From c39ea0b9dd24bf1bf2baa5cdbfa1905f3065347b Mon Sep 17 00:00:00 2001 From: Feng Tang Date: Tue, 14 May 2019 15:45:34 -0700 Subject: [PATCH] panic: avoid the extra noise dmesg 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: 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 overridden 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. Adding a flag "suppress_printk" which gets set in panic() to avoid those noisy messages, without changing current kernel behavior that both panic blinking and sysrq magic key can work as is, suggested by Petr Mladek. To verify this, make sure kernel is not configured to reboot on panic and in console # echo c > /proc/sysrq-trigger to see if console only prints out the panic call stack. Link: http://lkml.kernel.org/r/1551430186-24169-1-git-send-email-feng.tang@intel.com Signed-off-by: Feng Tang Suggested-by: Petr Mladek Reviewed-by: Petr Mladek Acked-by: Steven Rostedt (VMware) Acked-by: Sergey Senozhatsky Cc: Thomas Gleixner Cc: Kees Cook Cc: Borislav Petkov Cc: Andi Kleen Cc: Peter Zijlstra Cc: Greg Kroah-Hartman Cc: Jiri Slaby Cc: Sasha Levin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/tty/sysrq.c | 6 ++++++ include/linux/printk.h | 2 ++ kernel/panic.c | 3 +++ kernel/printk/printk.c | 10 ++++++++++ 4 files changed, 21 insertions(+) diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c index 59e82e6d776d..573b2055173c 100644 --- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -527,8 +527,12 @@ void __handle_sysrq(int key, bool check_mask) { struct sysrq_key_op *op_p; int orig_log_level; + int orig_suppress_printk; int i; + orig_suppress_printk = suppress_printk; + suppress_printk = 0; + rcu_sysrq_start(); rcu_read_lock(); /* @@ -574,6 +578,8 @@ void __handle_sysrq(int key, bool check_mask) } rcu_read_unlock(); rcu_sysrq_end(); + + suppress_printk = orig_suppress_printk; } void handle_sysrq(int key) diff --git a/include/linux/printk.h b/include/linux/printk.h index 84ea4d094af3..cefd374c47b1 100644 --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -82,6 +82,8 @@ static inline void console_verbose(void) extern char devkmsg_log_str[]; struct ctl_table; +extern int suppress_printk; + struct va_format { const char *fmt; va_list *va; diff --git a/kernel/panic.c b/kernel/panic.c index c1fcaad337b7..a6145050a8da 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -321,6 +321,9 @@ void panic(const char *fmt, ...) disabled_wait(); #endif pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf); + + /* Do not scroll important messages printed above */ + suppress_printk = 1; local_irq_enable(); for (i = 0; ; i += PANIC_TIMER_STEP) { touch_softlockup_watchdog(); diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 02ca827b8fac..17102fd4c136 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -86,6 +86,12 @@ static DEFINE_SEMAPHORE(console_sem); struct console *console_drivers; EXPORT_SYMBOL_GPL(console_drivers); +/* + * System may need to suppress printk message under certain + * circumstances, like after kernel panic happens. + */ +int __read_mostly suppress_printk; + #ifdef CONFIG_LOCKDEP static struct lockdep_map console_lock_dep_map = { .name = "console_lock" @@ -1943,6 +1949,10 @@ asmlinkage int vprintk_emit(int facility, int level, unsigned long flags; u64 curr_log_seq; + /* Suppress unimportant messages after panic happens */ + if (unlikely(suppress_printk)) + return 0; + if (level == LOGLEVEL_SCHED) { level = LOGLEVEL_DEFAULT; in_sched = true; -- 2.30.2