Hello, Petr. (cc'ing Johannes)
On Wed, Apr 13, 2016 at 11:42:16AM +0200, Petr Mladek wrote: ... > By other words, "memcg_move_char/2860" flushes a work. But it cannot > get flushed because one worker is blocked and another one could not > get created. All these operations are blocked by the very same > "memcg_move_char/2860". > > Note that also "systemd/1" is waiting for "cgroup_mutex" in > proc_cgroup_show(). But it seems that it is not in the main > cycle causing the deadlock. > > I am able to reproduce this problem quite easily (within few minutes). > There are often even more tasks waiting for the cgroups-related locks > but they are not causing the deadlock. > > > The question is how to solve this problem. I see several possibilities: > > + avoid using workqueues in lru_add_drain_all() > > + make lru_add_drain_all() killable and restartable > > + do not block fork() when lru_add_drain_all() is running, > e.g. using some lazy techniques like RCU, workqueues > > + at least do not block fork of workers; AFAIK, they have a limited > cgroups usage anyway because they are marked with PF_NO_SETAFFINITY > > > I am willing to test any potential fix or even work on the fix. > But I do not have that big insight into the problem, so I would > need some pointers. An easy solution would be to make lru_add_drain_all() use a WQ_MEM_RECLAIM workqueue. A better way would be making charge moving asynchronous similar to cpuset node migration but I don't know whether that's realistic. Will prep a patch to add a rescuer to lru_add_drain_all(). Thanks. -- tejun
