[PATCH RT 6/8] sched: Better debug output for might sleep

[Steven Rostedt]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

might sleep can tell us where interrupts have been disabled, but we
have no idea what disabled preemption. Add some debug infrastructure.

Cc: stable-rt@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 include/linux/sched.h |    4 ++++
 kernel/sched.c        |   23 +++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 03498cc..12317b6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1612,6 +1612,10 @@ struct task_struct {
 	int kmap_idx;
 	pte_t kmap_pte[KM_TYPE_NR];
 #endif
+
+#ifdef CONFIG_DEBUG_PREEMPT
+	unsigned long preempt_disable_ip;
+#endif
 };
 
 #ifdef CONFIG_PREEMPT_RT_FULL
diff --git a/kernel/sched.c b/kernel/sched.c
index 87654e6..cdf9484 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4487,8 +4487,13 @@ void __kprobes add_preempt_count(int val)
 	DEBUG_LOCKS_WARN_ON((preempt_count() & PREEMPT_MASK) >=
 				PREEMPT_MASK - 10);
 #endif
-	if (preempt_count() == val)
-		trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
+	if (preempt_count() == val) {
+		unsigned long ip = get_parent_ip(CALLER_ADDR1);
+#ifdef CONFIG_DEBUG_PREEMPT
+		current->preempt_disable_ip = ip;
+#endif
+		trace_preempt_off(CALLER_ADDR0, ip);
+	}
 }
 EXPORT_SYMBOL(add_preempt_count);
 
@@ -4530,6 +4535,13 @@ static noinline void __schedule_bug(struct task_struct *prev)
 	print_modules();
 	if (irqs_disabled())
 		print_irqtrace_events(prev);
+#ifdef DEBUG_PREEMPT
+	if (in_atomic_preempt_off()) {
+		pr_err("Preemption disabled at:");
+		print_ip_sym(current->preempt_disable_ip);
+		pr_cont("\n");
+	}
+#endif
 
 	if (regs)
 		show_regs(regs);
@@ -8912,6 +8924,13 @@ void __might_sleep(const char *file, int line, int preempt_offset)
 	debug_show_held_locks(current);
 	if (irqs_disabled())
 		print_irqtrace_events(current);
+#ifdef DEBUG_PREEMPT
+	if (!preempt_count_equals(preempt_offset)) {
+		pr_err("Preemption disabled at:");
+		print_ip_sym(current->preempt_disable_ip);
+		pr_cont("\n");
+	}
+#endif
 	dump_stack();
 }
 EXPORT_SYMBOL(__might_sleep);
-- 
1.7.10.4


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