Re: [PATCH v4] panic: Avoid the extra noise dmesg

[Petr Mladek]

On Fri 2019-02-15 13:56:54, Feng Tang wrote:
> Hi all,
> 
> On Tue, Dec 11, 2018 at 09:32:30AM +0100, Petr Mladek wrote:
> > On Mon 2018-12-10 10:49:22, Kees Cook wrote:
> > > On Mon, Dec 10, 2018 at 10:17 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > > >
> > > > On Fri,  7 Dec 2018 17:51:19 +0800
> > > > Feng Tang <feng.tang@xxxxxxxxx> wrote:
> > > >
> > > > > 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:
> 
> So currently, there are 2 proposals:
> 1. this v4 patch of "panic_keep_irq_on" flag (default off to be same
>    as the current kernel behavior)
> 2. Petr's suggestion of adding a flag to suppress printk after enterring
>    late panic phase (blinking time), while keeping the sysrq printk
>    working.
> 
> Following is the draft patch based on Petr's suggestion:
> 
> Please review, thanks. I'm fine with both solutions.
> 
> - Feng
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 1f03078..8921fed 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -528,6 +528,11 @@ void __handle_sysrq(int key, bool check_mask)
>  	struct sysrq_key_op *op_p;
>  	int orig_log_level;
>  	int i;
> +	int old_val;
> +
> +	/* save the old panic printk flag */

The comment is not needed. It is obvious.

> +	old_val = panic_suppress_printk;

s/old_val/orig_panic_suppress_printk/ to follow
the naming of orig_log_level.

> +	panic_suppress_printk = 1;

We want to enable the messages in sysrq. This should be:

	panic_suppress_printk = 0;


>  	rcu_sysrq_start();
>  	rcu_read_lock();
> @@ -574,6 +579,8 @@ void __handle_sysrq(int key, bool check_mask)
>  	}
>  	rcu_read_unlock();
>  	rcu_sysrq_end();
> +
> +	panic_suppress_printk = old_val;
>  }
>  
>  void handle_sysrq(int key)
> diff --git a/kernel/panic.c b/kernel/panic.c
> index f121e6b..0cd3a1b 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -326,6 +328,7 @@ void panic(const char *fmt, ...)
>  	}
>  #endif
>  	pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf);

/* Do not scroll important messages with errors from blinking code. */

> +	panic_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 d3d1703..c27bbf5 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1987,6 +1987,9 @@ asmlinkage __visible int printk(const char *fmt, ...)
>  	va_list args;
>  	int r;

	/* Suppress messages from panic blinking code. */

> +	if (unlikely(panic_suppress_printk))
> +		return 0;

This should go to vprintk_emit() so that it works for all
printk() interfaces.

> +
>  	va_start(args, fmt);
>  	r = vprintk_func(fmt, args);
>  	va_end(args);

Best Regards,
Petr