linux-kernel - Re: [RFC 2/2] printk: Add more information about the printk caller

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Message-ID: <20200924115814.GB29288@alley>
Date:   Thu, 24 Sep 2020 13:58:14 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Prarit Bhargava <prarit@...hat.com>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Orson Zhai <orsonzhai@...il.com>,
        Changki Kim <changki.kim@...sung.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/2] printk: Add more information about the printk caller

On Thu 2020-09-24 10:40:10, Sergey Senozhatsky wrote:
> On (20/09/23 15:56), Petr Mladek wrote:
> > The information about the printk caller has been added by the commit
> > 15ff2069cb7f967da ("printk: Add caller information to printk() output.").
> > The main motivation was to reconstruct original messages when they
> > longer output from different CPUs got mixed.
> > 
> > But there are more usecases. The number of CPUs and tasks running on
> > a single system is growing. It brings bigger challenges when debugging
> > problems. The context in which kernel reports its state is sometimes
> > pretty useful.
> > 
> > In particular, people suggest to remember the task id, cpu id, more details
> > about the interrupt context, and even the task name [0][1].
> > 
> > Prepare for extending the caller information by replacing caller_id
> > with struct printk_caller. And always store task id, cpu id, and
> > exact interrupt context.
> 
> Wild idea:
> 
> Currently, we have two sides to the problem, I think. There are tasks
> that store messages, but then there are tasks that print those messages
> on the consoles. And those tasks and contexts are not always the same.
> What I found helpful in the past was not only the caller_id, but the
> emitter_id (especially preemption count and irqs state of the context
> that prints messages on the slow consoles).

Yeah, it might be useful. But it should get discussed in a separate thread.

The information about the emitter do not need to get stored into the
metadata. It is mostly need only for debugging printk-related
problems. It should become less important if we succeed with
introducing the printk kthreads.

Best Regards,
Petr