Special Files

• /sys/devices/system/<name>
  • Creation: subsys_system_register(), @ drivers/base/bus.c
  • Note that this subdirectory is a legacy. Newer stuffer are added into other folders inside /sys.
  • /sys/devices/system/cpu/*, @ drivers/base/cpu.c
    • Root Object is cpu_root_attrs. The online file belongs to another sub-object
    • And this register_cpu() function is used to setup the directories for each cpu.

Many applications use /sys/devices/system/cpu/online to get the number of available CPUs. And it’s hard to change this behavior because it’s usually encoded inside glibc. Thus, if you want to “hide” certain CPUs from applications for some reason, you can write a kernel module that use set_cpu_active(cpu, false), and then use the following small patch. (Note that using set_cpu_online(cpu, false) will confuse CPU idle routine and panic.)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -220,7 +220,8 @@ static ssize_t show_cpus_attr(struct device *dev,

 /* Keep in sync with cpu_subsys_attrs */
 static struct cpu_attr cpu_attrs[] = {
-       _CPU_ATTR(online, &__cpu_online_mask),
+       _CPU_ATTR(online, &__cpu_active_mask),
        _CPU_ATTR(possible, &__cpu_possible_mask),
        _CPU_ATTR(present, &__cpu_present_mask),
 };

• /proc/pressure
  • https://lwn.net/Articles/759658/

–
Yizhou Shan
Created: Jul 26, 2019
Last Updated: Aug 03, 2019