PM: hibernate: add sysfs interface for hibernate_compression_threads

Add a sysfs attribute `/sys/power/hibernate_compression_threads` to
allow runtime configuration of the number of threads used for
compressing and decompressing hibernation images.

The new sysfs interface enables dynamic adjustment at runtime:

# cat /sys/power/hibernate_compression_threads
3
# echo 4 > /sys/power/hibernate_compression_threads

This change provides greater flexibility for debugging and performance
tuning of hibernation without requiring a reboot.

Signed-off-by: Xueqin Luo <[e-mail]>
Link: https://patch.msgid.link/c[e-mail]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/ABI/testing/sysfs-power

···
 		disables it.  Reads from the file return the current value.
 		The default is "1" if the build-time "SUSPEND_SKIP_SYNC" config
 		flag is unset, or "0" otherwise.
+
+What:           /sys/power/hibernate_compression_threads
+Date:           October 2025
+Contact:        <[e-mail]>
+Description:
+                Controls the number of threads used for compression
+                and decompression of hibernation images.
+
+                The value can be adjusted at runtime to balance
+                performance and CPU utilization.
+
+                The change takes effect on the next hibernation or
+                resume operation.
+
+                Minimum value: 1
+                Default value: 3

kernel/power/swap.c

···
 }
 #endif
 
+static ssize_t hibernate_compression_threads_show(struct kobject *kobj,
+				struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%d\n", hibernate_compression_threads);
+}
+
+static ssize_t hibernate_compression_threads_store(struct kobject *kobj,
+				struct kobj_attribute *attr,
+				const char *buf, size_t n)
+{
+	unsigned long val;
+
+	if (kstrtoul(buf, 0, &val))
+		return -EINVAL;
+
+	if (val < 1)
+		return -EINVAL;
+
+	hibernate_compression_threads = val;
+	return n;
+}
+power_attr(hibernate_compression_threads);
+
+static struct attribute *g[] = {
+	&hibernate_compression_threads_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group attr_group = {
+	.attrs = g,
+};
+
 static int __init swsusp_header_init(void)
 {
+	int error;
+
+	error = sysfs_create_group(power_kobj, &attr_group);
+	if (error)
+		return -ENOMEM;
+
 	swsusp_header = (struct swsusp_header*) __get_free_page(GFP_KERNEL);
 	if (!swsusp_header)
 		panic("Could not allocate memory for swsusp_header\n");