linux-kernel - Re: [PATCH printk v3 5/6] printk: introduce console_get_next_message() and console

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Message-ID: <87ilhnd5te.fsf@jogness.linutronix.de>
Date:   Tue, 03 Jan 2023 16:03:17 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v3 5/6] printk: introduce
 console_get_next_message() and console_message

On 2023-01-03, Petr Mladek <pmladek@...e.com> wrote:
> On Thu 2022-12-22 16:47:39, John Ogness wrote:
>> On 2022-12-21, John Ogness <john.ogness@...utronix.de> wrote:
>> > +static bool console_emit_next_record(struct console *con, bool *handover, int cookie)
>> > +{
>> > +	bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
>> > +	static char dropped_text[DROPPED_TEXT_MAX];
>> > +	static struct console_buffers cbufs;
>> > +	static struct console_message cmsg = {
>> > +		.cbufs = &cbufs,
>> > +	};
>> 
>> @cmsg should not be static. The whole point of the console_message
>> wrapper struct is so that it can sit on the stack.
>
> Well, it might actually be better to keep it static for now.
> It always points to static struct console_buffers cbufs anyway.
>
> It would make sense to have it on stack only when it uses
> different buffers.

I think we should avoid making things static when it serves no purpose.

> Which brings the question. Does it makes sense to use
> the same buffers by different struct console_message?
> Will it be safe in any situation?
>
> I did not want to complicate it yesterday. I think that
> I have already proposed this. But this brings back
> the question whether it makes sense to have two structures
> at all.
>
> I still think that it would be easier and even more safe
> to put everything into struct console_message.
>
> I mean to have:
>
> struct console_message {
> 	char			buf[CONSOLE_EXT_LOG_MAX];
> 	char			scratch_buf[LOG_LINE_MAX];
> 	unsigned int		len;
> 	u64			seq;
> 	unsigned long		dropped;
> };

The current atomic console proposal allocates 1x cbuf per-cpu and 4x
meta-data per-cpu. Different contexts of a cpu will have different
meta-data, but all the contexts of a cpu will share the same cbuf.

If cbufs become embedded in cmsg, then we would allocate 1x cmsg
per-cpu. But the atomic consoles would still need their own 4x per-cpu
meta-data.

When looking at the proposal code, it looks wrong to have meta-data
fields in the cmsg struct that are not being used. But maybe that is
acceptable during the "transition phase" until all legacy consoles are
gone.

For v4 I will drop the console_buffers struct. I will use your
suggestion.

John