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

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Date:   Wed, 14 Oct 2020 16:58:27 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
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>,
        John Ogness <john.ogness@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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 14/10/2020 16.16, Geert Uytterhoeven wrote:
> Hi Petr,
> 
> On Mon, Oct 12, 2020 at 4:50 PM Petr Mladek <pmladek@...e.com> 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
>     [...]
> 
> Seriously?!? Or am I being misled by the tools?
> 
>     linux-m68k-atari_defconfig$ size vmlinux.old vmlinux.lockless_ringbuffer
>        text    data     bss     dec     hex filename
>     3559108 941716 177772 4678596 4763c4 vmlinux.old
>     3563922 1152496 175276 4891694 4aa42e vmlinux.lockless_ringbuffer
> 
> Apparently not...

Hm, that's quite a lot. And the only reason the buffers don't live
entirely in .bss is because a few of their entries have non-zero
initializers.

Perhaps one could add a .init.text.initialize_static_data section of
function pointers, with the _DEFINE_PRINTKRB macro growing something like

static void __init __initialize_printkrb_##name(void) { \
  _##name##_descs[_DESCS_COUNT(descbits) - 1] = ...; \
  _##name##_infos[0] = ...; \
  _##name##_infos[_DESCS_COUNT(descbits) - 1] = ...; \
} \
static_data_initializer(__initialize_printkrb_##name);

with static_data_initalizer being the obvious yoga for putting a
function pointer in the .init.text.initialize_static_data section. Then
very early in start_kernel(), probably first thing, iterate that section
and call all the functions. But maybe that's not even early enough?

Rasmus