[PATCH] cpuset: restore sanity to cpuset_cpus_allowed_fallback()

[Joel Savitz]

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 <jsav...@redhat.com>
---
 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