linux-kernel - Re: [PATCH v3 2/2] dynamic debug: allow printing to trace event

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Message-ID: <9bfb4c4bd1415a8ce527a913730672508a8e8330.camel@perches.com>
Date:   Wed, 26 Aug 2020 12:53:57 -0700
From:   Joe Perches <joe@...ches.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        jbaron@...mai.com, mingo@...hat.com,
        Nicolas Boichat <drinkcat@...omium.org>, kernel@...s.com,
        corbet@....net, pmladek@...e.com, sergey.senozhatsky@...il.com,
        john.ogness@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] dynamic debug: allow printing to trace event

On Wed, 2020-08-26 at 15:32 -0400, Steven Rostedt wrote:
> On Tue, 25 Aug 2020 08:53:25 -0700
> Joe Perches <joe@...ches.com> wrote:
> 
> > > The print buffer is statically allocated and managed using code borrowed
> > > from __ftrace_trace_stack() and is limited to 256 bytes (four of these
> > > are allocated per CPU to handle the various contexts); anything larger
> > > will be truncated.  
> > 
> > There is an effort to avoid using trace_printk and the like
> > so perhaps this feature should have the same compile time
> > guard.
> 
> No, this is fine for being in a production kernel. Basically, these are
> simply debug printk()s that can also be put into the trace buffer. The
> key difference to trace_printk() is that they are an event that needs
> to be enabled to write into the buffer.

It just seems like a backdoor way to convert various pr_debug functions
(dev_dbg/netdev_dbg, et al) into tracing.

What's the real value?  Timing data?  Something else?