Unexpected high latency on bbb

* Unexpected high latency on bbb
@ 2020-02-20 14:42 Laurentiu-Cristian Duca
  2020-02-25 15:59 ` Laurentiu-Cristian Duca
  0 siblings, 1 reply; 2+ messages in thread
From: Laurentiu-Cristian Duca @ 2020-02-20 14:42 UTC (permalink / raw)
  To: linux-rt-users

Hello,

  I have made a rt-test on kernel preempt-rt 5.4.5
and got unexpected high latencies (5ms max).
Maybe I am doing something wrong ... please help
The same test got 120us max latency on EVL project
(on 3 hour test with one million samples)
and only the synchronization is different on preempt_rt
(referring to rt_gpio module described below).

scenario between beaglebone black computer and fpga board
====
fpga generates an interrupt and counts latency until ack
bbb receives and ack interrupt via rt_gpio module
fpga sends latency counter value via SPI to bbb
bbb prints on the screen
result: 5ms max latency.

rt_gpio.c module
====
...
static struct swait_queue_head head_swait;

static irqreturn_t rt_interrupt_handler(int irq, void *data)
{
	n_interrupts++;

	/* use swait.h model to signal interrupt arrived */
	interrupt_done = 1;
	smp_mb();
 	if (swait_active(&head_swait))
		swake_up_one(&head_swait);

	return IRQ_HANDLED;
}

static ssize_t rt_gpio_read(struct file *f, char __user *buf, size_t
	len, loff_t *off)
{
	int ret;
	DECLARE_SWAITQUEUE(swait);

	/* wait for interrupt using swait.h model */
	for (;;) {
		prepare_to_swait_exclusive(&head_swait, &swait, TASK_INTERRUPTIBLE);
 		/* smp_mb() from set_current_state() */
 		if (interrupt_done)
 			break;
 		schedule();
 	}
	finish_swait(&head_swait, &swait);
	interrupt_done = 0;
	smp_mb();

	gpiod_get_raw_value(gpiod_in)
	copy_to_user(...);
}

static ssize_t rt_gpio_write(struct file *f, const char __user *buf,
	size_t len, loff_t *off)
{
    copy_from_user(...);
	gpiod_set_raw_value(gpiod_out, user_value);
}

static int __init rt_gpio_init(void) /* Constructor */
{
	...
	request_irq(irq_line, rt_interrupt_handler,
		IRQ_TYPE_EDGE_RISING | IRQF_NO_THREAD, "rt_gpio interrupt\n", (void *)1));

	init_swait_queue_head(&head_swait);

}

static void __exit rt_gpio_exit(void)
{
	free_irq(irq_line, (void*)1);
	gpio_free(gpio_in_id);
	gpio_free(gpio_out_id);
	...
}

userspace
====
    printf("Setup this thread as a real time thread\n");
  	if((ret = mlockall(MCL_FUTURE|MCL_CURRENT)) < 0) {
       	printf("mlockall failed: %s\n", strerror(ret));
       	return -1;
   	}
    sp.sched_priority = 98;
    if((ret = pthread_setschedparam(pthread_self(), SCHED_FIFO, &sp)) != 0) {
        printf("Failed to set lat thread to real-time priority: %s\n",
strerror(ret));
		return -1;
    }
    ...
    use rt_gpio module to receive interrupt and ack.

Thank you,
L-C

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