[PATCH] sched: fix cpupri hotplug support

[Gregory Haskins]

Hi Ingo, Steven, Thomas,
  This patch is critical to fix a snafu in the root-domain/cpupri code.  It
  applies to 25.4-rt4, but as mentioned below it affects most recent -rt
  kernels as well as sched-devel.  If you accept this patch, and you have
  non-trivial merge issues with other flavors just let me know and I will port
  it to those kernels as well.

Regards
-Greg

--------------------------------
sched: fix cpupri hotplug support

The RT folks over at RedHat found an issue w.r.t. hotplug support which
was traced to problems with the cpupri infrastructure in the scheduler:

https://bugzilla.redhat.com/show_bug.cgi?id=449676

This bug affects 23-rt12+, 24-rtX, 25-rtX, and sched-devel.  This patch
applies to 25.4-rt4, though it should trivially apply to most cpupri enabled
kernels mentioned above.

It turned out that the issue was that offline cpus could get inadvertently
registered with cpupri so that they were erroneously selected during
migration decisions.  The end result would be an OOPS as the offline cpu
had tasks routed to it.

This patch adds a new per-sched-class pair of interfaces:
online/offline. They allow for the sched-class to register for
notification when a run-queue is taken online or offline.  Additionally,
the existing join/leave-domain code has been unified behind the new
online/offline handlers so that they do the right thing w.r.t. the online
status of a particular CPU.

I was able to easily reproduce the issue prior to this patch, and am no
longer able to reproduce it after this patch.  I can offline cpus
indefinately and everything seems to be in working order.

Thanks to Arnaldo (acme) and Thomas (tglx) for doing the legwork to point
me in the right direction. 

Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---

 include/linux/sched.h |    2 ++
 kernel/sched.c        |   19 +++++++++++++++++++
 kernel/sched_rt.c     |   16 ++++++++++++----
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index bb71371..9c5b8c9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -970,6 +970,8 @@ struct sched_class {
 
 	void (*join_domain)(struct rq *rq);
 	void (*leave_domain)(struct rq *rq);
+	void (*online)(struct rq *rq);
+	void (*offline)(struct rq *rq);
 
 	void (*switched_from) (struct rq *this_rq, struct task_struct *task,
 			       int running);
diff --git a/kernel/sched.c b/kernel/sched.c
index 0399411..fe96984 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6278,6 +6278,9 @@ static void unregister_sched_domain_sysctl(void)
 }
 #endif
 
+#define for_each_class(class) \
+   for (class = sched_class_highest; class; class = class->next)
+
 /*
  * migration_call - callback that gets triggered when a CPU is added.
  * Here we can start up the necessary migration thread for the new CPU.
@@ -6314,8 +6317,15 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		rq = cpu_rq(cpu);
 		spin_lock_irqsave(&rq->lock, flags);
 		if (rq->rd) {
+			const struct sched_class *class;
+
 			BUG_ON(!cpu_isset(cpu, rq->rd->span));
 			cpu_set(cpu, rq->rd->online);
+
+			for_each_class(class) {
+				if (class->online)
+					class->online(rq);
+			}
 		}
 		spin_unlock_irqrestore(&rq->lock, flags);
 		break;
@@ -6375,8 +6385,17 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		rq = cpu_rq(cpu);
 		spin_lock_irqsave(&rq->lock, flags);
 		if (rq->rd) {
+			const struct sched_class *class;
+
 			BUG_ON(!cpu_isset(cpu, rq->rd->span));
+
+			for_each_class(class) {
+				if (class->offline)
+					class->offline(rq);
+			}
+
 			cpu_clear(cpu, rq->rd->online);
+
 		}
 		spin_unlock_irqrestore(&rq->lock, flags);
 		break;
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 425cf01..8baf2e8 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1101,8 +1101,11 @@ static void set_cpus_allowed_rt(struct task_struct *p, cpumask_t *new_mask)
 }
 
 /* Assumes rq->lock is held */
-static void join_domain_rt(struct rq *rq)
+static void rq_online_rt(struct rq *rq)
 {
+	if (!cpu_isset(rq->cpu, rq->rd->online))
+		return;
+
 	if (rq->rt.overloaded)
 		rt_set_overload(rq);
 
@@ -1110,8 +1113,11 @@ static void join_domain_rt(struct rq *rq)
 }
 
 /* Assumes rq->lock is held */
-static void leave_domain_rt(struct rq *rq)
+static void rq_offline_rt(struct rq *rq)
 {
+	if (!cpu_isset(rq->cpu, rq->rd->online))
+		return;
+
 	if (rq->rt.overloaded)
 		rt_clear_overload(rq);
 
@@ -1278,8 +1284,10 @@ const struct sched_class rt_sched_class = {
 	.load_balance		= load_balance_rt,
 	.move_one_task		= move_one_task_rt,
 	.set_cpus_allowed       = set_cpus_allowed_rt,
-	.join_domain            = join_domain_rt,
-	.leave_domain           = leave_domain_rt,
+	.join_domain            = rq_online_rt,
+	.leave_domain           = rq_offline_rt,
+	.online                 = rq_online_rt,
+	.offline                = rq_offline_rt,
 	.pre_schedule		= pre_schedule_rt,
 	.post_schedule		= post_schedule_rt,
 	.task_wake_up		= task_wake_up_rt,

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