RCU_BOOST not working for me

[Joel Fernandes <joel@joelfernandes.org>]

Hi,
Me and Daniel were poking around with RCU_BOOST. I wrote a kernel module to
test it a bit and I don't see the boost happening (thanks to Daniel for idea
of writing a module). Haven't debugged it more yet. Will look more tomorrow.
But below is the kernel module code and it prints a FAIL message to kernel
logs in a few seconds.

I see the reader thread not getting CPU for several seconds. RCU_BOOST_DELAY
is set to 500.

Thoughts?

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

diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index c1860d35dc7e..ba34957dff26 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -2,7 +2,7 @@
 #
 # Makefile for misc devices that really don't fit anywhere else.
 #
-
+obj-m += ptest.o
 obj-$(CONFIG_IBM_ASM)		+= ibmasm/
 obj-$(CONFIG_IBMVMC)		+= ibmvmc.o
 obj-$(CONFIG_AD525X_DPOT)	+= ad525x_dpot.o
diff --git a/drivers/misc/ptest.c b/drivers/misc/ptest.c
new file mode 100644
index 000000000000..76cc9524ccac
--- /dev/null
+++ b/drivers/misc/ptest.c
@@ -0,0 +1,112 @@
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/vmalloc.h>
+#include <linux/kthread.h>
+#include <linux/delay.h>
+
+#define RCU_READER_DELAY 100 //ms
+#define RCU_BLOCKER_DELAY 600 //ms
+
+MODULE_LICENSE("GPL");
+
+struct sched_param {
+	int sched_priority;
+};
+
+int stop_test = 0;
+int test_pass = 1;
+int reader_exit = 0;
+s64 delta_fail;
+
+#define ns_to_ms(delta) (delta / 1000000ULL)
+
+static int rcu_reader(void *a)
+{
+	ktime_t start, end, reader_begin;
+	s64 delta;
+
+	reader_begin = ktime_get();
+
+	while (!kthread_should_stop() && !stop_test) {
+		start = ktime_get();
+		rcu_read_lock();
+		trace_printk("rcu_reader entering RSCS\n");
+		mdelay(RCU_READER_DELAY);
+		trace_printk("rcu_reader exiting RSCS\n");
+		rcu_read_lock();
+		end = ktime_get();
+		delta = ktime_to_ns(ktime_sub(end, start));
+
+		if (delta < 0 || (ns_to_ms(delta) > (2 * RCU_READER_DELAY))) {
+			delta_fail = delta;
+			test_pass = 0;
+			break;
+		}
+
+		// Don't let the rcu_reader() run more than 3s inorder to
+		// not starve the blocker incase reader prio > blocker prio.
+		delta = ktime_to_ns(ktime_sub(end, reader_begin));
+		if (ns_to_ms(delta) > 3000)
+			break;
+	}
+
+	stop_test = 1;
+	reader_exit = 1;
+	pr_err("Exiting reader\n");
+	return 0;
+}
+
+static int rcu_blocker(void *a)
+{
+	int loops = 5;
+
+	while (!kthread_should_stop() && loops-- && !stop_test) {
+		trace_printk("rcu_blocker entering\n");
+		mdelay(RCU_BLOCKER_DELAY);
+		trace_printk("rcu_blocker exiting\n");
+	}
+
+	pr_err("Exiting blocker\n");
+	stop_test = 1;
+
+	// Wait for reader to finish
+	while (!reader_exit)
+		schedule_timeout_uninterruptible(1);
+
+	if (test_pass)
+		pr_err("TEST PASSED\n");
+	else
+		pr_err("TEST FAILED, failing delta=%lldms\n", ns_to_ms(delta_fail));
+
+	return 0;
+}
+
+static int __init ptest_init(void){
+	struct sched_param params;
+	struct task_struct *reader, *blocker;
+
+	reader = kthread_create(rcu_reader, NULL, "reader");
+	params.sched_priority = 50;
+	sched_setscheduler(reader, SCHED_FIFO, &params);
+	kthread_bind(reader, smp_processor_id());
+
+	blocker = kthread_create(rcu_blocker, NULL, "blocker");
+	params.sched_priority = 60;
+	sched_setscheduler(blocker, SCHED_FIFO, &params);
+	kthread_bind(blocker, smp_processor_id());
+
+	wake_up_process(reader);
+
+	// Let reader run a little
+	mdelay(50);
+
+	wake_up_process(blocker);
+	return 0;
+}
+
+static void __exit ptest_exit(void){
+}
+
+module_init(ptest_init);
+module_exit(ptest_exit);
-- 
2.25.0.341.g760bfbb309-goog