Date:   Thu, 20 Aug 2020 21:57:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        devel@...verdev.osuosl.org, lkml <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v4 3/3] media: atomisp: Only use trace_printk if allowed

On Fri, 21 Aug 2020 09:39:19 +0800
Nicolas Boichat <drinkcat@...omium.org> wrote:

> On Fri, Aug 21, 2020 at 8:36 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > On Fri, 21 Aug 2020 08:13:00 +0800
> > Nicolas Boichat <drinkcat@...omium.org> wrote:
> >  
> > > On Thu, Aug 20, 2020 at 10:23 PM Steven Rostedt <rostedt@...dmis.org> wrote:  
> > > >
> > > > On Thu, 20 Aug 2020 17:14:12 +0800
> > > > Nicolas Boichat <drinkcat@...omium.org> wrote:
> > > >  
> > > > > Technically, we could only initialize the trace_printk buffers
> > > > > when the print env is switched, to avoid the build error and
> > > > > unconditional boot-time warning, but I assume this printing
> > > > > framework will eventually get removed when the driver moves out
> > > > > of staging?  
> > > >
> > > > Perhaps this should be converting into a trace event. Look at what bpf
> > > > did for their bpf_trace_printk().
> > > >
> > > > The more I think about it, the less I like this series.  
> > >
> > > To make it clear, the primary goal of this series is to get rid of
> > > trace_printk sprinkled in the kernel by making sure some randconfig
> > > builds fail. Since my v2, there already has been one more added (the
> > > one that this patch removes), so I'd like to land 2/3 ASAP to prevent
> > > even more from being added.
> > >
> > > Looking at your reply on 1/3, I think we are aligned on that goal? Is
> > > there some other approach you'd recommend?
> > >
> > > Now, I'm not pretending my fixes are the best possible ones, but I
> > > would much rather have the burden of converting to trace events on the
> > > respective driver maintainers. (btw is there a short
> > > documentation/tutorial that I could link to in these patches, to help
> > > developers understand what is the recommended way now?)
> > >  
> >
> > I like the goal, but I guess I never articulated the problem I have
> > with the methodology.
> >
> > trace_printk() is meant to be a debugging tool. Something that people
> > can and do sprinkle all over the kernel to help them find a bug in
> > areas that are called quite often (where printk() is way too slow).
> >
> > The last thing I want them to deal with is adding a trace_printk() with
> > their distro's config (or a config from someone that triggered the bug)
> > only to have the build to fail, because they also need to add a config
> > value.
> >
> > I add to the Cc a few developers I know that use trace_printk() in this
> > fashion. I'd like to hear their view on having to add a config option
> > to make trace_printk work before they test a config that is sent to
> > them.  
> 
> Gotcha, thanks. I have also used trace_printk in the past, as
> uncommitted changes (and understand the usefulness ,-)). And in Chrome
> OS team here, developers have also raised this concern: how do we make
> the developer flow convenient so that we can add trace_printk to our
> code for debugging, without having to flip back that config option,
> and _yet_ make sure that no trace_printk ever makes it into our
> production kernels. We have creative ways of making that work (portage
> USE flags and stuff). But I'm not sure about other flows, and your
> concern is totally valid...
> 
> Some other approaches/ideas:
>  1. Filter all lkml messages that contain trace_printk. Already found
> 1 instance, and I can easily reply to those with a semi-canned answer,
> if I remember to check that filter regularly (not sustainable in the
> long run...).

Added Joe Perches to the thread.

We can update checkpatch.pl to complain about a trace_printk() that it
finds in the added code.

>  2. Integration into some kernel test robot? (I will not roll my own
> for this ,-)) It may be a bit difficult as some debug config options
> do enable trace_printk, and that's ok.
>  3. In Chromium OS, I can add a unit test (i.e. something outside of
> the normal kernel build system), but that'll only catch regressions
> downstream (or when we happen to backport patches).
> 
> Down the line, #3 catches what I care about the most (Chromium OS
> issues: we had production kernels for a few days/weeks showing that
> splat on boot), but it'd be nice to have something upstream that
> benefits everyone.
> 

What about an opposite config. That is, not have a config to enable it.
But one to disable it. If it is disabled and a trace_printk is found,
it will fail the build. This way your builds will not allow your kernel
to get out the door with one.

#ifdef CONFIG_DISABLE_TRACE_PRINTK
#define trace_printk	__this_function_is_disabled
#endif

?

-- Steve

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