Re: [PATCH] cpuset: fix cpus_allowed mask for offline/online CPUs

[Zefan Li <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

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.

Commit be4c9dd7aee5 changed this behavior, but only for unified hierachy.