks; again: #ifdef CONFIG_FAIR_GROUP_SCHED @@ -4784,7 +4785,17 @@ simple: return p; idle: - if (idle_balance(rq)) /* drops rq->lock */ + /* + * Because idle_balance() releases (and re-acquires) rq->lock, it is + * possible for any higher priority task to appear. In that case we + * must re-start the pick_next_entity() loop. + */ + new_tasks = idle_balance(rq); + + if (rq->nr_running != rq->cfs.h_nr_running) + return RETRY_TASK; + + if (new_tasks) goto again; return NULL; diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 4d4b386..398b3f9 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev) struct task_struct *p; struct rt_rq *rt_rq = &rq->rt; - if (need_pull_rt_task(rq, prev)) + if (need_pull_rt_task(rq, prev)) { pull_rt_task(rq); + /* + * pull_rt_task() can drop (and re-acquire) rq->lock; this + * means a dl task can slip in, in which case we need to + * re-start task selection. + */ + if (unlikely(rq->dl.dl_nr_running)) + return RETRY_TASK; + } if (!rt_rq->rt_nr_running) return NULL; diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 046084e..1929deb 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1091,6 +1091,8 @@ static const u32 prio_to_wmult[40] = { #define DEQUEUE_SLEEP 1 +#define RETRY_TASK ((void *)-1UL) + struct sched_class { const struct sched_class *next; @@ -1105,6 +1107,9 @@ struct sched_class { * It is the responsibility of the pick_next_task() method that will * return the next task to call put_prev_task() on the @prev task or * something equivalent. + * + * May return RETRY_TASK when it finds a higher prio class has runnable + * tasks. */ struct task_struct * (*pick_next_task) (struct rq *rq, struct task_struct *prev); -- 2.7.4