Re: RCU_BOOST not working for me

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

On Fri, Jan 17, 2020 at 09:32:22PM -0500, Joel Fernandes wrote:
> 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.

Did you boot with kernel parameter rcu_tree.kthread_prio=61 or more?  If not,
RCU priority boosting is not going to be able to help.

The writer does nothing for you.  The priority of the task invoking
synchronize_rcu() is irrelevant because synchronize_rcu() is inherently
non-realtime.

							Thanx, Paul

>           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, &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());
> +
> +	writer = kthread_create(rcu_writer, NULL, "writer");
> +	params.sched_priority = 70;
> +	sched_setscheduler(writer, SCHED_FIFO, &params);
> +	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
>