[PATCH 2/2] printk: Make the console flush configurable in hotplug path

[Prasad Sodagudi]

From: Mohammed Khajapasha <mkhaja@xxxxxxxxxxxxxx>

The thread which initiates the hot plug can get scheduled
out, while trying to acquire the console lock,
thus increasing the hot plug latency. This option
allows to selectively disable the console flush and
in turn reduce the hot plug latency.

Signed-off-by: Mohammed Khajapasha <mkhaja@xxxxxxxxxxxxxx>
Signed-off-by: Prasad Sodagudi <psodagud@xxxxxxxxxxxxxx>
---
 init/Kconfig           | 10 ++++++++++
 kernel/printk/printk.c | 10 ++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index d6a0b31..9ce39ba 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -699,6 +699,16 @@ config LOG_BUF_SHIFT
 		     13 =>  8 KB
 		     12 =>  4 KB
 
+config CONSOLE_FLUSH_ON_HOTPLUG
+	bool "Enable console flush configurable in hot plug code path"
+	depends on HOTPLUG_CPU
+	def_bool n
+	help
+	In cpu hot plug path console lock acquire and release causes the
+	console to flush. If console lock is not free hot plug latency
+	increases. So make console flush configurable in hot plug path
+	and default disabled to help in cpu hot plug latencies.
+
 config LOG_CPU_MAX_BUF_SHIFT
 	int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)"
 	depends on SMP
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9b75f6b..f02d3ef 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2283,6 +2283,8 @@ void resume_console(void)
 	console_unlock();
 }
 
+#ifdef CONFIG_CONSOLE_FLUSH_ON_HOTPLUG
+
 /**
  * console_cpu_notify - print deferred console messages after CPU hotplug
  * @cpu: unused
@@ -2302,6 +2304,8 @@ static int console_cpu_notify(unsigned int cpu)
 	return 0;
 }
 
+#endif
+
 /**
  * console_lock - lock the console system for exclusive use.
  *
@@ -2974,7 +2978,7 @@ void __init console_init(void)
 static int __init printk_late_init(void)
 {
 	struct console *con;
-	int ret;
+	int ret = 0;
 
 	for_each_console(con) {
 		if (!(con->flags & CON_BOOT))
@@ -2996,13 +3000,15 @@ static int __init printk_late_init(void)
 			unregister_console(con);
 		}
 	}
+#ifdef CONFIG_CONSOLE_FLUSH_ON_HOTPLUG
 	ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
 					console_cpu_notify);
 	WARN_ON(ret < 0);
 	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "printk:online",
 					console_cpu_notify, NULL);
 	WARN_ON(ret < 0);
-	return 0;
+#endif
+	return ret;
 }
 late_initcall(printk_late_init);
 
-- 
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