linux-kernel - Re: [RFC] printk/sysrq: Don't play with console

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Date:   Tue, 28 May 2019 17:03:45 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC] printk/sysrq: Don't play with console_loglevel

On Tue 2019-05-28 23:21:17, Tetsuo Handa wrote:
> On 2019/05/28 22:42, Petr Mladek wrote:
> >> Ahh.. OK, now I sort of remember why I gave up on this idea (see [1]
> >> at the bottom, when it comes to uv_nmi_dump_state()) - printk_NMI and
> >> printk-safe redirections.
> >>
> >> 	NMI
> >> 		loglevel = NEW
> >> 		printk -> printk_safe_nmi
> >> 		loglevel = OLD
> >>
> >> 	iret
> >>
> >> 	IRQ
> >> 		flush printk_safe_nmi -> printk
> >> 		// At this point we don't remember about
> >> 		// loglevel manipulation anymore
> >> 	iret
> > 
> > printk_safe buffer preserves KERN_* headers. It should be
> > possible to insert KERN_UNSUPPRESSED there.
> 
> But is context dependent buffer large enough to hold SysRq-t output?
> I think that only main logbuf can become large enough to hold SysRq-t output.

SysRq messages are stored directly into the main log buffer.

The limited per-CPU buffers are needed only in printk_safe
and NMI context. We discussed it here because KERN_UNSUPPRESSED
allows to pass the information even from this context.

> We can add KERN_UNSUPPRESSED to SysRq's header line. But I don't think
> that we can automatically add KERN_UNSUPPRESSED to SysRq's body lines
> based on some context information. If we want to avoid manipulating
> console_loglevel, we need to think about how to make sure that
> KERN_UNSUPPRESSED is added to all lines from such context without
> overflowing capacity of that buffer.

We could set this context in printk_context per-CPU variable.

Then we could easily add the set per-message flag in
vprintk_store() for the normal/atomic context. And we
could store an extra KERN_UNSUPPRESSED in printk_safe_log_store()
for printk_safe and NMI context.

Best Regards,
Petr