Re: [PATCH] cpuset: restore sanity to cpuset_cpus_allowed_fallback()

[Joel Savitz]

On Wed, Mar 6, 2019 at 7:55 PM Joel Savitz <jsavitz@xxxxxxxxxx> wrote:
>
> If a process is limited by taskset (i.e. cpuset) to only be allowed to
> run on cpu N, and then cpu N is offlined via hotplug, the process will
> be assigned the current value of its cpuset cgroup's effective_cpus field
> in a call to do_set_cpus_allowed() in cpuset_cpus_allowed_fallback().
> This argument's value does not makes sense for this case, because
> task_cs(tsk)->effective_cpus is modified by cpuset_hotplug_workfn()
> to reflect the new value of cpu_active_mask after cpu N is removed from
> the mask. While this may make sense for the cgroup affinity mask, it
> does not make sense on a per-task basis, as a task that was previously
> limited to only be run on cpu N will be limited to every cpu _except_ for
> cpu N after it is offlined/onlined via hotplug.
>
> Pre-patch behavior:
>
>         $ grep Cpus /proc/$$/status
>         Cpus_allowed:   ff
>         Cpus_allowed_list:      0-7
>
>         $ taskset -p 4 $$
>         pid 19202's current affinity mask: f
>         pid 19202's new affinity mask: 4
>
>         $ grep Cpus /proc/self/status
>         Cpus_allowed:   04
>         Cpus_allowed_list:      2
>
>         # echo off > /sys/devices/system/cpu/cpu2/online
>         $ grep Cpus /proc/$$/status
>         Cpus_allowed:   0b
>         Cpus_allowed_list:      0-1,3
>
>         # echo on > /sys/devices/system/cpu/cpu2/online
>         $ grep Cpus /proc/$$/status
>         Cpus_allowed:   0b
>         Cpus_allowed_list:      0-1,3
>
> On a patched system, the final grep produces the following
> output instead:
>
>         $ grep Cpus /proc/$$/status
>         Cpus_allowed:   ff
>         Cpus_allowed_list:      0-7
>
> This patch changes the above behavior by instead simply resetting the mask
> to cpu_possible_mask.
>
> Signed-off-by: Joel Savitz <jsavitz@xxxxxxxxxx>
> ---
>  kernel/cgroup/cpuset.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 479743db6c37..5f65a2167bdf 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3243,7 +3243,7 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
>  {
>         rcu_read_lock();
> -       do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus);
> +       do_set_cpus_allowed(tsk, cpu_possible_mask);
>         rcu_read_unlock();
>
>         /*
> --
> 2.20.1
>