linux-kernel - Re: [PATCH printk v1 03/18] printk: Consolidate console deferred printing

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Date:   Wed, 8 Mar 2023 14:15:48 +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
Subject: Re: [PATCH printk v1 03/18] printk: Consolidate console deferred
 printing

On Thu 2023-03-02 21:02:03, John Ogness wrote:
> Printig to consoles can be deferred for several reasons:
> 
> - explicitly with printk_deferred()
> - printk() in NMI context
> - recursive printk() calls
> 
> The current implementation is not consistent. For printk_deferred(),
> irq work is scheduled twice. For NMI und recursive, panic CPU
> suppression and caller delays are not properly enforced.
> 
> Correct these inconsistencies by consolidating the deferred printing
> code so that vprintk_deferred() is the toplevel function for
> deferred printing and vprintk_emit() will perform whichever irq_work
> queueing is appropriate.
> 
> Also add kerneldoc for wake_up_klogd() and defer_console_output() to
> clarify their differences and appropriate usage.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2321,7 +2321,10 @@ asmlinkage int vprintk_emit(int facility, int level,
>  		preempt_enable();
>  	}
>  
> -	wake_up_klogd();
> +	if (in_sched)
> +		defer_console_output();
> +	else
> +		wake_up_klogd();

Nit: I would add an empty line here. Or I would move this up into the
     previous if (in_sched()) condition.

>  	return printed_len;
>  }
>  EXPORT_SYMBOL(vprintk_emit);
> @@ -3811,11 +3814,30 @@ static void __wake_up_klogd(int val)
>  	preempt_enable();
>  }
>  
> +/**
> + * wake_up_klogd - Wake kernel logging daemon
> + *
> + * Use this function when new records have been added to the ringbuffer
> + * and the console printing for those records is handled elsewhere. In

"elsewhere" is ambiguous. I would write:

"and the console printing for those records maybe handled in this context".

> + * this case only the logging daemon needs to be woken.
> + *
> + * Context: Any context.
> + */
>  void wake_up_klogd(void)
>  {
>  	__wake_up_klogd(PRINTK_PENDING_WAKEUP);
>  }
>  

Anyway, I like this change.

Best Regards,
Petr