[PATCH v2 1/2] sched/rt: Check to push task away when its affinity is changed

[Xunlei Pang <xlpang@126.com>]

From: Xunlei Pang <pang.xunlei@linaro.org>

We may suffer from extra rt overload rq due to the affinity,
so when the affinity of any runnable rt task is changed, we
should check to trigger balancing, otherwise it will cause
some unnecessary delayed real-time response. Unfortunately,
current RT global scheduler does nothing about this.

For example: a 2-cpu system with two runnable FIFO tasks(same
rt_priority) bound on CPU0, let's name them rt1(running) and
rt2(runnable) respectively; CPU1 has no RTs. Then, someone sets
the affinity of rt2 to 0x3(i.e. CPU0 and CPU1), but after this,
rt2 still can't be scheduled enters schedule(), this
definitely causes some/big response latency for rt2.

This patch introduces a new sched_class::post_set_cpus_allowed()
for RT called after set_cpus_allowed_rt(). In this new function,
if the task is runnable but not running, it tries to push it away
once it got migratable.

The patch also solves a problem about move_queued_task() called
in set_cpus_allowed_ptr():
When a lower priorioty rt task got migrated due to its curr cpu 
isn't in the new affinity mask, after move_queued_task() it will 
miss the chance of pushing away, because check_preempt_curr() 
called by move_queued_task() doens't set the "need resched flag" 
for lower priority tasks.

Parts-suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
---
v1->v2:
Removed cpupri_find(), as it will probably be executed in push_rt_tasks().

 kernel/sched/core.c  |  3 +++
 kernel/sched/rt.c    | 15 +++++++++++++++
 kernel/sched/sched.h |  1 +
 3 files changed, 19 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d13fc13..64a1603 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4773,6 +4773,9 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 
 	cpumask_copy(&p->cpus_allowed, new_mask);
 	p->nr_cpus_allowed = cpumask_weight(new_mask);
+
+	if (p->sched_class->post_set_cpus_allowed)
+		p->sched_class->post_set_cpus_allowed(p);
 }
 
 /*
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 8885b65..4176f33 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2280,6 +2280,20 @@ static void set_cpus_allowed_rt(struct task_struct *p,
 	update_rt_migration(&rq->rt);
 }
 
+static void post_set_cpus_allowed_rt(struct task_struct *p)
+{
+	struct rq *rq;
+
+	if (!task_on_rq_queued(p))
+		return;
+
+	rq = task_rq(p);
+	if (!task_running(rq, p) &&
+	    p->nr_cpus_allowed > 1 &&
+	    !test_tsk_need_resched(rq->curr))
+		push_rt_tasks(rq);
+}
+
 /* Assumes rq->lock is held */
 static void rq_online_rt(struct rq *rq)
 {
@@ -2494,6 +2508,7 @@ const struct sched_class rt_sched_class = {
 	.select_task_rq		= select_task_rq_rt,
 
 	.set_cpus_allowed       = set_cpus_allowed_rt,
+	.post_set_cpus_allowed  = post_set_cpus_allowed_rt,
 	.rq_online              = rq_online_rt,
 	.rq_offline             = rq_offline_rt,
 	.post_schedule		= post_schedule_rt,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e0e1299..6f90645 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1191,6 +1191,7 @@ struct sched_class {
 
 	void (*set_cpus_allowed)(struct task_struct *p,
 				 const struct cpumask *newmask);
+	void (*post_set_cpus_allowed)(struct task_struct *p);
 
 	void (*rq_online)(struct rq *rq);
 	void (*rq_offline)(struct rq *rq);
-- 
1.9.1