preempt: Track unmatched preempt_enable() to preempt_disable()

Back in the original real-time patch there was a nice debug feature that tracked where preemption was disabled and not matched by a preempt enable. This information made debugging scheduling while atomic much easier, especially when a function was buried in a function chain that happened just before the schedule. For example:

bar() {
preempt_disable();
[...]
if (ret < 0)

     return;

[...]
}

foo() {
[...]
bar();
[...]
}

foo();
schedule();

Getting the "scheduling while atomic" warning detects the bug, but it does not let you know where the bug happened. It may take several trials to figure out where the missing preempt_enable() was. Some times those functions are very deep and that makes it even more difficult to find.

Although this bug does not happen often, this feature is extremely helpful when it does. I decided to take Ingo's code, which was just written for debugging the -rt patch, and clean it up for something that is suitable for mainline.

I started with his code and modified it to be a bit simpler, as he never meant that code to be mainline. First, as preempt disabling is really more of a CPU state and not a process state, I made the stacks just per cpu arrays. This eliminates any changes needed by sched.h or the task_struct.

The only times the preempt trace is needed is when scheduling while atomic or might sleep bugs are found. This information is only printed in those cases (not on all bug reports).

I renamed the config option from PREEMPT_TRACE to PREEMPT_STACK to prevent confusion from the tracing facility PREEMPT_TRACER that traces the length that preemption is disabled.

Using a struct to save the ip and parent ip together also keeps the code a bit cleaner than using two different arrays.

Using a module that creates a kernel thread that calls a function bad_preempt_disable() that just calls preempt_disable() with no matching preempt_enable(), and then calls schedule, the output can be seen as the following:

BUG: scheduling while atomic: task1/3889/0x00000002
Modules linked in: sched_bug(O) [..]
CPU: 7 PID: 3889 Comm: task1 Tainted: G           O 3.13.0-rc8-test+ #28
Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v02.05 05/07/2012
 0000000000000007 ffff8800c87a1dd8 ffffffff81627676 ffff88011ebcf5d0
 ffff8800ca0eaf00 ffff8800c87a1df8 ffffffff81623825 0000000000000002
 ffff88011ebd3600 ffff8800c87a1e78 ffffffff8162b0f5 ffff8800c87a1e28
Call Trace:
 [<ffffffff81627676>] dump_stack+0x4f/0x7c
 [<ffffffff81623825>] __schedule_bug+0x59/0x6e
 [<ffffffff8162b0f5>] __schedule+0x6b5/0x7e0
 [<ffffffff816329f1>] ? preempt_count_sub+0xb1/0x100
 [<ffffffffa06d9020>] ? bad_preempt_disable+0x20/0x20 [sched_bug]
 [<ffffffff8162b339>] schedule+0x29/0x70
 [<ffffffffa06d9038>] dead_thread+0x18/0x78 [sched_bug]
 [<ffffffff81073559>] kthread+0xc9/0xe0
 [<ffffffff81620000>] ? pci_add_new_bus+0x150/0x3b0
 [<ffffffff81073490>] ? flush_kthread_worker+0xb0/0xb0
 [<ffffffff8163676c>] ret_from_fork+0x7c/0xb0
 [<ffffffff81073490>] ? flush_kthread_worker+0xb0/0xb0
---------------------------

| preempt count: 00000002 ]
| 2-level deep critical section nesting:
----------------------------------------
.. [<ffffffffa06d9013>] .... bad_preempt_disable+0x13/0x20 [sched_bug]
.....[<ffffffffa06d9033>] .. ( <= dead_thread+0x13/0x78 [sched_bug])
.. [<ffffffff8162aa89>] .... __schedule+0x49/0x7e0
.....[<ffffffff8162b339>] .. ( <= schedule+0x29/0x70)

Steven Rostedt (Red Hat) (2):
preempt: Add CONFIG_DEBUG_PREEMPT_COUNT to consolidate ifdef logic
preempt: Show preempt disable stack on schedule bugs

----

 include/linux/preempt.h |  2 +-
 kernel/sched/core.c     | 63 ++++++++++++++++++++++++++++++++++++++++++++++---
 kernel/trace/Kconfig    |  1 +
 lib/Kconfig.debug       | 20 ++++++++++++++++
 4 files changed, 82 insertions(+), 4 deletions(-)

From: "Steven Rostedt (Red Hat)" <[email protected]>

Add a Kconfig bool DEBUG_PREEMPT_COUNT that gets enabled by both DEBUG_PREEMPT and PREEMPT_TRACER, where they both enable the full add_preempt_count() function call to debug preempt count logic.

