[patch] Add a simple backtrace test module

[patch] Add a simple backtrace test module

[Arjan van de Ven]

Subject: Add a simple backtrace test module
From: Arjan van de Ven <arjan@linux.intel.com>

During the work on the x86 32 and 64 bit backtrace code I found it useful
to have a simple test module to test a process and irq context backtrace.
Since the existing backtrace code was buggy, I figure it might be useful
to have such a test module in the kernel so that maybe we can even
detect such bugs earlier..

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

---
 kernel/Makefile        |    1 +
 kernel/backtracetest.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
 lib/Kconfig.debug      |   12 ++++++++++++
 3 files changed, 60 insertions(+)

Index: linux-2.6.24-rc7/kernel/Makefile
===================================================================
--- linux-2.6.24-rc7.orig/kernel/Makefile
+++ linux-2.6.24-rc7/kernel/Makefile
@@ -52,6 +52,7 @@ obj-$(CONFIG_DETECT_SOFTLOCKUP) += softl
 obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
 obj-$(CONFIG_SECCOMP) += seccomp.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
+obj-$(BACKTRACE_SELF_TEST) += backtracetest.o
 obj-$(CONFIG_RELAY) += relay.o
 obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
 obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
Index: linux-2.6.24-rc7/kernel/backtracetest.c
===================================================================
--- /dev/null
+++ linux-2.6.24-rc7/kernel/backtracetest.c
@@ -0,0 +1,47 @@
+/*
+ * Simple stack backtrace regression test module
+ *
+ * (C) Copyright 2008 Intel Corporation
+ * Author: Arjan van de Ven <arjan@linux.intel.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2
+ * of the License.
+ */
+
+#include <linux/module.h>
+#include <linux/sched.h>
+
+static struct timer_list backtrace_timer;
+
+static void backtrace_test_timer(unsigned long data)
+{
+	printk("Testing a backtrace from irq context.\n");
+	printk("The following trace is a kernel self test and not a bug!\n");
+	dump_stack();
+}
+static int backtrace_regression_test(void)
+{
+	printk("====[ backtrace testing ]===========\n");
+	printk("Testing a backtrace from process context.\n");
+	printk("The following trace is a kernel self test and not a bug!\n");
+	dump_stack();
+
+	init_timer(&backtrace_timer);
+	backtrace_timer.function = backtrace_test_timer;
+	mod_timer(&backtrace_timer, jiffies + 10);
+
+	msleep(10);
+	printk("====[ end of backtrace testing ]====\n");
+	return 0;
+}
+
+static void exitf(void)
+{
+}
+
+module_init(backtrace_regression_test);
+module_exit(exitf);
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Arjan van de Ven <arjan@linux.intel.com>");
Index: linux-2.6.24-rc7/lib/Kconfig.debug
===================================================================
--- linux-2.6.24-rc7.orig/lib/Kconfig.debug
+++ linux-2.6.24-rc7/lib/Kconfig.debug
@@ -462,6 +462,18 @@ config RCU_TORTURE_TEST
 	  Say M if you want the RCU torture tests to build as a module.
 	  Say N if you are unsure.
 
+config BACKTRACE_SELF_TEST
+	tristate "Self test for the backtrace code"
+	depends on DEBUG_KERNEL
+	default n
+	help
+	  This option provides a kernel module that can be used to test
+	  the kernel stack backtrace code. This option is not useful
+	  for distributions or general kernels, but only for kernel
+	  developers working on architecture code.
+
+	  Say N if you are unsure.
+
 config LKDTM
 	tristate "Linux Kernel Dump Test Tool Module"
 	depends on DEBUG_KERNEL

-- 
If you want to reach me at my work email, use arjan@linux.intel.com
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

Re: [patch] Add a simple backtrace test module

[Christoph Hellwig]

On Wed, Jan 09, 2008 at 10:42:08PM -0800, Arjan van de Ven wrote:
> 
> Subject: Add a simple backtrace test module
> From: Arjan van de Ven <arjan@linux.intel.com>
> 
> During the work on the x86 32 and 64 bit backtrace code I found it useful
> to have a simple test module to test a process and irq context backtrace.
> Since the existing backtrace code was buggy, I figure it might be useful
> to have such a test module in the kernel so that maybe we can even
> detect such bugs earlier..

