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cgroup/cpuset: Don't filter offline CPUs in cpuset_cpus_allowed() for top cpuset tasks
Since commit 8f9ea86 ("sched: Always preserve the user
requested cpumask"), relax_compatible_cpus_allowed_ptr() is calling
__sched_setaffinity() unconditionally. This helps to expose a bug in
the current cpuset hotplug code where the cpumasks of the tasks in
the top cpuset are not updated at all when some CPUs become online or
offline. It is likely caused by the fact that some of the tasks in the
top cpuset, like percpu kthreads, cannot have their cpu affinity changed.
One way to reproduce this as suggested by Peter is:
- boot machine
- offline all CPUs except one
- taskset -p ffffffff $$
- online all CPUs
Fix this by allowing cpuset_cpus_allowed() to return a wider mask that
includes offline CPUs for those tasks that are in the top cpuset. For
tasks not in the top cpuset, the old rule applies and only online CPUs
will be returned in the mask since hotplug events will update their
cpumasks accordingly.
Fixes: 8f9ea86 ("sched: Always preserve the user requested cpumask")
Reported-by: Will Deacon <[email protected]>
Originally-from: Peter Zijlstra (Intel) <[email protected]>
Tested-by: Will Deacon <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
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