linux-kernel - [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

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Date:	Wed,  9 Dec 2015 14:21:05 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Daniel Thompson <daniel.thompson@...aro.org>,
	Jiri Kosina <jkosina@...e.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-cris-kernel@...s.com, linux-mips@...ux-mips.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
	Petr Mladek <pmladek@...e.com>
Subject: [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

Testing has shown that the backtrace sometimes does not fit
into the 4kB temporary buffer that is used in NMI context.
The warnings are gone when I double the temporary buffer size.

This patch doubles the buffer size and makes it configurable.

Note that this problem existed even in the x86-specific
implementation that was added by the commit a9edc8809328
("x86/nmi: Perform a safe NMI stack trace on all CPUs").
Nobody noticed it because it did not print any warnings.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 init/Kconfig        | 22 ++++++++++++++++++++++
 kernel/printk/nmi.c |  3 ++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index c1c0b6a2d712..efcff25a112d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -866,6 +866,28 @@ config LOG_CPU_MAX_BUF_SHIFT
 		     13 =>   8 KB for each CPU
 		     12 =>   4 KB for each CPU
 
+config NMI_LOG_BUF_SHIFT
+	int "Temporary per-CPU NMI log buffer size (12 => 4KB, 13 => 8KB)"
+	range 10 21
+	default 13
+	depends on PRINTK && HAVE_NMI
+	help
+	  Select the size of a per-CPU buffer where NMI messages are temporary
+	  stored. They are copied to the main log buffer in a safe context
+	  to avoid a deadlock. The value defines the size as a power of 2.
+
+	  NMI messages are rare and limited. The largest one is when
+	  a backtrace is printed. It usually fits into 4KB. Select
+	  8KB if you want to be on the safe side.
+
+	  Examples:
+		     17 => 128 KB for each CPU
+		     16 =>  64 KB for each CPU
+		     15 =>  32 KB for each CPU
+		     14 =>  16 KB for each CPU
+		     13 =>   8 KB for each CPU
+		     12 =>   4 KB for each CPU
+
 #
 # Architectures with an unreliable sched_clock() should select this:
 #
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 5465230b75ec..78c07d441b4e 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -41,7 +41,8 @@ DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
 static int printk_nmi_irq_ready;
 atomic_t nmi_message_lost;
 
-#define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
+#define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) -		\
+			 sizeof(atomic_t) - sizeof(struct irq_work))
 
 struct nmi_seq_buf {
 	atomic_t		len;	/* length of written data */
-- 
1.8.5.6

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