This cleans up the code and lets other users of add_preempt_count() without messing up the code as well.

---
 include/linux/preempt.h | 2 +-
 kernel/sched/core.c     | 3 +--
 kernel/trace/Kconfig    | 1 +
 lib/Kconfig.debug       | 4 ++++
 4 files changed, 7 insertions(+), 3 deletions(-)

index a3d9dc8..cd48ecc 100644
@@ -17,7 +17,7 @@
 
 #include <asm/preempt.h>
 
-#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_PREEMPT_TRACER)
+#ifdef CONFIG_DEBUG_PREEMPT_COUNT
 extern void preempt_count_add(int val);
 extern void preempt_count_sub(int val);
 #define preempt_count_dec_and_test() ({ preempt_count_sub(1); should_resched(); })

index a88f4a4..68b0c1f 100644
@@ -2339,8 +2339,7 @@ notrace unsigned long get_parent_ip(unsigned long addr)
 	return addr;
 }
 
-#if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
-				defined(CONFIG_PREEMPT_TRACER))
+#ifdef CONFIG_DEBUG_PREEMPT_COUNT
 
 void __kprobes preempt_count_add(int val)
 {

index 015f85a..3fb0331 100644
@@ -202,6 +202,7 @@ config PREEMPT_TRACER
 	select RING_BUFFER_ALLOW_SWAP
 	select TRACER_SNAPSHOT
 	select TRACER_SNAPSHOT_PER_CPU_SWAP
+	select DEBUG_PREEMPT_COUNT
 	help
 	  This option measures the time spent in preemption-off critical
 	  sections, with microsecond accuracy.

index db25707..8cf7819 100644
@@ -796,9 +796,13 @@ config TIMER_STATS
 	  (it defaults to deactivated on bootup and will only be activated
 	  if some application like powertop activates it explicitly).
 
+config DEBUG_PREEMPT_COUNT
+       bool
+
 config DEBUG_PREEMPT
 	bool "Debug preemptible kernel"
 	depends on DEBUG_KERNEL && PREEMPT && TRACE_IRQFLAGS_SUPPORT
+	select DEBUG_PREEMPT_COUNT
 	default y
 	help
 	  If you say Y here then the kernel will use a debug variant of the

1.8.4.3

From: "Steven Rostedt (Red Hat)" <[email protected]>

There's been a few times where a function was called that did not have a matching preempt_enable() for the preempt_disable().

For example, if a function deep in a call chain was called, and did a preempt_disable() but due to an early exit of the routine, did not do a matching preempt_enable(), it may not warn about this until later.

bar() {
preempt_disable();
[...]
if (ret < 0)

     return;

[...]
}

foo() {
[...]
bar();
[...]
}

foo();
schedule();

Getting the "scheduling while atomic" warning detects the bug, but it does not let you know where the bug happened. It may take several trials to figure out where the missing preempt_enable() was. Some times those functions are very deep and that makes it even more difficult to find.

The original real-time patch had a feature, written by Ingo Molnar, to trace all the locations that disabled preemption for each process. On a bug, it would report where the preemption was disabled.

I started with his code and modified it to be a bit simpler, as he never meant that code to be mainline. First, as preempt disabling is really more of a CPU state and not a process state, I made the stacks just per cpu arrays. This eliminates any changes needed by sched.h or the task_struct.

The only times the preempt trace is needed is when scheduling while atomic or might sleep bugs are found. This information is only printed in those cases (not on all bug reports).

I renamed the config option from PREEMPT_TRACE to PREEMPT_STACK to prevent confusion from the tracing facility PREEMPT_TRACER that traces the length that preemption is disabled.

Using a struct to save the ip and parent ip together also keeps the code a bit cleaner than using two different arrays.

