linux-kernel - Re: [PATCH v5 1/3] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE

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Date: Mon, 12 Feb 2024 20:01:21 +0100
From: Yoann Congal <yoann.congal@...le.fr>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kbuild@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org, x86@...nel.org,
 André Almeida <andrealmeid@...lia.com>,
 Borislav Petkov <bp@...en8.de>, Darren Hart <dvhart@...radead.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Davidlohr Bueso <dave@...olabs.net>,
 Geert Uytterhoeven <geert@...ux-m68k.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
 Jiri Slaby <jirislaby@...nel.org>, John Ogness <john.ogness@...utronix.de>,
 Josh Triplett <josh@...htriplett.org>, Matthew Wilcox <willy@...radead.org>,
 Peter Zijlstra <peterz@...radead.org>, Petr Mladek <pmladek@...e.com>,
 Sergey Senozhatsky <senozhatsky@...omium.org>,
 Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Vegard Nossum <vegard.nossum@...cle.com>
Subject: Re: [PATCH v5 1/3] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE_SMALL
 is enabled

Le 11/02/2024 à 00:41, Masahiro Yamada a écrit :
> On Thu, Feb 8, 2024 at 2:10 AM Yoann Congal <yoann.congal@...le.fr> wrote:
>>
>> LOG_CPU_MAX_BUF_SHIFT default value depends on BASE_SMALL:
>>   config LOG_CPU_MAX_BUF_SHIFT
>>         default 12 if !BASE_SMALL
>>         default 0 if BASE_SMALL
>> But, BASE_SMALL is a config of type int and "!BASE_SMALL" is always
>> evaluated to true whatever is the value of BASE_SMALL.
>>
>> This patch fixes this by using the correct conditional operator for int
>> type : BASE_SMALL != 0.
>>
>> Note: This changes CONFIG_LOG_CPU_MAX_BUF_SHIFT=12 to
>> CONFIG_LOG_CPU_MAX_BUF_SHIFT=0 for BASE_SMALL defconfigs, but that will
>> not be a big impact due to this code in kernel/printk/printk.c:
>>   /* by default this will only continue through for large > 64 CPUs */
>>   if (cpu_extra <= __LOG_BUF_LEN / 2)
>>           return;
>> Systems using CONFIG_BASE_SMALL and having 64+ CPUs should be quite
>> rare.
>>
>> John Ogness <john.ogness@...utronix.de> (printk reviewer) wrote:
>>> For printk this will mean that BASE_SMALL systems were probably
>>> previously allocating/using the dynamic ringbuffer and now they will
>>> just continue to use the static ringbuffer. Which is fine and saves
>>> memory (as it should).
>>
>> Petr Mladek <pmladek@...e.com> (printk maintainer) wrote:
>>> More precisely, it allocated the buffer dynamically when the sum
>>> of per-CPU-extra space exceeded half of the default static ring
>>> buffer. This happened for systems with more than 64 CPUs with
>>> the default config values.
>>
>> Signed-off-by: Yoann Congal <yoann.congal@...le.fr>
>> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> Closes: https://lore.kernel.org/all/CAMuHMdWm6u1wX7efZQf=2XUAHascps76YQac6rdnQGhc8nop_Q@mail.gmail.com/
>> Reported-by: Vegard Nossum <vegard.nossum@...cle.com>
>> Closes: https://lore.kernel.org/all/f6856be8-54b7-0fa0-1d17-39632bf29ada@oracle.com/
>> Fixes: 4e244c10eab3 ("kconfig: remove unneeded symbol_empty variable")
>>
> 
> 
> 
> All the Reviewed-by tags are dropped every time, annoyingly.

Hi!

Was I supposed to gather these tags from patch version N to patch version N+1?
In that case, I'm sorry, I did not know that :-/
Patch 1/3 is exactly the same but patch 2/3 is equivalent but different. Is there a rule written somewhere about when carrying the tags across revision and when not? (I could not find it)

> This is equivalent to v4, which had these tags:
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> Reviewed-by: Masahiro Yamada <masahiroy@...nel.org>

Thanks a lot!

> 
>> ---
>> v3->v4:
>> * Fix BASE_SMALL usage instead of switching to BASE_FULL because
>>   BASE_FULL will be removed in the next patches of this series.
>> ---
>>  init/Kconfig | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/init/Kconfig b/init/Kconfig
>> index deda3d14135bb..d50ebd2a2ce42 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -734,8 +734,8 @@ config LOG_CPU_MAX_BUF_SHIFT
>>         int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)"
>>         depends on SMP
>>         range 0 21
>> -       default 12 if !BASE_SMALL
>> -       default 0 if BASE_SMALL
>> +       default 0 if BASE_SMALL != 0
>> +       default 12
>>         depends on PRINTK
>>         help
>>           This option allows to increase the default ring buffer size
>> --
>> 2.39.2
>>
>>
> 
> 
> --
> Best Regards
> Masahiro Yamada

-- 
Yoann Congal
Smile ECS - Tech Expert