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Date: Thu, 27 Feb 2014 05:33:04 -0800
From: tip-bot for Peter Zijlstra <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, sasha.levin@...cle.com,
hpa@...or.com, mingo@...nel.org, peterz@...radead.org,
rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
tglx@...utronix.de, juri.lelli@...il.com
Subject: [tip:sched/core] sched: Guarantee task priority in pick_next_task
()
Commit-ID: 37e117c07b89194aae7062bc63bde1104c03db02
Gitweb: http://git.kernel.org/tip/37e117c07b89194aae7062bc63bde1104c03db02
Author: Peter Zijlstra <peterz@...radead.org>
AuthorDate: Fri, 14 Feb 2014 12:25:08 +0100
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 27 Feb 2014 12:41:02 +0100
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 <wangyun@...ux.vnet.ibm.com>
Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
Tested-by: Sasha Levin <sasha.levin@...cle.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Link: http://lkml.kernel.org/r/20140224121218.GR15586@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
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);
--
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