On 14.11.18 10:00, Baoquan He wrote: > Hi David, > > On 11/14/18 at 09:18am, David Hildenbrand wrote: >> Code seems to be waiting for the mem_hotplug_lock in read. >> We hold mem_hotplug_lock in write whenever we online/offline/add/remove >> memory. There are two ways to trigger offlining of memory: >> >> 1. Offlining via "cat offline > /sys/devices/system/memory/memory0/state" >> >> This always properly took the mem_hotplug_lock. Nothing changed >> >> 2. Offlining via "cat 0 > /sys/devices/system/memory/memory0/online" >> >> This didn't take the mem_hotplug_lock and I fixed that for this release. >> >> So if you were testing with 1., you should have seen the same error >> before this release (unless there is something else now broken in this >> release). > > Thanks a lot for looking into this. > > I triggered sysrq+t to check threads' state. You can see that we use > firmware to trigger ACPI event to go to acpi_bus_offline(), it truly > didn't take mem_hotplug_lock() and has taken it with your fix in > commit 381eab4a6ee mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock > > [ +0.007062] Workqueue: kacpi_hotplug acpi_hotplug_work_fn > [ +0.005398] Call Trace: > [ +0.002476] ? page_vma_mapped_walk+0x307/0x710 > [ +0.004538] ? page_remove_rmap+0xa2/0x340 > [ +0.004104] ? ptep_clear_flush+0x54/0x60 > [ +0.004027] ? enqueue_entity+0x11c/0x620 > [ +0.005904] ? schedule+0x28/0x80 > [ +0.003336] ? rmap_walk_file+0xf9/0x270 > [ +0.003940] ? try_to_unmap+0x9c/0xf0 > [ +0.003695] ? migrate_pages+0x2b0/0xb90 > [ +0.003959] ? try_offline_node+0x160/0x160 > [ +0.004214] ? __offline_pages+0x6ce/0x8e0 > [ +0.004134] ? memory_subsys_offline+0x40/0x60 > [ +0.004474] ? device_offline+0x81/0xb0 > [ +0.003867] ? acpi_bus_offline+0xdb/0x140 > [ +0.004117] ? acpi_device_hotplug+0x21c/0x460 > [ +0.004458] ? acpi_hotplug_work_fn+0x1a/0x30 > [ +0.004372] ? process_one_work+0x1a1/0x3a0 > [ +0.004195] ? worker_thread+0x30/0x380 > [ +0.003851] ? drain_workqueue+0x120/0x120 > [ +0.004117] ? kthread+0x112/0x130 > [ +0.003411] ? kthread_park+0x80/0x80 > [ +0.005325] ? ret_from_fork+0x35/0x40 > Yes, this is indeed another code path that was fixed (and I didn't actually realize it ;) ). Thanks for the callchain. Before my fix hotplug still would have never succeeded (offline_pages would have silently looped forever) as far as I can tell. > >> >> >> The real question is, however, why offlining of the last block doesn't >> succeed. In __offline_pages() we basically have an endless loop (while >> holding the mem_hotplug_lock in write). Now I consider this piece of >> code very problematic (we should automatically fail after X >> attempts/after X seconds, we should not ignore -ENOMEM), and we've had >> other BUGs whereby we would run into an endless loop here (e.g. related >> to hugepages I guess). > > Hmm, even though memory hotplug stalled there, there are still much > memory. E.g in this system, it has 8 nodes and each node has 64 GB > memory, it's 512 GB in all. Now I run "stress -m 200" to trigger 200 > processes to malloc then free 256 MB contiously, and it's eating 50 GB > in all. In theory, it still has much memory for migrating to. Maybe a NUMA issue? But I am just making wild guesses. Maybe it is not -ENOMEM but just some other migration condition that is not properly handled (see Michals reply). > >> >> You mentioned memory pressure, if our host is under memory pressure we >> can easily trigger running into an endless loop there, because we >> basically ignore -ENOMEM e.g. when we cannot get a page to migrate some >> memory to be offlined. I assume this is the case here. >> do_migrate_range() could be the bad boy if it keeps failing forever and >> we keep retrying. > > Not sure what other people think about this. If failed the memory removing > when still much free memory left, I worry customer will complain. Indeed, we have to look into this. > > Yeah, it stoped at do_migrate_range() when try to migrate the last > memory block. And each time it's the last memory block which can't be > offlined and hang. It would be interesting to see which error message we keep getting. > > If any message or information needed, I can provide. > > Thanks > Baoquan > -- Thanks, David / dhildenb
