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gpio: sysfs: change 'value' attribute to prealloc

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The GPIO 'value' attribute is time critical. A small bench with
'perf record' on the app below shows that 80% of the time spent in
sysfs_kf_seq_show() is spent in memset() for zeroising the buffer.

|--67.48%--sysfs_kf_seq_show
|          |
|          |--54.40%--memset
|          |
|          |--11.49%--dev_attr_show
|          |          |
|          |          |--10.06%--value_show
|          |          |          |
|          |          |          |--4.75%--sprintf
|          |          |          |          |

This patch changes the attribute type to prealloc, eliminating the
need to zeroise the buffer at each read. 'perf record' gives the
following result.

|--42.41%--sysfs_kf_read
|          |
|          |--39.73%--dev_attr_show
|          |          |
|          |          |--38.23%--value_show
|          |          |          |
|          |          |          |--29.22%--sprintf
|          |          |          |          |

Test done with the following small app:

int main(int argc, char **argv)
{
	int fd = open(argv[1], O_RDONLY);

	for (;;) {
		int buf[512];

		read(fd, buf, 512);
		lseek(fd, 0, SEEK_SET);
	}
	exit(0);
}

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
This commit is contained in:
Christophe Leroy 2017-12-18 11:08:29 +01:00 • committed by Linus Walleij
parent fcf273e580
commit 7fda9100bb
2 changed files with 4 additions and 1 deletions
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2
drivers/gpio/gpiolib-sysfs.c
View file

@ -138,7 +138,7 @@ static ssize_t value_store(struct device *dev,
return status;
}
static DEVICE_ATTR_RW(value);
static DEVICE_ATTR_PREALLOC(value, S_IWUSR | S_IRUGO, value_show, value_store);
static irqreturn_t gpio_sysfs_irq(int irq, void *priv)
{

3
include/linux/device.h
View file

@ -575,6 +575,9 @@ ssize_t device_store_bool(struct device *dev, struct device_attribute *attr,
#define DEVICE_ATTR(_name, _mode, _show, _store) \
struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)
#define DEVICE_ATTR_PREALLOC(_name, _mode, _show, _store) \
struct device_attribute dev_attr_##_name = \
__ATTR_PREALLOC(_name, _mode, _show, _store)
#define DEVICE_ATTR_RW(_name) \
struct device_attribute dev_attr_##_name = __ATTR_RW(_name)
#define DEVICE_ATTR_RO(_name) \