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Date: Mon, 27 Apr 2015 01:10:58 +0800
From: Xunlei Pang <xlpang@....com>
To: linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Juri Lelli <juri.lelli@...il.com>,
Ingo Molnar <mingo@...hat.com>,
Xunlei Pang <pang.xunlei@...aro.org>
Subject: [RFC PATCH 6/6] sched/rt: Requeue p back if the preemption of check_preempt_equal_prio_common() failed
From: Xunlei Pang <pang.xunlei@...aro.org>
In check_preempt_equal_prio_common(), it requeues "next" ahead
in the "run queue" and want to push current away. But when doing
the actual pushing, if the system state changes, the pushing may
fail as a result.
In this case, p finally becomes the new current and gets running,
while previous current was queued back waiting in the same "run
queue". This broke FIFO.
This patch adds a flag named RT_PREEMPT_PUSHAWAY for task_struct::
rt_preempt, sets it when doing check_preempt_equal_prio_common(),
and clears it if current is away(when it is dequeued). Thus we can
test this flag in p's post_schedule_rt() to judge if the pushing
has happened. If the pushing failed, requeue previous current back
to the head of its "run queue" and start a rescheduling.
Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>
---
kernel/sched/rt.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 67 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index e7d66eb..94789f1 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -258,6 +258,8 @@ int alloc_rt_sched_group(struct task_group *tg, struct task_group *parent)
#ifdef CONFIG_SMP
#define RT_PREEMPT_QUEUEAHEAD 1UL
+#define RT_PREEMPT_PUSHAWAY 2UL
+#define RT_PREEMPT_MASK 3UL
/*
* p(current) was preempted, and to be put ahead of
@@ -268,6 +270,22 @@ static inline bool rt_preempted(struct task_struct *p)
return !!(p->rt_preempt & RT_PREEMPT_QUEUEAHEAD);
}
+static inline struct task_struct *rt_preempting_target(struct task_struct *p)
+{
+ return (struct task_struct *) (p->rt_preempt & ~RT_PREEMPT_MASK);
+}
+
+/*
+ * p(new current) is preempting and pushing previous current away.
+ */
+static inline bool rt_preempting(struct task_struct *p)
+{
+ if ((p->rt_preempt & RT_PREEMPT_PUSHAWAY) && rt_preempting_target(p))
+ return true;
+
+ return false;
+}
+
static inline void clear_rt_preempt(struct task_struct *p)
{
p->rt_preempt = 0;
@@ -375,13 +393,17 @@ static inline int has_pushable_tasks(struct rq *rq)
return !plist_head_empty(&rq->rt.pushable_tasks);
}
-static inline void set_post_schedule(struct rq *rq)
+static inline void set_post_schedule(struct rq *rq, struct task_struct *p)
{
- /*
- * We detect this state here so that we can avoid taking the RQ
- * lock again later if there is no need to push
- */
- rq->post_schedule = has_pushable_tasks(rq);
+ if (rt_preempting(p))
+ /* Forced post schedule */
+ rq->post_schedule = 1;
+ else
+ /*
+ * We detect this state here so that we can avoid taking
+ * the RQ lock again later if there is no need to push
+ */
+ rq->post_schedule = has_pushable_tasks(rq);
}
static void
@@ -430,6 +452,11 @@ static inline bool rt_preempted(struct task_struct *p)
return false;
}
+static inline bool rt_preempting(struct task_struct *p)
+{
+ return false;
+}
+
static inline void clear_rt_preempt(struct task_struct *p)
{
}
@@ -472,7 +499,7 @@ static inline int pull_rt_task(struct rq *this_rq)
return 0;
}
-static inline void set_post_schedule(struct rq *rq)
+static inline void set_post_schedule(struct rq *rq, struct task_struct *p)
{
}
#endif /* CONFIG_SMP */
@@ -1330,6 +1357,7 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
dequeue_rt_entity(rt_se);
dequeue_pushable_task(rq, p);
+ clear_rt_preempt(p);
}
/*
@@ -1468,6 +1496,11 @@ static void check_preempt_equal_prio_common(struct rq *rq)
* to try and push current away.
*/
requeue_task_rt(rq, next, 1);
+
+ get_task_struct(curr);
+ curr->rt_preempt |= RT_PREEMPT_PUSHAWAY;
+ next->rt_preempt = (unsigned long) curr;
+ next->rt_preempt |= RT_PREEMPT_PUSHAWAY;
resched_curr_preempted_rt(rq);
}
@@ -1590,7 +1623,7 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
/* The running task is never eligible for pushing */
dequeue_pushable_task(rq, p);
- set_post_schedule(rq);
+ set_post_schedule(rq, p);
return p;
}
@@ -2151,6 +2184,32 @@ skip:
static void post_schedule_rt(struct rq *rq)
{
push_rt_tasks(rq);
+
+ if (rt_preempting(current)) {
+ struct task_struct *target;
+
+ current->rt_preempt = 0;
+ target = rt_preempting_target(current);
+ if (!(target->rt_preempt & RT_PREEMPT_PUSHAWAY))
+ goto out;
+
+ /*
+ * target still has RT_PREEMPT_PUSHAWAY set which
+ * means it wasn't pushed away successfully if it
+ * is still on this rq. thus restore former status
+ * of current and target if so.
+ */
+ if (!task_on_rq_queued(target) ||
+ task_cpu(target) != rq->cpu)
+ goto out;
+
+ /* target is previous current, requeue it back ahead. */
+ requeue_task_rt(rq, target, 1);
+ /* Let's preempt current, loop back to __schedule(). */
+ resched_curr_preempted_rt(rq);
+out:
+ put_task_struct(target);
+ }
}
/*
--
1.9.1
--
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