linux-kernel - Re: [PATCH next v3 07/15] printk: introduce CONSOLE_LOG

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Date:   Tue, 02 Mar 2021 15:34:49 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH next v3 07/15] printk: introduce CONSOLE_LOG_MAX for improved multi-line support

Hi Geert,

On 2021-03-02, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Tue, Mar 2, 2021 at 2:54 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> On Thu, Feb 25, 2021 at 9:30 PM John Ogness <john.ogness@...utronix.de> wrote:
>>> Instead of using "LOG_LINE_MAX + PREFIX_MAX" for temporary buffer
>>> sizes, introduce CONSOLE_LOG_MAX. This represents the maximum size
>>> that is allowed to be printed to the console for a single record.
>>>
>>> Rather than setting CONSOLE_LOG_MAX to "LOG_LINE_MAX + PREFIX_MAX"
>>> (1024), increase it to 4096. With a larger buffer size, multi-line
>>> records that are nearly LOG_LINE_MAX in length will have a better
>>> chance of being fully printed. (When formatting a record for the
>>> console, each line of a multi-line record is prepended with a copy
>>> of the prefix.)
>>>
>>> Signed-off-by: John Ogness <john.ogness@...utronix.de>
>>> Reviewed-by: Petr Mladek <pmladek@...e.com>
>>
>> Thanks for your patch!
>>
>> This increases kernel size by more than 3 KiB, which affects small
>> devices (e.g. SoCs with 10 MiB of SRAM inside).

Petr was concerned that this patch might raise issues for the small
devices.

>> Who is printing such long lines to the console?

Some printk users like to print large multi-line messages into a single
record. They can get pretty long. But since no one is complaining with
the current 1024, we can assume it is big enough.

For v4 I will return it back to 1024 bytes.

> BTW, printing a single line of 1024 characters to a serial console at
> 115200 bps takes almost 100 ms.

Yes. Although once we move to threaded printers, I don't think anyone
will care. Also, I think the netconsole will become quite attractive
when we move to threaded printers.

John Ogness