On 2016/1/1 20:09, Chen Yu wrote:
Commit be4c9dd7aee5 ("cpuset: enable onlined cpu/node in effective
masks") leverages cpuset's cpus_allowed and its parent's
effective_cpus to calculate the new_cpus by:
cpumask_and(&new_cpus, cs->cpus_allowed,
parent_cs(cs)->effective_cpus);
However cpus_allowed will also be updated after the CPU is offline, in
hotplug_update_tasks_legacy, so when the CPU is online again, it will
use the old cpus_allowed mask to calculate the new_cpus, thus new_cpus
will get incorrect value after each round of offline/online.
This problem is found on ubuntu 15.10 with cpuset mounted:
1. echo 0 > /sys/devices/system/cpu/cpu2/online
2. echo 1 > /sys/devices/system/cpu/cpu2/online
3. cat /sys/fs/cgroup/cpuset/cpuset.cpus
0-3
4. cat /sys/fs/cgroup/cpuset/user.slice/cpuset.cpus
0-1,3
5. taskset -c 2 ls
taskset: failed to set pid 0's affinity: Invalid argument
This is the expected behavior...In legacy hierachy onlining an offlined cpu
won't restore cpuset configurations automatically.
[Yu] Ah, I see, we were just a little confused with it before.
So if we want to online an offlined cpu in legacy hierarchy, we
should not only echo 1 to 'online' sysfs, but also restore the mask manually?
Right. You have to take care of this by yourself.
It seems that we should not rely on /sys/devices/system/cpu/cpu2/online,
if cpuset has been mounted.
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