linux-kernel - Re: [RFC] printk's sync mode for ftrace

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Message-ID: <87y26c9ja6.fsf@jogness.linutronix.de>
Date:   Fri, 29 Oct 2021 13:18:33 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] printk's sync mode for ftrace_dump()

On 2021-10-29, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> ftrace_dump() is used for instance by alt-sysrq-z / sysrq_ftrace_dump().
> The function itself dumps the whole trace buffer in an irq-off section so
> no need to talk about max latencies unless this is going to change.
>
> The output on the serial is more or less brief and starts with
>     "** 397774 printk messages dropped **"
> and so I do see only the end of it. Might be okay.
> Any reason not to use the sync mode + atomic console while ftrace_dump()
> is in progress?

Since latencies are not a concern, I see no reason not to use
sync+atomic.

However, it does put us into the situation of beginning to define
non-emergency scenarios where sync+atomic is used. That rabbit hole
might go quite deep. I wonder if it might be better to introduce a new
sysrq to toggle sync mode.

John