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References: <20240206001333.1710070-1-yoann.congal@smile.fr> <20240206001333.1710070-2-yoann.congal@smile.fr>
In-Reply-To: <20240206001333.1710070-2-yoann.congal@smile.fr>
From: Masahiro Yamada <masahiroy@kernel.org>
Date: Wed, 7 Feb 2024 08:03:12 +0900
Message-ID: <CAK7LNAQ22qMJi3jQD8R_+7bi5DTz-pgMAEs91m-cDcnfOmES+A@mail.gmail.com>
Subject: Re: [PATCH v4 1/3] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE_SMALL
 is enabled
To: Yoann Congal <yoann.congal@smile.fr>
Cc: linux-fsdevel@vger.kernel.org, linux-kbuild@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, x86@kernel.org, 
	=?UTF-8?Q?Andr=C3=A9_Almeida?= <andrealmeid@igalia.com>, 
	Borislav Petkov <bp@alien8.de>, Darren Hart <dvhart@infradead.org>, 
	Dave Hansen <dave.hansen@linux.intel.com>, Davidlohr Bueso <dave@stgolabs.net>, 
	Geert Uytterhoeven <geert@linux-m68k.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
	"H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>, Jiri Slaby <jirislaby@kernel.org>, 
	John Ogness <john.ogness@linutronix.de>, Josh Triplett <josh@joshtriplett.org>, 
	Matthew Wilcox <willy@infradead.org>, Peter Zijlstra <peterz@infradead.org>, 
	Petr Mladek <pmladek@suse.com>, Sergey Senozhatsky <senozhatsky@chromium.org>, 
	Steven Rostedt <rostedt@goodmis.org>, Thomas Gleixner <tglx@linutronix.de>, 
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>, Vegard Nossum <vegard.nossum@oracle.com>
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On Tue, Feb 6, 2024 at 9:13=E2=80=AFAM Yoann Congal <yoann.congal@smile.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 !=3D 0.
>
> Note: This changes CONFIG_LOG_CPU_MAX_BUF_SHIFT=3D12 to
> CONFIG_LOG_CPU_MAX_BUF_SHIFT=3D0 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 <=3D __LOG_BUF_LEN / 2)
>           return;
> Systems using CONFIG_BASE_SMALL and having 64+ CPUs should be quite
> rare.
>
> John Ogness <john.ogness@linutronix.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@suse.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@smile.fr>
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Closes: https://lore.kernel.org/all/CAMuHMdWm6u1wX7efZQf=3D2XUAHascps76YQ=
ac6rdnQGhc8nop_Q@mail.gmail.com/
> Reported-by: Vegard Nossum <vegard.nossum@oracle.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@kernel.org>


--=20
Best Regards
Masahiro Yamada