On 25.02.20 15:58, Michal Hocko wrote: > On Thu 12-12-19 18:11:36, David Hildenbrand wrote: >> We already have a way to trigger reclaiming of all reclaimable slab objects >> from user space (echo 2 > /proc/sys/vm/drop_caches). Let's allow drivers >> to also trigger this when they really want to make progress and know what >> they are doing. > > I cannot say I would be fan of this. This is a global action with user > visible performance impact. I am worried that we will find out that all > sorts of drivers have a very good idea that dropping slab caches is > going to help their problem whatever it is. We have seen the same patter > in the userspace already and that is the reason we are logging the usage > to the log and count invocations in the counter. Yeah, I decided to hold back patch 11-13 for the v1 (which I am planning to post in March after more testing). What we really want is to make memory offlining an alloc_contig_range() work better with reclaimable objects. > >> virtio-mem wants to use these functions when it failed to unplug memory >> for quite some time (e.g., after 30 minutes). It will then try to >> free up reclaimable objects by dropping the slab caches every now and >> then (e.g., every 30 minutes) as long as necessary. There will be a way to >> disable this feature and info messages will be logged. >> >> In the future, we want to have a drop_slab_range() functionality >> instead. Memory offlining code has similar demands and also other >> alloc_contig_range() users (e.g., gigantic pages) could make good use of >> this feature. Adding it, however, requires more work/thought. > > We already do have a memory_notify(MEM_GOING_OFFLINE) for that purpose > and slab allocator implements a callback (slab_mem_going_offline_callback). > The callback is quite dumb and it doesn't really try to free objects > from the given memory range or even try to drop active objects which > might turn out to be hard but this sounds like a more robust way to > achieve what you want. Two things: 1. memory_notify(MEM_GOING_OFFLINE) is called after trying to isolate the page range and checking if we only have movable pages. Won't help much I guess. 2. alloc_contig_range() won't benefit from that. Something like drop_slab_range() would be better, and calling it from the right spots in the core (e.g., set_migratetype_isolate() see below). Especially, have a look at mm/page_isolation.c:set_migratetype_isolate() "FIXME: Now, memory hotplug doesn't call shrink_slab() by itself. We just check MOVABLE pages." -- Thanks, David / dhildenb
