Re: [PATCH 4/4] printk: Add config option for disabling printk offloading

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 19 Aug 2015 17:38:31 +0200 Jan Kara <jack@suse.com> wrote:

> From: Jan Kara <jack@suse.cz>
> 
> Necessity for offloading of printing was observed only for large
> systems. So add a config option (disabled by default)

The Kconfig has "default y"?

> which removes most
> of the overhead added by this functionality.
> 
> ...
>
> +#ifdef CONFIG_PRINTK_OFFLOAD
>  /*
>   * Returns true iff there is other cpu waiting to take over printing. This
>   * function also takes are of setting PRINTK_HANDOVER_B if we want to hand over
> @@ -2278,6 +2283,14 @@ static bool cpu_stop_printing(int printed_chars)
>  
>  	return false;
>  }
> +#else
> +
> +static bool cpu_stop_printing(int printed_chars, bool *woken)
> +{
> +	return false;
> +}
> +
> +#endif
>  
>  /**
>   * console_unlock - unlock the console system
> @@ -2316,7 +2329,9 @@ void console_unlock(void)
>  	/* flush buffered message fragment immediately to console */
>  	console_cont_flush(text, sizeof(text));
>  again:
> +#ifdef CONFIG_PRINTK_OFFLOAD
>  	spin_lock(&print_lock);
> +#endif

You could nuke a couple of ugly ifdefs by adding
spin_[un]lock_printk_lock() wrappers into that ifdef/else/endif block
which holds cpu_stop_printing().

>  	for (;;) {
>  		struct printk_log *msg;
>  		size_t ext_len = 0;
> @@ -2399,12 +2414,14 @@ skip:
>  
>  	console_locked = 0;
>  	up_console_sem();
> +#ifdef CONFIG_PRINTK_OFFLOAD
>  	/*
>  	 * Release print_lock after console_sem so that printing_task()
>  	 * succeeds in getting console_sem (unless someone else takes it and
>  	 * then he'll be responsible for printing).
>           */
>  	spin_unlock(&print_lock);
> +#endif
>  
>  	/*
>  	 * Subtlety: We have interrupts disabled iff hand_over == false (to
> @@ -2770,6 +2787,7 @@ int unregister_console(struct console *console)
>  }
>  EXPORT_SYMBOL(unregister_console);
>  
> +#ifdef CONFIG_PRINTK_OFFLOAD
>  /* Kthread which takes over printing from a CPU which asks for help */
>  static int printing_task(void *arg)
>  {
> @@ -2838,6 +2856,7 @@ static int offload_chars_set(const char *val, const struct kernel_param *kp)
>  	mutex_unlock(&printk_kthread_mutex);
>  	return 0;
>  }
> +#endif	/* CONFIG_PRINTK_OFFLOAD */
>  
>  static int __init printk_late_init(void)
>  {
> @@ -2850,9 +2869,11 @@ static int __init printk_late_init(void)
>  	}
>  	hotcpu_notifier(console_cpu_notify, 0);
>  
> +#ifdef CONFIG_PRINTK_OFFLOAD
>  	mutex_lock(&printk_kthread_mutex);
>  	printk_start_offload_kthreads();
>  	mutex_unlock(&printk_kthread_mutex);
> +#endif

Possibly ditto here.  Maybe move the mutex_lock() into
printk_start_offload_kthreads() too?