Care to add new testcases to a new tests/ subdirectory so we can have
them in one place?

Re: [patch] Add a simple backtrace test module

[Ingo Molnar]

* Christoph Hellwig <hch@infradead.org> wrote:

> On Wed, Jan 09, 2008 at 10:42:08PM -0800, Arjan van de Ven wrote:
> > 
> > Subject: Add a simple backtrace test module
> > From: Arjan van de Ven <arjan@linux.intel.com>
> > 
> > During the work on the x86 32 and 64 bit backtrace code I found it 
> > useful to have a simple test module to test a process and irq 
> > context backtrace. Since the existing backtrace code was buggy, I 
> > figure it might be useful to have such a test module in the kernel 
> > so that maybe we can even detect such bugs earlier..
> 
> Care to add new testcases to a new tests/ subdirectory so we can have 
> them in one place?

yeah, agreed, we'll clean this all up, and move the other testcode there 
too, ok? There's rcutorture, lock-selftests, rt-tester, 
kprobes-smoke-test and now backtracetest.

Arjan might as well want to use the opportunity and break new kernel 
namespace ground by creating the tests/ directory for the first time in 
Linux history and introduce tests/backtracetest.c :-)

	Ingo

Re: [patch] Add a simple backtrace test module

[Christoph Hellwig]

On Thu, Jan 10, 2008 at 10:04:29AM +0100, Ingo Molnar wrote:
> yeah, agreed, we'll clean this all up, and move the other testcode there 
> too, ok? There's rcutorture, lock-selftests, rt-tester, 
> kprobes-smoke-test and now backtracetest.
> 
> Arjan might as well want to use the opportunity and break new kernel 
> namespace ground by creating the tests/ directory for the first time in 
> Linux history and introduce tests/backtracetest.c :-)

Yeah, I'm happy to volunteer Arjan for creating test/ and moving the
existing one there :)

Re: [patch] Add a simple backtrace test module

[Ingo Molnar]

* Arjan van de Ven <arjan@infradead.org> wrote:

> During the work on the x86 32 and 64 bit backtrace code I found it 
> useful to have a simple test module to test a process and irq context 
> backtrace. Since the existing backtrace code was buggy, I figure it 
> might be useful to have such a test module in the kernel so that maybe 
> we can even detect such bugs earlier..

cool patch, applied!

a few suggestions:

a fundamental one: could you do a save_stack_trace() and check that both 
the process context and the irq context functions are present in that 
trace? If not then flag it as a regression and emit a real WARN_ON() 
warning.

i.e. use save_stack_trace() to do a "silent" test - instead of emitting 
backtraces during bootup. (which are marked via 'this is not a bug' but 
which are visually active nevertheless.)

the locking selftests use similar techniques to never emit real 
warnings, just a readable table of test results:

 | Locking API testsuite:
 ----------------------------------------------------------------------------
                                  | spin |wlock |rlock |mutex | wsem | rsem |
   --------------------------------------------------------------------------
                      A-A deadlock:  ok  |  ok  |  ok  |  ok  |  ok  |  ok  |
                  A-B-B-A deadlock:  ok  |  ok  |  ok  |  ok  |  ok  |  ok  |

internally, while the test is running, lockdep is triggered for real but 
the debug output and the backtraces are supressed.

and a few small details:

> +	printk("====[ backtrace testing ]===========\n");
> +	printk("Testing a backtrace from process context.\n");
> +	printk("The following trace is a kernel self test and not a bug!\n");

the printks need a KERN_ attribute.

> +	dump_stack();
> +
> +	init_timer(&backtrace_timer);
> +	backtrace_timer.function = backtrace_test_timer;
> +	mod_timer(&backtrace_timer, jiffies + 10);
> +
> +	msleep(10);
> +	printk("====[ end of backtrace testing ]====\n");

would be nice to have a testcase for the NMI watchdog and the softlockup 
watchdog as well: do they properly detect lockups on all CPUs?

> +static void exitf(void)

s/exitf/exit_backtrace_test

> +	  This option provides a kernel module that can be used to test
> +	  the kernel stack backtrace code. This option is not useful
> +	  for distributions or general kernels, but only for kernel
> +	  developers working on architecture code.

s/but only/only

	Ingo