On Tue, Jul 14, 2015 at 05:18:23PM +0200, Michal Hocko wrote: > On Fri 10-07-15 16:05:33, Michal Hocko wrote: > > JFYI: I've found some more issues while hamerring this more. > > OK so the main issue is quite simple but I have completely missed it when > thinking about the patch before. clone(CLONE_VM) without CLONE_THREAD is > really nasty and it will easily lockup the machine with preempt. disabled > for ever. It goes like this: > taskA (in memcg A) > taskB = clone(CLONE_VM) > taskB > A -> B # Both tasks charge to B now > exit() # No tasks in B -> can be > # offlined now > css_offline() > mem_cgroup_try_charge > get_mem_cgroup_from_mm > rcu_read_lock() > do { > } while css_tryget_online(mm->memcg) # will never succeed > rcu_read_unlock() > > taskA and taskB are basically independent entities wrt. the life > cycle (unlike threads which are bound to the group leader). The > previous code handles this by re-ownering during exit by the monster > mm_update_next_owner. > > I can see the following options without reintroducing reintroducing > some form of mm_update_next_owner: > > 1) Do not allow offlining a cgroup if we have active users in it. This > would require a callback from the cgroup core to the subsystem called if > there are no active tasks tracked by the cgroup core. Tracking on the memcg > side doesn't sound terribly hard - just mark a mm_struct as an alien and > count the number of aliens during the move in mem_cgroup. mm_drop_memcg > then drops the counter. We could end up with EBUSY cgroup without any > visible tasks which is a bit awkward. You couldn't remove the group, and you wouldn't know which task needs to move to get the mm out of there. That's no good. > 2) update get_mem_cgroup_from_mm and others to fallback to the parent > memcg if the current one is offline. This would be in line with charge > reparenting we used to do. I cannot say I would like this because it > allows for easy runaway to the root memcg if the hierarchy is not > configured cautiously. The code would be also quite tricky because each > direct consumer of mm->memcg would have to be aware of this. This is > awkward. In the unified hierarchy, there won't be tasks inside intermediate nodes, so reparenting would lead to surprising behavior. > 3) fail mem_cgroup_can_attach if we are trying to migrate a task sharing > mm_struct with a process outside of the tset. If I understand the > tset properly this would require all the sharing tasks to be migrated > together and we would never end up with task_css != &task->mm->css. > __cgroup_procs_write doesn't seem to support multi pid move currently > AFAICS, though. cgroup_migrate_add_src, however, seems to be intended > for this purpose so this should be doable. Without that support we would > basically disallow migrating these tasks - I wouldn't object if you ask > me. I'd prefer not adding controller-specific failure modes for attaching, and this too would lead to very non-obvious behavior. > Do you see other options? From the above three options the 3rd one > sounds the most sane to me and the 1st quite easy to implement. Both will > require some cgroup core work though. But maybe we would be good enough > with 3rd option without supporting moving schizophrenic tasks and that > would be reduced to memcg code. A modified form of 1) would be to track the mms referring to a memcg but during offline search the process tree for a matching task. This is heavy-handed, but it's a rare case and this work would be done in the cgroup removal path rather than during task exit. This is stolen from the current mm_update_next_owner(): list_for_each_entry(mm, memcg->mms, memcg_list) { for_each_process(g) { if (g->flags & PF_KTHREAD) continue; for_each_thread(g, c) { if (c->mm == mm) goto assign; if (c->mm) break; } } assign: memcg = mem_cgroup_from_task(c); mm->memcg = memcg; list_move(&mm->memcg_list, &memcg->mms); } (plus appropriate synchronization, of course) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>