linux-kernel - Re: [PATCH v3 1/2] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE

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Date: Mon, 5 Feb 2024 14:34:30 +0100
From: Petr Mladek <pmladek@...e.com>
To: Yoann Congal <yoann.congal@...le.fr>
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>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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 v3 1/2] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE_SMALL
 is enabled

On Mon 2024-02-05 00:29:44, Yoann Congal 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 BASE_FULL (type bool) which is equivalent
> to 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).

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.

I believe that this patch won't have any effect in practice.
It is hard to imagine a system with >64 CPUs which would require
BASE_SMALL kernel. Well, never say never ;-)

> 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")

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr