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From: Linus Walleij <linus.walleij@linaro.org>
To: Maxim Kochetkov <fido_max@inbox.ru>
Cc: "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>
Subject: Re: gpiolib gpio_chrdev_release duration is about 30 ms
Date: Fri, 17 Jul 2020 15:37:12 +0200	[thread overview]
Message-ID: <CACRpkda-pXF71vr5v90yipKubc14tbZW5Ryw1o7rdn4FbWwsTw@mail.gmail.com> (raw)
In-Reply-To: <7eb11c0d-cd11-f873-c336-4ec955a7bdb3@inbox.ru>

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

  reply	other threads:[~2020-07-17 13:37 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17 12:56 gpiolib gpio_chrdev_release duration is about 30 ms Maxim Kochetkov
2020-07-17 13:37 ` Linus Walleij [this message]
2020-07-17 13:44   ` Maxim Kochetkov
2020-07-17 14:17   ` Maxim Kochetkov
2020-07-17 15:07     ` Andy Shevchenko
2020-07-18  4:25       ` Kent Gibson
2020-07-20  8:14         ` Maxim Kochetkov
2020-07-24 19:36           ` Bartosz Golaszewski
2020-07-25  3:52             ` Kent Gibson
2020-07-26 11:04               ` Bartosz Golaszewski
2020-07-26 22:32               ` Linus Walleij

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