Re: [PATCH 1/1] tracing: Fix event_trace_printk loss on printk_format.

From: Mars Cheng
Date: Tue Feb 12 2019 - 23:54:26 EST

Hi Steve

On Tue, 2019-02-12 at 20:41 -0500, Steven Rostedt wrote:
> On Tue, 12 Feb 2019 19:41:57 +0800
> Mars Cheng <mars.cheng@xxxxxxxxxxxx> wrote:
> > From: Andress Kuo <andress.kuo@xxxxxxxxxxxx>
> >
> > If fmt on event_trace_printk is not a constant, It means that something not
> > guaranteed, so the compiler optimizes the fmt out, and then the
> > __trace_printk_fmt section is not filled. if __trace_printk_fmt is not
> > filled, the kernel will not allocate the special buffers needed for the
> > event_trace_printk() and then not shown in the file output
> > sys/kernel/debug/tracing/print_formats.
> >
> > Adding a "__used" to the variable in the __trace_printk_fmt section on
> > event_trace_printk() will keep it around, even though it is set to NULL.
> > This will keep the string from being printed in the
> > sys/kernel/debug/tracing/printk_formats section.
> >
> > We can also refer to commit 3debb0a9ddb1 ("tracing: Fix trace_printk()
> > to print when not using bprintk()")that it had similar issue on path of
> > trace_printk().
> Honestly, I don't even remember why that macro was created. I think
> it's a remnant from the creation of the trace events. I think the best
> solution is just to nuke it. It shouldn't be used anymore.
> I'll dig a bit deeper into the history of that macro, but I'm thinking
> it shouldn't exist anymore.
> [ /me returns from walking down memory lane ]
> Wow, that brings back some wild memories. Yes, that macro must die, and
> I wish the history of it could die along with it ;-) The horror of the
> old ways I tell you. The event_trace_printk() was the original way we
> displayed events! No real formatting, no parsing my userspace tools. It
> was just a glamorized printk. It was called TRACE_FORMAT() which was
> deprecated by today's TRACE_EVENT().
> That macro should have been removed by commit b8e65554d80b4.
> Please just send a patch to delete that macro. Let's not be maintaining
> it. It gives me nightmares.
> -- Steve

Got it, I will send another patch to remove the nightmares. :-)