When change the 'cpuset.mems' under some cgroup, system will hung
for a long time. From the dmesg, many processes or theads are
stuck in fork/exit. The reason is show as follows.
thread A:
cpuset_write_resmask /* takes cpuset_rwsem */
...
update_tasks_nodemask
mpol_rebind_mm /* waits mmap_lock */
thread B:
worker_thread
...
cpuset_migrate_mm_workfn
do_migrate_pages /* takes mmap_lock */
thread C:
cgroup_procs_write /* takes cgroup_mutex and cgroup_threadgroup_rwsem */
...
cpuset_can_attach
percpu_down_write /* waits cpuset_rwsem */
Once update the nodemasks of cpuset, thread A wakes up thread B to
migrate mm. But when thread A iterates through all tasks, including
child threads and group leader, it has to wait the mmap_lock which
has been take by thread B. Unfortunately, thread C wants to migrate
tasks into cgroup at this moment, it must wait thread A to release
cpuset_rwsem. If thread B spends much time to migrate mm, the
fork/exit which acquire cgroup_threadgroup_rwsem also need to
wait for a long time.
There is no need to migrate the mm of child threads which is
shared with group leader. Just iterate through the group
leader only.
Signed-off-by: haifeng.xu <haifeng.xu@xxxxxxxxxx>
---
kernel/cgroup/cpuset.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 589827ccda8b..43cbd09546d0 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1968,6 +1968,9 @@ static void update_tasks_nodemask(struct cpuset *cs)
cpuset_change_task_nodemask(task, &newmems);
+ if (!thread_group_leader(task))
+ continue;
+
mm = get_task_mm(task);
if (!mm)
continue;
--
2.25.1
