linux-kernel - Re: [PATCH -printk] printk, tracing: fix console tracepoint

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Message-ID: <20220504094636.GA8069@pathway.suse.cz>
Date:   Wed, 4 May 2022 11:46:36 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Marco Elver <elver@...gle.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Johannes Berg <johannes.berg@...el.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Linux Kernel Functional Testing <lkft@...aro.org>
Subject: Re: [PATCH -printk] printk, tracing: fix console tracepoint

On Tue 2022-05-03 21:20:44, John Ogness wrote:
> On 2022-05-03, Marco Elver <elver@...gle.com> wrote:
> > One notable difference is that by moving tracing into printk_sprint(),
> > the 'text' will no longer include the "header" (loglevel and timestamp),
> > but only the raw message. Arguably this is less of a problem now that
> > the console tracepoint happens on the printk() call and isn't delayed.
> 
> Another slight difference is that messages composed of LOG_CONT pieces
> will trigger the tracepoint for each individual piece and _never_ as a
> complete line.
> 
> It was never guaranteed that all LOG_CONT pieces make it into the final
> printed line anyway, but with this change it will be guaranteed that
> they are always handled separately.
> 
> I am OK with this change, but like Steven, I agree the the users of that
> tracepoint need to chime in.

My feeling is that the feature is not used much. Otherwise people
would complain that it was asynchronous and hard to use.

I mean that the printk() messages appeared in the trace log
asynchronously. So it required some post processing to correctly
sort them against other tracing messages. The same result can be
achieved by processing printk log buffer, dmesg.log, journalctl.

I guess that we will only find the answer when we push the change
into linux-next and mainline. I am going to do so.

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr