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Message-Id: <201109271909.p8RJ9Fxk010028@int-mx02.intmail.prod.int.phx2.redhat.com>
Date: Tue, 27 Sep 2011 15:09:15 -0400
From: Jason Baron <jbaron@...hat.com>
To: arjan@...ux.intel.com, mingo@...e.hu
Cc: peterz@...radead.org, rostedt@...dmis.org,
linux-kernel@...r.kernel.org
Subject: [PATCH] introduce CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC
Currently, CONFIG_DEBUG_NOTIFIERS, is only a build-time option, since
it can be costly to have to check all modules and kernel text for valid
notifiers on each notifier callback. We can turn it into a run-time check
that has no over-head in the disabled case by using jump labels (leaving
a single no-op in the disabled code path). In this way, we can enable
it at run-time, via 0/1 in: <debugfs>/notifiers/debug_enable or on the kernel
command line via: "debug_notifiers".
In general, I'd like to turn a number of the build-time debug options into
run-time options, where possible. For Red Hat, which ships a separate -debug
kernel, having debug options in our production kernel is desireable. Both from
a customer perspective (having less kernels to deal with), and from a testing
point of view.
I've also tested this change using a module load/unload loop, which hits
the notifier chain. Below is the combinations for
CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC enabled/disabled and jump label
enabled/disabled, when notifier debug is off.
CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC enabled/jump label enabled:
# taskset -c 0 perf stat --repeat 100 -e instructions,cycles,branches /tmp/load-unload
Performance counter stats for '/tmp/load-unload' (100 runs):
2,420,961,130 instructions # 0.587 IPC ( +- 0.018% )
4,122,380,182 cycles ( +- 0.094% )
463,442,670 branches ( +- 0.020% )
1.622449703 seconds time elapsed ( +- 0.099% )
CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC/jump label disabled:
taskset -c 0 perf stat --repeat 100 -e instructions,cycles,branches /tmp/load-unload
Performance counter stats for '/tmp/load-unload' (100 runs):
2,425,139,714 instructions # 0.580 IPC ( +- 0.018% )
4,178,078,387 cycles ( +- 0.114% )
465,802,135 branches ( +- 0.020% )
1.642928148 seconds time elapsed ( +- 0.105% )
CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC disabled/jump label enabled:
# taskset -c 0 perf stat --repeat 100 -e instructions,cycles,branches /tmp/load-unload
Performance counter stats for '/tmp/load-unload' (100 runs):
2,421,957,621 instructions # 0.585 IPC ( +- 0.019% )
4,142,800,048 cycles ( +- 0.137% )
463,568,665 branches ( +- 0.021% )
1.628438721 seconds time elapsed ( +- 0.130% )
CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC disabled/jump label disabled:
# taskset -c 0 perf stat --repeat 100 -e instructions,cycles,branches /tmp/load-unload
Performance counter stats for '/tmp/load-unload' (100 runs):
2,429,450,979 instructions # 0.578 IPC ( +- 0.022% )
4,202,429,137 cycles ( +- 0.126% )
466,506,471 branches ( +- 0.022% )
1.651379469 seconds time elapsed ( +- 0.119% )
Signed-off-by: Jason Baron <jbaron@...hat.com>
---
kernel/notifier.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
lib/Kconfig.debug | 13 ++++++++
2 files changed, 92 insertions(+), 1 deletions(-)
diff --git a/kernel/notifier.c b/kernel/notifier.c
index 8d7b435..44ec29d 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -5,6 +5,9 @@
#include <linux/rcupdate.h>
#include <linux/vmalloc.h>
#include <linux/reboot.h>
+#include <linux/jump_label.h>
+#include <linux/debugfs.h>
+#include <linux/uaccess.h>
/*
* Notifier list for kernel code which wants to be called
@@ -59,6 +62,79 @@ static int notifier_chain_unregister(struct notifier_block **nl,
return -ENOENT;
}
+#ifdef CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC
+static struct jump_label_key debug_notifiers_key;
+static int early_debug_enable;
+
+static ssize_t read_notifier_debug(struct file *file, char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ char buf[3];
+
+ if (jump_label_enabled(&debug_notifiers_key))
+ buf[0] = '1';
+ else
+ buf[0] = '0';
+ buf[1] = '\n';
+ buf[2] = 0x00;
+
+ return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
+}
+
+static ssize_t write_notifier_debug(struct file *file,
+ const char __user *user_buf, size_t count, loff_t *ppos)
+{
+ char buf[32];
+ int buf_size;
+
+ buf_size = min(count, (sizeof(buf)-1));
+ if (copy_from_user(buf, user_buf, buf_size))
+ return -EFAULT;
+ if ((buf[0] == '1') && !jump_label_enabled(&debug_notifiers_key))
+ jump_label_inc(&debug_notifiers_key);
+ else if ((buf[0] == '0') && jump_label_enabled(&debug_notifiers_key))
+ jump_label_dec(&debug_notifiers_key);
+
+ return count;
+}
+
+static const struct file_operations fops_debug_notifiers = {
+ .read = read_notifier_debug,
+ .write = write_notifier_debug,
+ .llseek = default_llseek,
+};
+
+static int __init debug_notifiers_setup(char *str)
+{
+ early_debug_enable = 1;
+ return 0;
+}
+early_param("debug_notifiers", debug_notifiers_setup);
+
+static int __init debug_notifiers_setup_debugfs(void)
+{
+ struct dentry *dir, *file;
+
+ /* we can't enable jump labels until after early_initcall() */
+ if (early_debug_enable)
+ jump_label_inc(&debug_notifiers_key);
+
+ dir = debugfs_create_dir("notifiers", NULL);
+ if (!dir)
+ return -ENOMEM;
+ file = debugfs_create_file("debug_enable", 0644, dir, NULL,
+ &fops_debug_notifiers);
+ if (!file) {
+ debugfs_remove(dir);
+ return -ENOMEM;
+ }
+ return 0;
+}
+/* must be after debugfs is setup */
+postcore_initcall(debug_notifiers_setup_debugfs);
+
+#endif
+
/**
* notifier_call_chain - Informs the registered notifiers about an event.
* @nl: Pointer to head of the blocking notifier chain
@@ -82,8 +158,10 @@ static int __kprobes notifier_call_chain(struct notifier_block **nl,
while (nb && nr_to_call) {
next_nb = rcu_dereference_raw(nb->next);
-
#ifdef CONFIG_DEBUG_NOTIFIERS
+#ifdef CONFIG_DEBUG_NOTIFIERS_MAKE_DYNAMIC
+ if (static_branch(&debug_notifiers_key))
+#endif
if (unlikely(!func_ptr_is_kernel_text(nb->notifier_call))) {
WARN(1, "Invalid notifier called!");
nb = next_nb;
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c0cb9c4..4d57783 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -818,6 +818,19 @@ config DEBUG_NOTIFIERS
This is a relatively cheap check but if you care about maximum
performance, say N.
+config DEBUG_NOTIFIERS_MAKE_DYNAMIC
+ bool "Debug notifier call chains"
+ depends on DEBUG_KERNEL
+ depends on DEBUG_NOTIFIERS
+ depends on DEBUG_FS
+ help
+ Enable this to turn on sanity checking for notifier call chains.
+ This is most useful for kernel developers to make sure that
+ modules properly unregister themselves from notifier chains.
+ This is a relatively cheap check but if you care about maximum
+ performance, say N.
+
+
config DEBUG_CREDENTIALS
bool "Debug credential management"
depends on DEBUG_KERNEL
--
1.7.5.4
--
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