[PATCH 2/4] printk/lib: simulate slow consoles

[Sergey Senozhatsky]

*** FOR TESTING ***

Add a hack to delay console_drivers and simulate slow console(s). Doing
something like

	preempt_disable();
	while (...) {
		printk();
		delay();
	}
	preempt_enable();

is not correct. First, not every printk() ends up in console_unlock(),
second - delay should happen in console_unlock() if we want to test
printk() offloading, not outside of printk().

A simple test_printk.sh script to run the tests:

8<-----------------------------------------------------------------------------

TEST_CASE=1
_MAX_NUM_MESSAGES=1024
_CONSOLE_DRIVERS_DELAY=3500

sysctl kernel.watchdog_thresh=5

if [ "z$MAX_NUM_MESSAGES" != "z" ]; then
	_MAX_NUM_MESSAGES=$MAX_NUM_MESSAGES
fi

if [ "z$CONSOLE_DRIVERS_DELAY" != "z" ]; then
	_CONSOLE_DRIVERS_DELAY=$CONSOLE_DRIVERS_DELAY
fi

while [ $TEST_CASE -le 256 ]; do
	echo 1 > /sys/kernel/debug/tracing/events/printk/offloading/enable
	echo 1 > /sys/kernel/debug/tracing/trace

	echo "Executing test $TEST_CASE"

	modprobe test_printk max_num_messages=$_MAX_NUM_MESSAGES \
		console_drivers_delay=$_CONSOLE_DRIVERS_DELAY \
		tests_mask=$TEST_CASE

	TEST_DONE=`cat /sys/test_printk/test_done`
	while [ $TEST_DONE -ne 1 ]; do
		sleep 1s;
		let TEST_DONE=`cat /sys/test_printk/test_done`
	done

	rmmod test_printk

	echo 0 > /sys/kernel/debug/tracing/events/printk/offloading/enable
	cat /sys/kernel/debug/tracing/trace > /tmp/trace-test_case-$TEST_CASE

	echo "Done... cat /tmp/trace-test_case-$TEST_CASE"
	cat /tmp/trace-test_case-$TEST_CASE

	echo "================================================================"

	let TEST_CASE=$TEST_CASE*2
done

sysctl kernel.watchdog_thresh=10

8<-----------------------------------------------------------------------------

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 kernel/printk/printk.c | 15 +++++++++++++++
 lib/test_printk.c      | 10 +++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d4e1abb36d3f..01626f2f42bd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -75,6 +75,9 @@ int console_printk[4] = {
 int oops_in_progress;
 EXPORT_SYMBOL(oops_in_progress);
 
+int __CONSOLE_DRIVERS_DELAY__ = 0;
+EXPORT_SYMBOL(__CONSOLE_DRIVERS_DELAY__);
+
 /*
  * console_sem protects the console_drivers list, and also
  * provides serialisation for access to the entire console
@@ -2584,6 +2587,18 @@ void console_unlock(void)
 
 		stop_critical_timings();	/* don't trace print latency */
 		call_console_drivers(ext_text, ext_len, text, len);
+
+		/* pretend we have a slow console */
+		{
+			volatile int num_chars, num_iter;
+
+			for (num_chars = 0; num_chars < len; num_chars++) {
+				num_iter = 0;
+				while (num_iter++ < __CONSOLE_DRIVERS_DELAY__)
+					cpu_relax();
+			}
+		}
+
 		start_critical_timings();
 
 		/* Must be called under printk_safe */
diff --git a/lib/test_printk.c b/lib/test_printk.c
index 9b01a03ef385..a030f1e61745 100644
--- a/lib/test_printk.c
+++ b/lib/test_printk.c
@@ -22,9 +22,10 @@
 #define MAX_MESSAGES	4242
 #define ALL_TESTS	(~0UL)
 
+static int console_drivers_delay;
+
 static unsigned long max_num_messages;
 static unsigned long tests_mask;
-
 static DEFINE_MUTEX(hog_mutex);
 
 static struct hrtimer printk_timer;
@@ -148,6 +149,8 @@ static void test_noirq_printk_storm(void)
 	local_irq_restore(flags);
 }
 
+extern int __CONSOLE_DRIVERS_DELAY__;
+
 /*
  * hogger printk() tests are based on Tejun Heo's code
  */
@@ -381,6 +384,8 @@ static int __init test_init(void)
 	if (!max_num_messages)
 		max_num_messages = MAX_MESSAGES;
 
+	__CONSOLE_DRIVERS_DELAY__ = console_drivers_delay;
+
 	if (!tests_mask)
 		tests_mask = ALL_TESTS;
 
@@ -409,6 +414,9 @@ MODULE_PARM_DESC(num_devices, "Number of messages to printk() in each test");
 module_param(tests_mask, ulong, 0);
 MODULE_PARM_DESC(tests_mask, "Which tests to run");
 
+module_param(console_drivers_delay, int, 0);
+MODULE_PARM_DESC(console_drivers_delay, "Delay console drivers");
+
 module_init(test_init);
 module_exit(test_exit);
 
-- 
2.15.1