On 11/23/22 12:05, Tejun Heo wrote:
On Wed, Nov 23, 2022 at 08:21:57AM +0000, haifeng.xu wrote: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.This is only a problem in cgroup1 and cgroup1 doesn't require the threads of a given task to be in the same cgroup. I don't think you can optimize it this way.
I think it is an issue anyway if different threads of a process are in different cpusets with different node mask. It is not a configuration that should be used at all.
This patch makes update_tasks_nodemask() somewhat similar to cpuset_attach() where all tasks are iterated to update the node mask but only the task leaders are required to update the mm. For a non-group leader task, maybe we can check if the group leader is in the same cpuset. If so, we can skip the mm update. Do we need similar change in cpuset_attach()?
I do think the "migrate = is_memory_migrate(cs);" line can be moved outside of the loop, though. Of course, that won't help much in this case.
Cheers, Longman
