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

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Date: Tue,  6 Feb 2024 01:13:31 +0100
From: Yoann Congal <yoann.congal@...le.fr>
To: linux-fsdevel@...r.kernel.org,
	linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	x86@...nel.org
Cc: 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>,
	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>,
	Yoann Congal <yoann.congal@...le.fr>,
	Vegard Nossum <vegard.nossum@...cle.com>
Subject: [PATCH v4 1/3] printk: Fix LOG_CPU_MAX_BUF_SHIFT when BASE_SMALL is enabled

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")

---
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