linux-kernel - flush was: Re: [PATCH printk v2 22/26] printk: nbcon: Implement emergency sections

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Date: Fri, 1 Mar 2024 16:49:52 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: flush was: Re: [PATCH printk v2 22/26] printk: nbcon: Implement
 emergency sections

On Sun 2024-02-18 20:03:22, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> In emergency situations (something has gone wrong but the
> system continues to operate), usually important information
> (such as a backtrace) is generated via printk(). Each
> individual printk record has little meaning. It is the
> collection of printk messages that is most often needed by
> developers and users.
> 
> In order to help ensure that the collection of printk messages
> in an emergency situation are all stored to the ringbuffer as
> quickly as possible, disable console output for that CPU while
> it is in the emergency situation. When exiting the emergency
> situation, trigger the consoles to be flushed.
> 
> Add per-CPU emergency nesting tracking because an emergency
> can arise while in an emergency situation.
> 
> Add functions to mark the beginning and end of emergency
> sections where the urgent messages are generated.
> 
> Do not print if the current CPU is in an emergency state.
> 
> Trigger console flushing when exiting all emergency nesting.
> 
> Note that the emergency state is not system-wide. While one CPU
> is in an emergency state, another CPU may continue to print
> console messages.
> 
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1105,6 +1134,58 @@ void nbcon_atomic_flush_unsafe(void)
>  	__nbcon_atomic_flush_all(prb_next_reserve_seq(prb), true);
>  }
>  
> +/**
> + * nbcon_cpu_emergency_enter - Enter an emergency section where printk()
> + *	messages for that CPU are only stored
> + *
> + * Upon exiting the emergency section, all stored messages are flushed.
> + *
> + * Context:	Any context. Disables preemption.
> + *
> + * When within an emergency section, no printing occurs on that CPU. This
> + * is to allow all emergency messages to be dumped into the ringbuffer before
> + * flushing the ringbuffer. The actual printing occurs when exiting the
> + * outermost emergency section.
> + */
> +void nbcon_cpu_emergency_enter(void)
> +{
> +	unsigned int *cpu_emergency_nesting;
> +
> +	preempt_disable();
> +
> +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> +	(*cpu_emergency_nesting)++;
> +}
> +
> +/**
> + * nbcon_cpu_emergency_exit - Exit an emergency section and flush the
> + *	stored messages
> + *
> + * Flushing only occurs when exiting all nesting for the CPU.
> + *
> + * Context:	Any context. Enables preemption.
> + */
> +void nbcon_cpu_emergency_exit(void)
> +{
> +	unsigned int *cpu_emergency_nesting;
> +	bool do_trigger_flush = false;
> +
> +	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
> +
> +	WARN_ON_ONCE(*cpu_emergency_nesting == 0);
> +
> +	if (*cpu_emergency_nesting == 1)
> +		do_trigger_flush = true;
> +
> +	/* Undo the nesting count of nbcon_cpu_emergency_enter(). */
> +	(*cpu_emergency_nesting)--;
> +
> +	preempt_enable();
> +
> +	if (do_trigger_flush)
> +		printk_trigger_flush();

Just an idea. printk_trigger_flush() calls defer_console_output().
It always moves the flushing into IRQ context.

It might make sense to add a flush() function which would try
to flush the messages directly from this context and
queue the IRQ work only when it fails. Something like:

void printk_emergency_flush()
{
	/* nbcon consoles could be flushed in any context. */
	if (have_nbcon_console)
		nbcon_flush_all();

	if (have_legacy_console) {
		if (console_trylock())
			console_unlock();
		else
			defer_console_output();
	}
}

But wait, nbcon_flush_all() might have troubles to get the per-console
lock because it would be called with NBCON_PRIO_NORMAL.


Wait, wait, wait.

defer_console_output() schedules wake_up_klogd_work_func(). It flushes
only legacy consoles. It means that even emergency messages would
need to wait for the printk kthread.

By other words, it seems that the emergency context does not have any
effect for nbcon consoles.

It brings a question if any code would need to acquire the emergency
context at all.

I quess that this does not work as expected.

Or do I miss anything, please?

Best Regards,
Petr