linux-kernel - Re: [GIT PULL] printk for 5.10 (includes lockless ringbuffer)

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Date:   Wed, 14 Oct 2020 23:01:30 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: [GIT PULL] printk for 5.10 (includes lockless ringbuffer)

On 2020-10-14, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> - Fully lockless ringbuffer implementation, including the support for
>>   continuous lines. It will allow to store and read messages in any
>>   situation wihtout the risk of deadlocks and without the need
>>   of temporary per-CPU buffers.
>
>     linux-m68k-atari_defconfig$ bloat-o-meter vmlinux.old vmlinux.lockless_ringbuffer
>     add/remove: 39/16 grow/shrink: 9/15 up/down: 214075/-4362 (209713)
>     Function                                     old     new   delta
>     _printk_rb_static_infos                        -  180224 +180224
>     _printk_rb_static_descs                        -   24576  +24576

131072 of _printk_rb_static_infos is reserved for dictionary usage. The
rest (49152) is for the record meta data. Previously any dictionary
usage and record meta data was embedded in the message buffer (log_buf,
65536).

Since the meta data is now separate, setting CONFIG_LOG_BUF_SHIFT=15
would provide roughly the same amount of record storage as
CONFIG_LOG_BUF_SHIFT=16 did with vmlinux.old. Then there would be:

32768: message buffer
24576: meta data
65536: dictionary data
12288: descriptor data

Excluding the dictionary data, the total is 65536.

(With vmlinux.old there is 65536 total with CONFIG_LOG_BUF_SHIFT=16.)

It is the reserved dictionary data that is hurting us here.

Should we provide a config option to kill the dictionary data?

Should CONFIG_LOG_BUF_SHIFT do an implicit -1 so that the sizes (without
dictionary data) are about the same as before?

Maybe dictionaries should only exist in the dynamically allocated
ringbuffer?

John Ogness