From: Steven Rostedt <rostedt@goodmis.org>
Date: Thu, 29 Sep 2011 12:24:30 -0500
Subject: tracing: Account for preempt off in preempt_schedule()
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: a832f72ba1743141fb0b04c1647741912fc32164
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

The preempt_schedule() uses the preempt_disable_notrace() version
because it can cause infinite recursion by the function tracer as
the function tracer uses preempt_enable_notrace() which may call
back into the preempt_schedule() code as the NEED_RESCHED is still
set and the PREEMPT_ACTIVE has not been set yet.

See commit: d1f74e20b5b064a130cd0743a256c2d3cfe84010 that made this
change.

The preemptoff and preemptirqsoff latency tracers require the first
and last preempt count modifiers to enable tracing. But this skips
the checks. Since we can not convert them back to the non notrace
version, we can use the idle() hooks for the latency tracers here.
That is, the start/stop_critical_timings() works well to manually
start and stop the latency tracer for preempt off timings.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Clark Williams <williams@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 kernel/sched/core.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3687,7 +3687,16 @@ asmlinkage __visible void __sched notrac
 		 * an infinite recursion.
 		 */
 		prev_ctx = exception_enter();
+		/*
+		 * The add/subtract must not be traced by the function
+		 * tracer. But we still want to account for the
+		 * preempt off latency tracer. Since the _notrace versions
+		 * of add/subtract skip the accounting for latency tracer
+		 * we must force it manually.
+		 */
+		start_critical_timings();
 		__schedule(true);
+		stop_critical_timings();
 		exception_exit(prev_ctx);
 
 		preempt_latency_stop(1);