Re: RCU_BOOST not working for me

["Paul E. McKenney" <paulmck@kernel.org>]

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.

But rcutorture already has tests for RCU priority boosting.  Or are those
failing in some way?

Either way, it is good to see interest in RCU priority boosting.  ;-)

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 
> > ---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(cu_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
> >