Hello, Balbir. On Tue, Aug 09, 2016 at 02:19:01PM +1000, Balbir Singh wrote: > > cgroup_threadgroup_rwsem is acquired in read mode during process exit and fork. > It is also grabbed in write mode during __cgroups_proc_write > > I've recently run into a scenario with lots of memory pressure and OOM > and I am beginning to see > > systemd > > __switch_to+0x1f8/0x350 > __schedule+0x30c/0x990 > schedule+0x48/0xc0 > percpu_down_write+0x114/0x170 > __cgroup_procs_write.isra.12+0xb8/0x3c0 > cgroup_file_write+0x74/0x1a0 > kernfs_fop_write+0x188/0x200 > __vfs_write+0x6c/0xe0 > vfs_write+0xc0/0x230 > SyS_write+0x6c/0x110 > system_call+0x38/0xb4 > > This thread is waiting on the reader of cgroup_threadgroup_rwsem to exit. > The reader itself is under memory pressure and has gone into reclaim after > fork. There are times the reader also ends up waiting on oom_lock as well. > ... > copy_page_range+0x4ec/0x950 > copy_process.isra.5+0x15a0/0x1870 > _do_fork+0xa8/0x4b0 > ppc_clone+0x8/0xc Yeah, we definitely don't wanna be holding the rwsem during the actual fork. ... > There are other theoretical issues with this semaphore > > systemd can do > > 1. cgroup_mutex (cgroup_kn_lock_live) > 2. cgroup_threadgroup_rwsem (W) (__cgroup_procs_write) > > and other threads can go > > 1. cgroup_threadgroup_rwsem (R) (copy_process) > 2. mem_cgroup_iter (as a part of reclaim) (cgroup_mutex -- rcu lock or cgroup_mutex) Hmm? Where does mem_cgroup_iter grab cgroup_mutex? cgroup_mutex nests outside cgroup_threadgroup_rwsem or most other mutexes for that matter and isn't exposed from cgroup core. > However, I've not examined them in too much detail or looked at lockdep > wait chains for those paths. > > I am sure there is a good reason for placing cgroup_threadgroup_rwsem > where it is today and I might be missing something. I am also surprised I could be missing something too but the positioning is largely historic. > no-one else has run into it so far. Maybe it might matter that much on a system which is already heavily thrasing, but yeah, we definitely want to tighten down the reader sections so that it doesn't get in the way of making forward progress. > Comments? The change looks good to me on the first glance but I'll think more about it tomorrow. Thanks! -- tejun -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
