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 <pmla...@suse.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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