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Subject[PATCH for v3.14.58 3/7] sched: Allow balance callbacks for check_class_changed()
Date
From: Peter Zijlstra <peterz@infradead.org>

In order to remove dropping rq->lock from the
switched_{to,from}()/prio_changed() sched_class methods, run the
balance callbacks after it.

We need to remove dropping rq->lock because its buggy,
suppose using sched_setattr()/sched_setscheduler() to change a running
task from FIFO to OTHER.

By the time we get to switched_from_rt() the task is already enqueued
on the cfs runqueues. If switched_from_rt() does pull_rt_task() and
drops rq->lock, load-balancing can come in and move our task @p to
another rq.

The subsequent switched_to_fair() still assumes @p is on @rq and bad
things will happen.

By using balance callbacks we delay the load-balancing operations
{rt,dl}x{push,pull} until we've done all the important work and the
task is fully set up.

Furthermore, the balance callbacks do not know about @p, therefore
they cannot get confused like this.

Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: ktkhai@parallels.com
Cc: rostedt@goodmis.org
Cc: juri.lelli@gmail.com
Cc: pang.xunlei@linaro.org
Cc: oleg@redhat.com
Cc: wanpeng.li@linux.intel.com
Link: http://lkml.kernel.org/r/20150611124742.615343911@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Byungchul Park <byungchul.park@lge.com>

Conflicts:
kernel/sched/core.c
---
kernel/sched/core.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cc1be56..459cc86 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -937,6 +937,13 @@ inline int task_curr(const struct task_struct *p)
return cpu_curr(task_cpu(p)) == p;
}

+/*
+ * switched_from, switched_to and prio_changed must _NOT_ drop rq->lock,
+ * use the balance_callback list if you want balancing.
+ *
+ * this means any call to check_class_changed() must be followed by a call to
+ * balance_callback().
+ */
static inline void check_class_changed(struct rq *rq, struct task_struct *p,
const struct sched_class *prev_class,
int oldprio)
@@ -1423,8 +1430,12 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)

p->state = TASK_RUNNING;
#ifdef CONFIG_SMP
- if (p->sched_class->task_woken)
+ if (p->sched_class->task_woken) {
+ /*
+ * XXX can drop rq->lock; most likely ok.
+ */
p->sched_class->task_woken(rq, p);
+ }

if (rq->idle_stamp) {
u64 delta = rq_clock(rq) - rq->idle_stamp;
@@ -3006,7 +3017,11 @@ void rt_mutex_setprio(struct task_struct *p, int prio)

check_class_changed(rq, p, prev_class, oldprio);
out_unlock:
+ preempt_disable(); /* avoid rq from going away on us */
__task_rq_unlock(rq);
+
+ balance_callback(rq);
+ preempt_enable();
}
#endif

@@ -3512,10 +3527,17 @@ change:
enqueue_task(rq, p, 0);

check_class_changed(rq, p, prev_class, oldprio);
+ preempt_disable(); /* avoid rq from going away on us */
task_rq_unlock(rq, p, &flags);

rt_mutex_adjust_pi(p);

+ /*
+ * Run balance callbacks after we've adjusted the PI chain.
+ */
+ balance_callback(rq);
+ preempt_enable();
+
return 0;
}

--
1.9.1


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