Re: [PATCH 2/2] printk: Make the console flush configurable in hotplug path

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Sep 23, 2020 at 05:08:32PM -0700, Prasad Sodagudi wrote:
> From: Mohammed Khajapasha <mkhaja@codeaurora.org>
> 
> The thread which initiates the hot plug can get scheduled
> out, while trying to acquire the console lock,
> thus increasing the hot plug latency. This option
> allows to selectively disable the console flush and
> in turn reduce the hot plug latency.
> 
> Signed-off-by: Mohammed Khajapasha <mkhaja@codeaurora.org>
> Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>
> ---
>  init/Kconfig           | 10 ++++++++++
>  kernel/printk/printk.c | 10 ++++++++--
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index d6a0b31..9ce39ba 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -699,6 +699,16 @@ config LOG_BUF_SHIFT
>  		     13 =>  8 KB
>  		     12 =>  4 KB
>  
> +config CONSOLE_FLUSH_ON_HOTPLUG
> +	bool "Enable console flush configurable in hot plug code path"
> +	depends on HOTPLUG_CPU
> +	def_bool n

n is the default, no need to list it.

> +	help
> +	In cpu hot plug path console lock acquire and release causes the
> +	console to flush. If console lock is not free hot plug latency
> +	increases. So make console flush configurable in hot plug path
> +	and default disabled to help in cpu hot plug latencies.

Why would you not want this option?

Why isn't this just a bugfix?

> +
>  config LOG_CPU_MAX_BUF_SHIFT
>  	int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)"
>  	depends on SMP
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 9b75f6b..f02d3ef 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2283,6 +2283,8 @@ void resume_console(void)
>  	console_unlock();
>  }
>  
> +#ifdef CONFIG_CONSOLE_FLUSH_ON_HOTPLUG
> +
>  /**
>   * console_cpu_notify - print deferred console messages after CPU hotplug
>   * @cpu: unused
> @@ -2302,6 +2304,8 @@ static int console_cpu_notify(unsigned int cpu)
>  	return 0;
>  }
>  
> +#endif
> +
>  /**
>   * console_lock - lock the console system for exclusive use.
>   *
> @@ -2974,7 +2978,7 @@ void __init console_init(void)
>  static int __init printk_late_init(void)
>  {
>  	struct console *con;
> -	int ret;
> +	int ret = 0;
>  
>  	for_each_console(con) {
>  		if (!(con->flags & CON_BOOT))
> @@ -2996,13 +3000,15 @@ static int __init printk_late_init(void)
>  			unregister_console(con);
>  		}
>  	}
> +#ifdef CONFIG_CONSOLE_FLUSH_ON_HOTPLUG

#ifdef in .c code is a mess to maintain.

>  	ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
>  					console_cpu_notify);
>  	WARN_ON(ret < 0);
>  	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "printk:online",
>  					console_cpu_notify, NULL);
>  	WARN_ON(ret < 0);
> -	return 0;
> +#endif

What happens if we don't make these calls entirely?  Why not just remove
them as who wants extra latency for their system?

thanks,

greg k-h