Re: [PATCH -printk] printk, tracing: fix console tracepoint

[Steven Rostedt <rostedt@goodmis.org>]

On Tue,  3 May 2022 09:38:44 +0200
Marco Elver <elver@google.com> wrote:

> The original intent of the 'console' tracepoint per 95100358491a
> ("printk/tracing: Add console output tracing") had been to "[...] record
> any printk messages into the trace, regardless of the current console
> loglevel. This can help correlate (existing) printk debugging with other
> tracing."
> 
> Petr points out [1] that calling trace_console_rcuidle() in
> call_console_driver() had been the wrong thing for a while, because
> "printk() always used console_trylock() and the message was flushed to
> the console only when the trylock succeeded. And it was always deferred
> in NMI or when printed via printk_deferred()."
> 
> With 09c5ba0aa2fc ("printk: add kthread console printers"), things only
> got worse, and calls to call_console_driver() no longer happen with
> typical printk() calls but always appear deferred [2].
> 
> As such, the tracepoint can no longer serve its purpose to clearly
> correlate printk() calls and other tracing, as well as breaks usecases
> that expect every printk() call to result in a callback of the console
> tracepoint. Notably, the KFENCE and KCSAN test suites, which want to
> capture console output and assume a printk() immediately gives us a
> callback to the console tracepoint.
> 
> Fix the console tracepoint by moving it into printk_sprint() [3].
> 
> One notable difference is that by moving tracing into printk_sprint(),
> the 'text' will no longer include the "header" (loglevel and timestamp),
> but only the raw message. Arguably this is less of a problem now that
> the console tracepoint happens on the printk() call and isn't delayed.
> 

I'm OK with this change, but I don't know everyone that uses the trace
printk feature. I am worried that this could cause regressions in
people's workloads.

I'd like to hear more feedback from others, but for me:

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve


> Link: https://lore.kernel.org/all/Ym+WqKStCg%2FEHfh3@alley/ [1]
> Link: https://lore.kernel.org/all/CA+G9fYu2kS0wR4WqMRsj2rePKV9XLgOU1PiXnMvpT+Z=c2ucHA@mail.gmail.com/ [2]
> Link: https://lore.kernel.org/all/87fslup9dx.fsf@jogness.linutronix.de/ [3]
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Signed-off-by: Marco Elver <elver@google.com>
> Cc: John Ogness <john.ogness@linutronix.de>
> Cc: Petr Mladek <pmladek@suse.com>
> ---
>  kernel/printk/printk.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index f66d6e72a642..a3e1035929b0 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2064,8 +2064,6 @@ static void call_console_driver(struct console *con, const char *text, size_t le
>  {
>  	size_t dropped_len;
>  
> -	trace_console_rcuidle(text, len);
> -
>  	if (con->dropped && dropped_text) {
>  		dropped_len = snprintf(dropped_text, DROPPED_TEXT_MAX,
>  				       "** %lu printk messages dropped **\n",
> @@ -2240,6 +2238,8 @@ static u16 printk_sprint(char *text, u16 size, int facility,
>  		}
>  	}
>  
> +	trace_console_rcuidle(text, text_len);
> +
>  	return text_len;
>  }
>