linux-kernel - Re: [PATCH v4 1/3] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE

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Message-ID: <CAK7LNAQ22qMJi3jQD8R_+7bi5DTz-pgMAEs91m-cDcnfOmES+A@mail.gmail.com>
Date: Wed, 7 Feb 2024 08:03:12 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
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>, 
	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 v4 1/3] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE_SMALL
 is enabled

On Tue, Feb 6, 2024 at 9:13 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")



Reviewed-by: Masahiro Yamada <masahiroy@...nel.org>





-- 
Best Regards
Masahiro Yamada