This patch adds hooks into the schedule switch tracing to
allow other latency traces to hook into the schedule switches.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
 lib/tracing/trace_sched_switch.c |  123 +++++++++++++++++++++++++++++++++------
 lib/tracing/tracer.h             |   14 ++++
 2 files changed, 119 insertions(+), 18 deletions(-)

Index: linux-mcount.git/lib/tracing/tracer.h
===================================================================
--- linux-mcount.git.orig/lib/tracing/tracer.h	2008-01-25 21:47:25.000000000 -0500
+++ linux-mcount.git/lib/tracing/tracer.h	2008-01-25 21:47:27.000000000 -0500
@@ -112,4 +112,18 @@ static inline notrace cycle_t now(void)
 	return get_monotonic_cycles();
 }
 
+#ifdef CONFIG_CONTEXT_SWITCH_TRACER
+typedef void (*tracer_switch_func_t)(void *private,
+				     struct task_struct *prev,
+				     struct task_struct *next);
+struct tracer_switch_ops {
+	tracer_switch_func_t func;
+	void *private;
+	struct tracer_switch_ops *next;
+};
+
+extern int register_tracer_switch(struct tracer_switch_ops *ops);
+extern int unregister_tracer_switch(struct tracer_switch_ops *ops);
+#endif /* CONFIG_CONTEXT_SWITCH_TRACER */
+
 #endif /* _LINUX_MCOUNT_TRACER_H */
Index: linux-mcount.git/lib/tracing/trace_sched_switch.c
===================================================================
--- linux-mcount.git.orig/lib/tracing/trace_sched_switch.c	2008-01-25 21:47:25.000000000 -0500
+++ linux-mcount.git/lib/tracing/trace_sched_switch.c	2008-01-25 21:47:27.000000000 -0500
@@ -16,33 +16,21 @@
 
 static struct tracing_trace *tracer_trace;
 static int trace_enabled __read_mostly;
+static DEFINE_SPINLOCK(sched_switch_func_lock);
 
-static notrace void sched_switch_callback(const struct marker *mdata,
-					  void *private_data,
-					  const char *format, ...)
+static void notrace sched_switch_func(void *private,
+				      struct task_struct *prev,
+				      struct task_struct *next)
 {
-	struct tracing_trace **p = mdata->private;
-	struct tracing_trace *tr = *p;
+	struct tracing_trace **ptr = private;
+	struct tracing_trace *tr = *ptr;
 	struct tracing_trace_cpu *data;
-	struct task_struct *prev;
-	struct task_struct *next;
 	unsigned long flags;
-	va_list ap;
 	int cpu;
 
-	if (likely(!atomic_read(&trace_record_cmdline)))
-		return;
-
-	tracing_record_cmdline(current);
-
 	if (likely(!trace_enabled))
 		return;
 
-	va_start(ap, format);
-	prev = va_arg(ap, typeof(prev));
-	next = va_arg(ap, typeof(next));
-	va_end(ap);
-
 	raw_local_irq_save(flags);
 	cpu = raw_smp_processor_id();
 	data = tr->data[cpu];
@@ -55,6 +43,105 @@ static notrace void sched_switch_callbac
 	raw_local_irq_restore(flags);
 }
 
+static struct tracer_switch_ops sched_switch_ops __read_mostly =
+{
+	.func = sched_switch_func,
+	.private = &tracer_trace,
+};
+
+static tracer_switch_func_t tracer_switch_func __read_mostly =
+	sched_switch_func;
+
+static struct tracer_switch_ops *tracer_switch_func_ops __read_mostly =
+	&sched_switch_ops;
+
+static void notrace sched_switch_func_loop(void *private,
+					   struct task_struct *prev,
+					   struct task_struct *next)
+{
+	struct tracer_switch_ops *ops = tracer_switch_func_ops;
+
+	for (; ops != NULL; ops = ops->next)
+		ops->func(ops->private, prev, next);
+}
+
+notrace int register_tracer_switch(struct tracer_switch_ops *ops)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&sched_switch_func_lock, flags);
+	ops->next = tracer_switch_func_ops;
+	smp_wmb();
+	tracer_switch_func_ops = ops;
+
+	if (ops->next == &sched_switch_ops)
+		tracer_switch_func = sched_switch_func_loop;
+
+	spin_unlock_irqrestore(&sched_switch_func_lock, flags);
+
+	return 0;
+}
+
+notrace int unregister_tracer_switch(struct tracer_switch_ops *ops)
+{
+	unsigned long flags;
+	struct tracer_switch_ops **p = &tracer_switch_func_ops;
+	int ret;
+
+	spin_lock_irqsave(&sched_switch_func_lock, flags);
+
+	/*
+	 * If the sched_switch is the only one left, then
+	 *  only call that function.
+	 */
+	if (*p == ops && ops->next == &sched_switch_ops) {
+		tracer_switch_func = sched_switch_func;
+		tracer_switch_func_ops = &sched_switch_ops;
+		goto out;
+	}
+
+	for (; *p != &sched_switch_ops; p = &(*p)->next)
+		if (*p == ops)
+			break;
+
+	if (*p != ops) {
+		ret = -1;
+		goto out;
+	}
+
+	*p = (*p)->next;
+
+ out:
+	spin_unlock_irqrestore(&sched_switch_func_lock, flags);
+
+	return 0;
+}
+
+static notrace void sched_switch_callback(const struct marker *mdata,
+					  void *private_data,
+					  const char *format, ...)
+{
+	struct task_struct *prev;
+	struct task_struct *next;
+	va_list ap;
+
+	if (likely(!atomic_read(&trace_record_cmdline)))
+		return;
+
+	tracing_record_cmdline(current);
+
+	va_start(ap, format);
+	prev = va_arg(ap, typeof(prev));
+	next = va_arg(ap, typeof(next));
+	va_end(ap);
+
+	/*
+	 * If tracer_switch_func only points to the local
+	 * switch func, it still needs the ptr passed to it.
+	 */
+	tracer_switch_func(mdata->private, prev, next);
+}
+
 static notrace void sched_switch_reset(struct tracing_trace *tr)
 {
 	int cpu;

-- 
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