linux-kernel - Re: [PATCH] dynamic debug: allow printing to trace event

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Date:   Wed, 22 Jul 2020 16:49:52 +0200
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     John Ogness <john.ogness@...utronix.de>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        "jbaron@...mai.com" <jbaron@...mai.com>,
        "mingo@...hat.com" <mingo@...hat.com>, kernel <kernel@...s.com>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] dynamic debug: allow printing to trace event

On Wed, Jul 22, 2020 at 03:52:22PM +0200, John Ogness wrote:
> On 2020-07-21, Steven Rostedt <rostedt@...dmis.org> wrote:
> >> diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
> >> index 321437bbf87d..9f6d8867af7c 100644
> >> --- a/lib/dynamic_debug.c
> >> +++ b/lib/dynamic_debug.c
> [..]
> >> +static void dynamic_printk(unsigned int flags, const char *fmt, ...)
> >> +{
> >> +	if (flags & _DPRINTK_FLAGS_TRACE) {
> >> +		va_list args;
> >> +
> >> +		va_start(args, fmt);
> >> +		/*
> >> +		 * All callers include the KERN_DEBUG prefix to keep the
> >> +		 * vprintk case simple; strip it out for tracing.
> >> +		 */
> >> +		dynamic_trace(fmt + strlen(KERN_DEBUG), args);
> 
> Do we really need a separate tracing event for this? Why not just:
> 
>                 ftrace_vprintk(fmt + strlen(KERN_DEBUG), args);

Thanks, I tried that out now and it seems to work, but it results in the
trace_printk() splat (even if the feature is not used), though:

 **********************************************************
 **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
 **                                                      **
 ** trace_printk() being used. Allocating extra memory.  **
 **                                                      **
 ** This means that this is a DEBUG kernel and it is     **
 ** unsafe for production use.                           **
 **                                                      **
 ** If you see this message and you are not debugging    **
 ** the kernel, report this immediately to your vendor!  **
 **                                                      **
 **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
 **********************************************************