Re: gpiolib gpio_chrdev_release duration is about 30 ms

[Maxim Kochetkov <fido_max@inbox.ru>]

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#define GPIOS_NUM 8
static char* gpio_names[GPIOS_NUM] = {"IO0", "IO1", "IO2", "IO3", "WR", 
"CS", "OE", "ADD_EN"};
................
gpio_init() {

     int i;
     for (i = 0; i < GPIOS_NUM; ++i) {
         syslog(LOG_INFO,"%s %d",__FUNCTION__,__LINE__);
         gpio_lines[i] = gpiod_line_find(gpio_names[i]);
         syslog(LOG_INFO,"%s %d",__FUNCTION__,__LINE__);
     }
}
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I used syslog/printk to measure time.
syslog in example shows about 300ms on each gpiod_line_find call.
17.07.2020 16:37, Linus Walleij пишет:
> Hi Maxim,
> 
> On Fri, Jul 17, 2020 at 2:56 PM Maxim Kochetkov <fido_max@inbox.ru> wrote:
> 
>> I'm using libgpiod in userspace.
>> I have 6 gpiochip's on my board.
>> gpiod_line_find takes about 300ms to find GPIO line.
>>
>> gpiod_line_find calls gpiod_foreach_chip
>> then gpiod_chip_iter_next
>> then gpiod_chip_close then close(chip->fd)
>> then we are going to kernel gpiolib gpio_chrdev_release
>> then atomic_notifier_chain_unregister
>> then synchronize_rcu()
>>
>> synchronize_rcu takes about 30 ms (6*30ms=280ms)
>>
>> I tried to remove synchronize_rcu from atomic_notifier_chain_unregister
>> and gpiod_line_find takes about 2ms now.
> 
> Interesting! Can you provide some context? Are you just testing because
> curious or do you need to meet a design objective?
> 
> Did you use ftrace or similar instrumentation to drill down and find
> where time is spent?
> 
> Yours,
> Linus Walleij
>