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

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Date:   Wed, 22 Jul 2020 11:25:25 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        jbaron@...mai.com, mingo@...hat.com, kernel@...s.com,
        corbet@....net, 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, 22 Jul 2020 15:58:22 +0206
John Ogness <john.ogness@...utronix.de> wrote:

> >> +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);

It must be an event, one that can be enabled or disabled separately
from trace_printk().

If you are asking if this could be something like trace_printk(), which
ftrace_vprintk() is. The reason for that nasty banner when people use
trace_printk() is to keep developers from using it as their personal
debugging tool in production.

A trace_printk() can not be discretely disabled. It's either totally
on, or totally off. And since it is used for debugging, if there's
trace_printk()s all over the kernel, you will have to deal with the
noise of everyone else's trace_printk(), making trace_printk() useless.

Thus, NAK on using ftrace_vprintk() here.

-- Steve