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Subject[PATCH v4] panic: Avoid the extra noise dmesg
Date
When kernel panic happens, it will first print the panic call stack,
then the ending msg like:

[ 35.743249] ---[ end Kernel panic - not syncing: Fatal exception
[ 35.749975] ------------[ cut here ]------------

The above message are very useful for debugging.

But if system is configured to not reboot on panic, say the "panic_timeout"
parameter equals 0, it will likely print out many noisy message like
WARN() call stack for each and every CPU except the panic one, messages
like below:

WARNING: CPU: 1 PID: 280 at kernel/sched/core.c:1198 set_task_cpu+0x183/0x190
Call Trace:
<IRQ>
try_to_wake_up
default_wake_function
autoremove_wake_function
__wake_up_common
__wake_up_common_lock
__wake_up
wake_up_klogd_work_func
irq_work_run_list
irq_work_tick
update_process_times
tick_sched_timer
__hrtimer_run_queues
hrtimer_interrupt
smp_apic_timer_interrupt
apic_timer_interrupt

For people working in console mode, the screen will first show the panic
call stack, but immediately overridded by these noisy extra messages, which
makes debugging much more difficult, as the original context gets lost on
screen.

Also these noisy messages will confuse some users, as I have seen many bug
reporters posted the noisy message into bugzilla, instead of the real panic
call stack and context.

Keeping the interrupt disabled will avoid the noisy message.

When code runs to this point, it means user has chosed to not reboot, or
do any special handling by using the panic notifier method, the only reason
to enable the interrupt may be sysrq migic key and panic_blink function
(though may not work even with irq enabled).

So make the irq disabled by default and add a cmdline parameter
"panic_keep_irq_on" to turn it on when needed.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: stable@kernel.org
---
Changelog:

v4:
- make the local_irq_enable conditional and default off
to cover possible use of interrupt/scheduling, as
mentioned by Sergey and Petr
v3:
- Make the change log clearer as suggested by Andrew Morton
v2:
- Move the solution from hacking arch/scheduler code back
to panic.c

Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
kernel/panic.c | 9 ++++++++-
2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ad8dc61..45621e9 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3093,6 +3093,12 @@
panic_on_warn panic() instead of WARN(). Useful to cause kdump
on a WARN().

+ panic_keep_irq_on
+ To keep the panic call stack info clean and suppress
+ noisy kernel message, the interrupt on panic cpu is
+ disabled by default. Add this to cmdline to enable
+ interrupt for panic cpu.
+
crash_kexec_post_notifiers
Run kdump after running panic-notifiers and dumping
kmsg. This only for the users who doubt kdump always
diff --git a/kernel/panic.c b/kernel/panic.c
index 85c4d14..04ace9b 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -40,6 +40,7 @@ static int pause_on_oops;
static int pause_on_oops_flag;
static DEFINE_SPINLOCK(pause_on_oops_lock);
bool crash_kexec_post_notifiers;
+bool panic_keep_irq_on;
int panic_on_warn __read_mostly;

int panic_timeout = CONFIG_PANIC_TIMEOUT;
@@ -322,7 +323,12 @@ void panic(const char *fmt, ...)
}
#endif
pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf);
- local_irq_enable();
+
+ if (panic_keep_irq_on)
+ local_irq_enable();
+ else
+ pr_emerg("Please add panic_keep_irq_on to cmdline if you want blink/sysrq to work.");
+
for (i = 0; ; i += PANIC_TIMER_STEP) {
touch_softlockup_watchdog();
if (i >= i_next) {
@@ -684,6 +690,7 @@ core_param(panic, panic_timeout, int, 0644);
core_param(panic_print, panic_print, ulong, 0644);
core_param(pause_on_oops, pause_on_oops, int, 0644);
core_param(panic_on_warn, panic_on_warn, int, 0644);
+core_param(panic_keep_irq_on, panic_keep_irq_on, bool, 0644);
core_param(crash_kexec_post_notifiers, crash_kexec_post_notifiers, bool, 0644);

static int __init oops_setup(char *s)
--
2.7.4
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