Using a module that creates a kernel thread that calls a function bad_preempt_disable() that just calls preempt_disable() with no matching preempt_enable(), and then calls schedule, the output can be seen as the following:

BUG: scheduling while atomic: task1/3889/0x00000002
Modules linked in: sched_bug(O) [..]
CPU: 7 PID: 3889 Comm: task1 Tainted: G           O 3.13.0-rc8-test+ #28
Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v02.05 05/07/2012
 0000000000000007 ffff8800c87a1dd8 ffffffff81627676 ffff88011ebcf5d0
 ffff8800ca0eaf00 ffff8800c87a1df8 ffffffff81623825 0000000000000002
 ffff88011ebd3600 ffff8800c87a1e78 ffffffff8162b0f5 ffff8800c87a1e28
Call Trace:
 [<ffffffff81627676>] dump_stack+0x4f/0x7c
 [<ffffffff81623825>] __schedule_bug+0x59/0x6e
 [<ffffffff8162b0f5>] __schedule+0x6b5/0x7e0
 [<ffffffff816329f1>] ? preempt_count_sub+0xb1/0x100
 [<ffffffffa06d9020>] ? bad_preempt_disable+0x20/0x20 [sched_bug]
 [<ffffffff8162b339>] schedule+0x29/0x70
 [<ffffffffa06d9038>] dead_thread+0x18/0x78 [sched_bug]
 [<ffffffff81073559>] kthread+0xc9/0xe0
 [<ffffffff81620000>] ? pci_add_new_bus+0x150/0x3b0
 [<ffffffff81073490>] ? flush_kthread_worker+0xb0/0xb0
 [<ffffffff8163676c>] ret_from_fork+0x7c/0xb0
 [<ffffffff81073490>] ? flush_kthread_worker+0xb0/0xb0
---------------------------

| preempt count: 00000002 ]
| 2-level deep critical section nesting:
----------------------------------------
.. [<ffffffffa06d9013>] .... bad_preempt_disable+0x13/0x20 [sched_bug]
.....[<ffffffffa06d9033>] .. ( <= dead_thread+0x13/0x78 [sched_bug])
.. [<ffffffff8162aa89>] .... __schedule+0x49/0x7e0
.....[<ffffffff8162b339>] .. ( <= schedule+0x29/0x70)

---
 kernel/sched/core.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 lib/Kconfig.debug   | 16 ++++++++++++++
 2 files changed, 75 insertions(+), 1 deletion(-)

index 68b0c1f..336b2c0 100644
@@ -2341,8 +2341,51 @@ notrace unsigned long get_parent_ip(unsigned long addr)
 
 #ifdef CONFIG_DEBUG_PREEMPT_COUNT
 
+#ifdef CONFIG_PREEMPT_STACK
+# define MAX_PREEMPT_STACK 25
+struct preempt_stack {
+	unsigned long		ip;
+	unsigned long		parent_ip;
+};
+
+static DEFINE_PER_CPU(struct preempt_stack,
+		      preempt_stack_trace[MAX_PREEMPT_STACK]);
+
+void print_preempt_trace(void)
+{
+	struct preempt_stack *stack;
+	unsigned int count;
+	unsigned int i, lim;
+
+	stack = this_cpu_ptr(preempt_stack_trace);
+
+	count = preempt_count();
+
+	lim = count & PREEMPT_MASK;
+
+	if (lim >= MAX_PREEMPT_STACK)
+		lim = MAX_PREEMPT_STACK-1;
+	printk(KERN_ERR "---------------------------\n");
+	printk(KERN_CONT "| preempt count: %08x ]\n", count);
+	printk(KERN_CONT "| %d-level deep critical section nesting:\n", lim);
+	printk(KERN_CONT "----------------------------------------\n");
+	for (i = 1; i <= lim; i++) {
+		printk(KERN_CONT ".. [<%08lx>] .... %pS\n",
+		       stack[i].ip, (void *)stack[i].ip);
+		printk(KERN_CONT ".....[<%08lx>] ..   ( <= %pS)\n",
+		       stack[i].parent_ip, (void *)stack[i].parent_ip);
+	}
+	printk(KERN_CONT "\n");
+}
+#else
+static inline void print_preempt_trace(void) { }
+#endif
+
 void __kprobes preempt_count_add(int val)
 {
+	unsigned long ip = CALLER_ADDR0;
+	unsigned long parent_ip = get_parent_ip(CALLER_ADDR1);
+
 #ifdef CONFIG_DEBUG_PREEMPT
 	/*
 	 * Underflow?

