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

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Date:   Fri, 25 Sep 2020 09:54:00 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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 (20/09/24 15:38), Petr Mladek wrote:
[..]
>
> Grrrr, I wonder why I thought that in_irq() covered also the situation
> when IRQ was disabled. It was likely my wish because disabled
> interrupts are problem for printk() because the console might
> cause a softlockup.

preempt_disable() can also trigger softlockup.

> in_irq() actually behaves like in_serving_softirq().
>
> I am confused and puzzled now. I wonder what contexts are actually
> interesting for developers.  It goes back to the ideas from Sergey
> about preemption disabled, ...

Are we talking about context tracking for LOG_CONT or context on
the serial console and /dev/kmsg?

If the latter, then my 5 cents, is that something like preemptible(),
which checks

	(preempt_count() == 0 && !irqs_disabled())

does not look completely unreasonable.

We had a rather OK context tracking in printk() before, but for a
completely different purpose:

       console_may_schedule = !oops_in_progress &&
                       preemptible() &&
                       !rcu_preempt_depth();

We know that printk() can cause RCU stalls [0]. Tracking this part
of the context state is sort of meaningful.

Let's look at this from this POV - why do we add in_irq()/etc tracking
info? Perhaps because we want to connect the dots between printk() caller
state and watchdog reports. Do we cover all watchdogs? No, I don't think
so. RCU stalls, local_irq_disable(), preempt_disable() are not covered.

Do we have any technical reasons not to add those missing bits?

[0] https://lkml.org/lkml/2018/1/9/485

	-ss