On Wed 14-11-18 09:18:09, David Hildenbrand wrote: > On 14.11.18 08:09, Baoquan He wrote: > > Hi, > > > > Tested memory hotplug on a bare metal system, hot removing always > > trigger a lock. Usually need hot plug/unplug several times, then the hot > > removing will hang there at the last block. Surely with memory pressure > > added by executing "stress -m 200". > > > > Will attach the log partly. Any idea or suggestion, appreciated. > > > > Thanks > > Baoquan > > > > 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. This discrepancy should go. > 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). > > > 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). We used to have number of retries previous and it was too fragile. If you need a timeout then you can easily do that from userspace. Just do timeout $TIME echo 0 > $MEM_PATH/online I have seen an issue when the migration cannot make a forward progress because of a glibc page with a reference count bumping up and down. Most probable explanation is the faultaround code. I am working on this and will post a patch soon. In any case the migration should converge and if it doesn't do then there is a bug lurking somewhere. Failing on ENOMEM is a questionable thing. I haven't seen that happening wildly but if it is a case then I wouldn't be opposed. > 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. My hotplug debugging patches [1] should help to tell us. [1] http://lkml.kernel.org/r/20181107101830.17405-1-mhocko@xxxxxxxxxx -- Michal Hocko SUSE Labs
