[PATCH 5/7] printk: Add config option for disabling printk offloading

[Jan Kara]

From: Jan Kara <jack@xxxxxxx>

Necessity for offloading of printing was observed only for large
systems. So add a config option (disabled by default) which removes most
of the overhead added by this functionality.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 Documentation/kernel-parameters.txt | 13 +++++++------
 init/Kconfig                        | 14 ++++++++++++++
 kernel/printk/printk.c              | 35 +++++++++++++++++++++++++++++++++--
 3 files changed, 54 insertions(+), 8 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index df8adee975ba..913c166fdfea 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2958,18 +2958,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
 			default: disabled
 
-	printk.offload_chars=
+	printk.offload_chars=	[KNL]
 			Printing to console can be relatively slow especially
 			in case of serial console. When there is intensive
 			printing happening from several cpus (as is the case
 			during boot), a cpu can be spending significant time
 			(seconds or more) doing printing. To avoid softlockups,
 			lost interrupts, and similar problems other cpus
-			will take over printing after the currently printing
-			cpu has printed 'printk.offload_chars' characters.
-			Higher value means possibly longer interrupt and other
-			latencies but lower overhead of printing due to handing
-			over of printing.
+			will take over printing (if CONFIG_PRINTK_OFFLOAD=y)
+			after the currently printing cpu has printed
+			'printk.offload_chars' characters. Higher value means
+			possibly longer interrupt and other latencies but
+			lower overhead of printing due to handing over of
+			printing.
 			Format: <number> (0 = disabled)
 			default: 1000
 
diff --git a/init/Kconfig b/init/Kconfig
index c24b6f767bf0..fa9749da5fc8 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1456,6 +1456,20 @@ config PRINTK
 	  very difficult to diagnose system problems, saying N here is
 	  strongly discouraged.
 
+config PRINTK_OFFLOAD
+	default n
+	bool "Enable support for offloading printing to different CPU"
+	depends on PRINTK && SMP
+	help
+	  Printing to console can be relatively slow especially in case of
+	  serial console. On large machines when there is intensive printing
+	  happening from several cpus (as is the case during boot), a cpu can
+	  be spending significant time (seconds or more) doing printing. To
+	  avoid softlockups, lost interrupts, and similar problems other cpus
+	  will take over printing after the currently printing cpu has printed
+	  certain number of characters (tunable via 'printk.offload_chars'
+	  kernel parameter).
+
 config BUG
 	bool "BUG() support" if EXPERT
 	default y
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e404c429fe87..5153c6518b9d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -79,6 +79,7 @@ static DEFINE_SEMAPHORE(console_sem);
 struct console *console_drivers;
 EXPORT_SYMBOL_GPL(console_drivers);
 
+#ifdef CONFIG_PRINTK_OFFLOAD
 /*
  * This spinlock is taken when printing to console. It is used only so that
  * we can spin on it when some other thread wants to take over printing to
@@ -105,6 +106,7 @@ static DEFINE_MUTEX(printk_kthread_mutex);
 
 /* Wait queue printing kthreads sleep on when idle */
 static DECLARE_WAIT_QUEUE_HEAD(print_queue);
+#endif	/* CONFIG_PRINTK_OFFLOAD */
 
 #ifdef CONFIG_LOCKDEP
 static struct lockdep_map console_lock_dep_map = {
@@ -308,6 +310,7 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
 static char *log_buf = __log_buf;
 static u32 log_buf_len = __LOG_BUF_LEN;
 
+#ifdef CONFIG_PRINTK_OFFLOAD
 static int offload_chars_set(const char *val, const struct kernel_param *kp);
 static struct kernel_param_ops offload_chars_ops = {
 	.set = offload_chars_set,
@@ -326,6 +329,7 @@ module_param_cb(offload_chars, &offload_chars_ops, &printk_offload_chars,
 		   S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(offload_chars, "offload printing to console to a different"
 	" cpu after this number of characters");
+#endif
 
 /* Return log buffer address */
 char *log_buf_addr_get(void)
@@ -2255,6 +2259,7 @@ out:
 	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 }
 
+#ifdef CONFIG_PRINTK_OFFLOAD
 /*
  * Returns true iff there is other cpu waiting to take over printing. This
  * function also takes are of setting PRINTK_HANDOVER_B if we want to hand over
@@ -2279,6 +2284,23 @@ static bool cpu_stop_printing(int printed_chars)
 	return false;
 }
 
+#define spin_lock_print_lock(flags) spin_lock_irqsave(&print_lock, flags)
+
+#define spin_unlock_print_lock(flags) spin_unlock_irqrestore(&print_lock, flags)
+
+#else
+
+static bool cpu_stop_printing(int printed_chars)
+{
+	return false;
+}
+
+#define spin_lock_print_lock(flags) local_irq_save(flags)
+
+#define spin_unlock_print_lock(flags) local_irq_restore(flags)
+
+#endif
+
 /**
  * console_unlock - unlock the console system
  *
@@ -2317,7 +2339,7 @@ void console_unlock(void)
 	console_cont_flush(text, sizeof(text));
 again:
 	retry = false;
-	spin_lock_irqsave(&print_lock, flags);
+	spin_lock_print_lock(flags);
 	for (;;) {
 		struct printk_log *msg;
 		size_t ext_len = 0;
@@ -2416,7 +2438,7 @@ skip:
 	 * succeeds in getting console_sem (unless someone else takes it and
 	 * then he'll be responsible for printing).
          */
-	spin_unlock_irqrestore(&print_lock, flags);
+	spin_unlock_print_lock(flags);
 
 	/*
 	 * In case we cannot trylock the console_sem again, there's a new owner
@@ -2776,6 +2798,7 @@ int unregister_console(struct console *console)
 }
 EXPORT_SYMBOL(unregister_console);
 
+#ifdef CONFIG_PRINTK_OFFLOAD
 /* Kthread which takes over printing from a CPU which asks for help */
 static int printing_task(void *arg)
 {
@@ -2868,6 +2891,14 @@ static void printk_offload_init(void)
 	mutex_unlock(&printk_kthread_mutex);
 }
 
+#else	/* CONFIG_PRINTK_OFFLOAD */
+
+static void printk_offload_init(void)
+{
+}
+
+#endif	/* CONFIG_PRINTK_OFFLOAD */
+
 static int __init printk_late_init(void)
 {
 	struct console *con;
-- 
2.1.4

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