Re: [PATCH v8 09/21] printk: Optimise printk_delay() to avoid walking consoles under SRCU

[Petr Mladek <pmladek@suse.com>]

On Fri 2025-11-28 03:43:42, Chris Down wrote:
> As part of the per-console loglevel implementation, printk_delay()
> introduces a call to suppress_message_printing_everywhere() within
> printk_delay(). This check acquires the console SRCU lock and iterates
> through all registered consoles to determine if the message implies a
> delay based on per-console loglevels.
> 
> Since printk_delay() is called in the hotpath of vprintk_emit(), this
> introduces measurable overhead (locking and pointer chasing) for every
> printk message, even on systems where no delay is configured.
> 
> Optimise this by short-circuiting the function. If neither a boot delay
> nor a runtime printk_delay_msec is configured, return immediately. This
> avoids the expensive console list iteration in the common case where
> delays are disabled.
> 
> Signed-off-by: Chris Down <chris@chrisdown.name>
> ---
>  kernel/printk/printk.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 0d7f3dac8177..13e17d892ec9 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2233,20 +2233,31 @@ int printk_delay_msec __read_mostly;
>  
>  static inline void printk_delay(int level)
>  {
> +	bool boot_delay_active = false;
> +	int m;
> +
> +#ifdef CONFIG_BOOT_PRINTK_DELAY
> +	boot_delay_active = boot_delay && system_state < SYSTEM_RUNNING;
> +#endif

The same code is used also in boot_delay_msec(). It can be removed
there. But still, it would be better to create a helper function
and avoid the #ifdef in the middle of the function.

> +	if (!boot_delay_active && !READ_ONCE(printk_delay_msec))

There is another READ_ONCE(printk_delay_msec) below. It would
be better to read the value at the beginning of the function
and use it everywhere.

> +		return;
> +
>  	/* If the message is forced (e.g. panic), we must delay */
>  	if (!is_printk_force_console() &&
>  	    suppress_message_printing_everywhere(level))
>  		return;
>  
> -	boot_delay_msec();
> +	if (boot_delay_active)
> +		boot_delay_msec();
>  
> -	if (unlikely(printk_delay_msec)) {
> -		int m = printk_delay_msec;
> +	m = READ_ONCE(printk_delay_msec);
> +	if (!m)
> +		return;
>  
> -		while (m--) {
> -			mdelay(1);
> -			touch_nmi_watchdog();
> -		}
> +	while (m--) {
> +		mdelay(1);
> +		touch_nmi_watchdog();
>  	}
>  }

I played with the code. I am going to send my variant as a reply to
the 1st patch where I proposed 2 alternative solutions, see
https://lore.kernel.org/all/aThQ5mZNIijc1G94@pathway

Best Regards,
Petr