[PATCH] sched: add a stacktrace on enqueue_pushable error

[Gregory Haskins <ghaskins@novell.com>]

Hi Chirag,
  Please apply this patch to your 26.5-rt9 tree for ppc64, enable
  CONFIG_PROVE_LOCKING (which enables CONFIG_STACKTRACE) and give it a whirl.
  If you get an oops, please post the console output.

Thanks!

-Greg

------------------
sched: add a stacktrace on enqueue_pushable error

NOT FOR INCLUSION!

This is to help debug an issue discovered by Chirag Jog in the thread

http://lkml.org/lkml/2008/9/25/189

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

 include/linux/sched.h |    6 ++++++
 kernel/sched_rt.c     |   20 +++++++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 67da014..53e8a6a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -87,6 +87,7 @@ struct sched_param {
 #include <linux/task_io_accounting.h>
 #include <linux/kobject.h>
 #include <linux/latencytop.h>
+#include <linux/stacktrace.h>
 
 #include <asm/processor.h>
 
@@ -1170,6 +1171,11 @@ struct task_struct {
 
 	struct list_head tasks;
 	struct plist_node pushable_tasks;
+	struct {
+		unsigned long data[15];
+		struct stack_trace trace;
+		pid_t pid;
+	} pushable_stack;
 
 	/*
 	 * ptrace_list/ptrace_children forms the list of my children
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 57a0c0d..0e6a88c 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -55,7 +55,25 @@ static void update_rt_migration(struct rq *rq)
 
 static void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
 {
-	plist_del(&p->pushable_tasks, &rq->rt.pushable_tasks);
+	if (plist_node_empty(&p->pushable_tasks)) {
+		struct stack_trace *trace = &p->pushable_stack.trace;
+
+		trace->nr_entries = 0;
+		trace->max_entries = sizeof(p->pushable_stack.data)/sizeof(p->pushable_stack.data[0]);
+		trace->entries = &p->pushable_stack.data[0];
+		trace->skip = 0;
+
+		save_stack_trace(trace);
+
+		p->pushable_stack.pid = current->pid;
+	} else {
+		printk(KERN_CRIT "redundant enqueue by %d detected\n",
+		       p->pushable_stack.pid);
+		print_stack_trace(&p->pushable_stack.trace, 5);
+
+		BUG_ON(!plist_node_empty(&p->pushable_tasks));
+	}
+
 	plist_node_init(&p->pushable_tasks, p->prio);
 	plist_add(&p->pushable_tasks, &rq->rt.pushable_tasks);
 }