On Fri, Jan 17, 2020 at 09:10:49PM -0500, Joel Fernandes wrote:
> On Fri, Jan 17, 2020 at 03:17:56PM -0800, Paul E. McKenney wrote:
> > On Fri, Jan 17, 2020 at 05:18:04PM -0500, Joel Fernandes wrote:
> > > On Fri, Jan 17, 2020 at 04:58:14PM -0500, Joel Fernandes wrote:
> > > > 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?
> > >
> > > So this could be because I did not start a grace period which is quite silly.
> > > I am sorry about that. I will add another thread to start grace periods as
> > > well and let you know if I still see a problem.
> >
> > In addition, the RCU_READER_DELAY isn't long enough to trigger RCU priority
> > boosting. And RCU_BLOCKER_DELAY would be, except that I am not seeing
> > an RCU read-side critical section surrounding it.
>
> I was assuming that only the thread being preempted needs an RCU read-side
> critical section. That preempted section would inherit the priority of the
> thread preempting it (the blocking thread). So the blocking thread would not
> need a read-side critical section, it just would need to be higher priority
> than the thread it is preempting and preempt it long enough to trigger the
> boosting.
>
> Did I miss something?
I started GPs and still see the issue, trace as follows showing GP took 4
seconds. This time I increased RCU_BLOCK_DELAY to 800ms. Updated test patch
appended below.
reader-240 [004] .... 5.389458: rcu_reader: rcu_reader entering RSCS
blocker-241 [004] .... 5.443703: rcu_blocker: rcu_blocker entering
writer-242 [004] .... 5.543743: rcu_writer: starting gp
blocker-241 [004] .... 6.243457: rcu_blocker: rcu_blocker exiting
blocker-241 [004] .... 6.243458: rcu_blocker: rcu_blocker entering
blocker-241 [004] .... 7.043453: rcu_blocker: rcu_blocker exiting
^^^ --- reader got blocked by blocker and did not get CPU...
blocker-241 [004] .... 7.043453: rcu_blocker: rcu_blocker entering
blocker-241 [004] .... 7.843450: rcu_blocker: rcu_blocker exiting
blocker-241 [004] .... 7.843450: rcu_blocker: rcu_blocker entering
blocker-241 [004] .... 8.643454: rcu_blocker: rcu_blocker exiting
blocker-241 [004] .... 8.643454: rcu_blocker: rcu_blocker entering
blocker-241 [004] .... 9.443448: rcu_blocker: rcu_blocker exiting
reader-240 [004] .... 9.490381: rcu_reader: rcu_reader exiting RSCS
^^^ --- ...until the blocker thread quit.
writer-242 [004] .... 9.492684: rcu_writer: ending gp
---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..c7456b2f5570
--- /dev/null
+++ b/drivers/misc/ptest.c
@@ -0,0 +1,134 @@
+#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 800 //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_writer(void *a)
+{
+ while (!kthread_should_stop() && !stop_test) {
+ trace_printk("starting gp\n");
+ synchronize_rcu();
+ trace_printk("ending gp\n");
+ msleep(10);
+ }
+
+ return 0;
+}
+
+static int rcu_reader(void *a)
+{
+ ktime_t start, end, reader_begin;
+ s64 delta;
+
+ reader_begin = ktime_get();
+
+ while (!kthread_should_stop() && !stop_test) {
+ rcu_read_lock();
+ trace_printk("rcu_reader entering RSCS\n");
+ start = ktime_get();
+ mdelay(RCU_READER_DELAY);
+ end = ktime_get();
+ trace_printk("rcu_reader exiting RSCS\n");
+ rcu_read_lock();
+ 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, *writer;
+
+ reader = kthread_create(rcu_reader, NULL, "reader");
+ params.sched_priority = 50;
+ sched_setscheduler(reader, SCHED_FIFO, ¶ms);
+ kthread_bind(reader, smp_processor_id());
+
+ blocker = kthread_create(rcu_blocker, NULL, "blocker");
+ params.sched_priority = 60;
+ sched_setscheduler(blocker, SCHED_FIFO, ¶ms);
+ kthread_bind(blocker, smp_processor_id());
+
+ writer = kthread_create(rcu_writer, NULL, "writer");
+ params.sched_priority = 70;
+ sched_setscheduler(writer, SCHED_FIFO, ¶ms);
+ kthread_bind(writer, smp_processor_id());
+
+ wake_up_process(reader);
+
+ // Let reader run a little
+ mdelay(50);
+
+ wake_up_process(blocker);
+
+ // Let blocker run a little
+ mdelay(100);
+
+ wake_up_process(writer);
+ return 0;
+}
+
+static void __exit ptest_exit(void){
+}
+
+module_init(ptest_init);
+module_exit(ptest_exit);
--
2.25.0.341.g760bfbb309-goog