@@ -2351,6 +2394,19 @@ void __kprobes preempt_count_add(int val)
 		return;
 #endif
 	__preempt_count_add(val);
+
+#ifdef CONFIG_PREEMPT_STACK
+	if (val < SOFTIRQ_OFFSET) {
+		unsigned int idx = preempt_count() & PREEMPT_MASK;
+		if (idx < MAX_PREEMPT_STACK) {
+			struct preempt_stack *stack;
+			stack = this_cpu_ptr(preempt_stack_trace);
+			stack[idx].ip = ip;
+			stack[idx].parent_ip = parent_ip;
+		}
+	}
+#endif
+
 #ifdef CONFIG_DEBUG_PREEMPT
 	/*
 	 * Spinlock count overflowing soon?

@@ -2359,7 +2415,7 @@ void __kprobes preempt_count_add(int val)
 				PREEMPT_MASK - 10);
 #endif
 	if (preempt_count() == val)
-		trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
+		trace_preempt_off(ip, parent_ip);
 }
 EXPORT_SYMBOL(preempt_count_add);
 

@@ -2403,6 +2459,7 @@ static noinline void __schedule_bug(struct task_struct *prev)
 	if (irqs_disabled())
 		print_irqtrace_events(prev);
 	dump_stack();
+	print_preempt_trace();
 	add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
 }
 

@@ -6388,6 +6445,7 @@ void __might_sleep(const char *file, int line, int preempt_offset)
 	if (irqs_disabled())
 		print_irqtrace_events(current);
 	dump_stack();
+	print_preempt_trace();
 }
 EXPORT_SYMBOL(__might_sleep);
 #endif

index 8cf7819..b7999a4 100644
@@ -810,6 +810,22 @@ config DEBUG_PREEMPT
 	  if kernel code uses it in a preemption-unsafe way. Also, the kernel
 	  will detect preemption count underflows.
 
+config PREEMPT_STACK
+	bool "Keep a record of preempt disabled locations"
+	depends on DEBUG_KERNEL
+	depends on PREEMPT
+	select DEBUG_PREEMPT_COUNT
+	help
+	 When enabled, it keeps track of the last 25 locations that disabled
+	 preemption. This is useful to debug a "scheduling while atomic" error.
+	 Sometimes it is hard to find what left preemption disabled, and this
+	 utility can help.
+
+	 Saying yes to this will introduce an overhead, as all preempt disabled
+	 calls are being tracked.
+
+	 If unsure, say N.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config DEBUG_RT_MUTEXES

1.8.4.3

Hmm, I rewrote this prologue a few times, and even though patch 2 states this, I want to make it clear that the original feature, that this is based on, was written by Ingo Molnar. I had it clear in my original version of the prologue, but somehow it was deleted.

-- Steve

On Thu, Jan 16, 2014 at 12:18:24PM -0500, Steven Rostedt wrote:

Didn't see anything horrible in there, so fine with me.

Another thing I recently ran into was:

  local_irq_disable();
  preempt_disable();
  local_irq_enable();

  local_irq_disable();
  preempt_enable();
  local_irq_enable();

I'm not sure any of our debugging catches that, I meant to extend lockdep, but maybe it could also be hooked into something else.

Hmm, what's wrong with the above?

Note, the preemptirqsoff tracer will trace that as not being preemptible between that first local_irq_disable() to the local_irq_enable(). In fact, that's basically what the preemptirqsoff tracer was made for ;-)

-- Steve

Suppose a reschedule interrupt happens in the middle; we set NEED_RESCHED but won't act because preempt is disabled.

When we hit preempt_enable() we'll call into preempt_schedule() but fail because preemptible() finds irqs_disabled().

And in the end we do not schedule, but have NEED_RESCHED set.

FAIL.

Ah! Interesting. Actually, you don't even need the first part. That happens simply with:

	preempt_disable();

	<interrupt - set need_resched>

	local_irq_save();
	preempt_enable();
	local_irq_restore();

I think we can get lockdep to trigger on that. I have ideas to make preempt tracer do that, which shouldn't be too hard to convert to lockdep.

It looks that all we need to do is check for anytime we have interrupts becoming disabled with preemption disabled, and then enable preemption without first enabling interrupts. This should be able to be driven by state of the task or CPU even.

I can see if I can whip up a patch to do that.

-- Steve

fwiw - I have found this feature useful in the past, and would love to see it mainstream.

Thanks

John Kacur