linux-kernel - Re: printk: Add process name information to printk() output.

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Date:   Fri, 04 Sep 2020 12:27:23 -0700
From:   Joe Perches <joe@...ches.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        John Ogness <john.ogness@...utronix.de>
Cc:     Changki Kim <changki.kim@...sung.com>, pmladek@...e.com,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        changbin.du@...el.com, masahiroy@...nel.org, rd.dunlap@...il.com,
        krzk@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: printk: Add process name information to printk() output.

On Fri, 2020-09-04 at 12:35 +0200, Greg KH wrote:
> On Fri, Sep 04, 2020 at 11:53:42AM +0206, John Ogness wrote:
> > On 2020-09-04, Changki Kim <changki.kim@...sung.com> wrote:
> > > Printk() meesages are the most basic and useful debug method.
> > > However, additional information needs in multi-processor.
> > > If we add messages with processor id and process name, we can find
> > > a problem only with messages when the problem occurs with H/W IP or CPU.
> > > This is very useful in narrowing down the scope of the problems.
> > 
> > [...]
> > 
> > > diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
> > > index e6302da041f9..fcefe9516606 100644
> > > --- a/kernel/printk/printk_ringbuffer.h
> > > +++ b/kernel/printk/printk_ringbuffer.h
> > > @@ -21,6 +22,12 @@ struct printk_info {
> > >  	u8	flags:5;	/* internal record flags */
> > >  	u8	level:3;	/* syslog level */
> > >  	u32	caller_id;	/* thread id or processor id */
> > > +#ifdef CONFIG_PRINTK_PROCESS
> > > +	int	pid;			/* process id */
> > > +	u8	cpu_id;			/* processor id */
> > > +	u8	in_interrupt;		/* interrupt conext */
> > > +	char process[TASK_COMM_LEN];	/* process name */
> > > +#endif
> > >  };
> > 
> > I can understand the desire to have more information with messages. But
> > IMHO adding it to the ringbuffer descriptor is the wrong place for
> > it. The descriptor should really be limited to data that the printk
> > subsystem needs for _itself_. With respect to LOG_CONT, I think we can
> > agree that @caller_id is not enough. But there has been discussions [0]
> > of having @caller_id provide a better context representation.
> > 
> > If we want to support adding more meta information to messages, I would
> > prefer that the information is either prepended directly to the message
> > text string or appended to the dictionary text string. We could even go
> > so far as providing a boot argument where a list of information could be
> > specified, what should be automatically added to the text/dict strings
> > of each message. That would not require any ringbuffer changes and would
> > allow new types of information to be added later.
> > 
> > Something like:
> > 
> >     printk.format=ts,cpu,comm,pid,in_atomic
> > 
> > John Ogness
> > 
> > [0] https://lkml.kernel.org/r/20200719143527.GA566@jagdpanzerIV.localdomain
> 
> Ah, finally a good use of the "dictionary" that we all can agree makes
> sense :)
> 
> This does seem like a better solution overall, thanks.

__func__ too please so that it could optionally be enabled per
subsystem using %pS, __builtin_return_address(<appropriate_level>)

That way a lot of %s:..., __func__ could be removed from
specific printk instances.