sysrq: Allow immediate Magic SysRq output for PREEMPT_RT_FULL

Add a CONFIG option to allow the output from Magic SysRq to be output
immediately, even if this causes large latencies.

If PREEMPT_RT_FULL, printk() will not try to acquire the console lock
when interrupts or preemption are disabled.  If the console lock is
not acquired the printk() output will be buffered, but will not be
output immediately. Some drivers call into the Magic SysRq code
with interrupts or preemption disabled, so the output of Magic SysRq
will be buffered instead of printing immediately if this option is
not selected.

Even with this option selected, Magic SysRq output will be delayed
if the attempt to acquire the console lock fails.

Signed-off-by: Frank Rowand <frank.rowand@am.sony.com>
Link: http://lkml.kernel.org/r/4E7CEF60.5020508@am.sony.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

5 files changed, 54 insertions, 3 deletions

diff --git a/drivers/tty/serial/cpm_uart/cpm_uart_core.c b/drivers/tty/serial/cpm_uart/cpm_uart_core.c
index 9488da74d4f7..2a75c8418f9d 100644
--- a/drivers/tty/serial/cpm_uart/cpm_uart_core.c
+++ b/drivers/tty/serial/cpm_uart/cpm_uart_core.c
@@ -1225,7 +1225,7 @@ static void cpm_uart_console_write(struct console *co, const char *s,
 {
 	struct uart_cpm_port *pinfo = &cpm_uart_ports[co->index];
 	unsigned long flags;
-	int nolock = oops_in_progress;
+	int nolock = oops_in_progress || sysrq_in_progress;
 
 	if (unlikely(nolock)) {
 		local_irq_save(flags);
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 43db715f1502..5219738ffd11 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -492,6 +492,23 @@ static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
                 sysrq_key_table[i] = op_p;
 }
 
+#ifdef CONFIG_MAGIC_SYSRQ_FORCE_PRINTK
+
+int sysrq_in_progress;
+
+static void set_sysrq_in_progress(int value)
+{
+	sysrq_in_progress = value;
+}
+
+#else
+
+static void set_sysrq_in_progress(int value)
+{
+}
+
+#endif
+
 void __handle_sysrq(int key, bool check_mask)
 {
 	struct sysrq_key_op *op_p;
@@ -500,6 +517,9 @@ void __handle_sysrq(int key, bool check_mask)
 	unsigned long flags;
 
 	spin_lock_irqsave(&sysrq_key_table_lock, flags);
+
+	set_sysrq_in_progress(1);
+
 	/*
 	 * Raise the apparent loglevel to maximum so that the sysrq header
 	 * is shown to provide the user with positive feedback.  We do not
@@ -541,6 +561,9 @@ void __handle_sysrq(int key, bool check_mask)
 		printk("\n");
 		console_loglevel = orig_log_level;
 	}
+
+	set_sysrq_in_progress(0);
+
 	spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
 }
 
diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
index 7faf933cced7..d224c0bae8df 100644
--- a/include/linux/sysrq.h
+++ b/include/linux/sysrq.h
@@ -38,6 +38,11 @@ struct sysrq_key_op {
 	int enable_mask;
 };
 
+#ifdef CONFIG_MAGIC_SYSRQ_FORCE_PRINTK
+extern int sysrq_in_progress;
+#else
+#define sysrq_in_progress 0
+#endif
 #ifdef CONFIG_MAGIC_SYSRQ
 
 /* Generic SysRq interface -- you may call it from any device driver, supplying
diff --git a/kernel/printk.c b/kernel/printk.c
index 1ecab0d8662d..a50af4e6f94f 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -21,6 +21,7 @@
 #include <linux/tty.h>
 #include <linux/tty_driver.h>
 #include <linux/console.h>
+#include <linux/sysrq.h>
 #include <linux/init.h>
 #include <linux/jiffies.h>
 #include <linux/nmi.h>
@@ -831,8 +832,8 @@ static int console_trylock_for_printk(unsigned int cpu, unsigned long flags)
 	__releases(&logbuf_lock)
 {
 #ifdef CONFIG_PREEMPT_RT_FULL
-	int lock = !early_boot_irqs_disabled && !irqs_disabled_flags(flags) &&
-		!preempt_count();
+	int lock = (!early_boot_irqs_disabled && !irqs_disabled_flags(flags) &&
+		!preempt_count()) || sysrq_in_progress;
 #else
 	int lock = 1;
 #endif
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1685473bdf96..383b565ed1af 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -62,6 +62,28 @@ config MAGIC_SYSRQ
 	  keys are documented in <file:Documentation/sysrq.txt>. Don't say Y
 	  unless you really know what this hack does.
 
+config MAGIC_SYSRQ_FORCE_PRINTK
+	bool "Force printk from Magic SysRq"
+	depends on MAGIC_SYSRQ && PREEMPT_RT_FULL
+	default n
+	help
+	  Allow the output from Magic SysRq to be output immediately, even if
+	  this causes large latencies.  This can cause performance problems
+	  for real-time processes.
+
+	  If PREEMPT_RT_FULL, printk() will not try to acquire the console lock
+	  when interrupts or preemption are disabled.  If the console lock is
+	  not acquired the printk() output will be buffered, but will not be
+	  output immediately.  Some drivers call into the Magic SysRq code
+	  with interrupts or preemption disabled, so the output of Magic SysRq
+	  will be buffered instead of printing immediately if this option is
+	  not selected.
+
+	  Even with this option selected, Magic SysRq output will be delayed
+	  if the attempt to acquire the console lock fails.
+
+	  Don't say Y unless you really know what this hack does.
+
 config STRIP_ASM_SYMS
 	bool "Strip assembler-generated symbols during link"
 	default n