On 2/20/20, Laurentiu-Cristian Duca <laurentiu.duca@xxxxxxxxx> wrote:
> 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;
> }
the mistake was corrected by calling:
sched_setscheduler(getpid(), SCHED_FIFO, &sp);
> ...
> use rt_gpio module to receive interrupt and ack.
>
>
>
> Thank you,
> L-C
>
