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
