On 25.02.20 18:06, Michal Hocko wrote: > On Tue 25-02-20 16:09:29, David Hildenbrand wrote: >> 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. > > You are right, I have missed that. Can we reorder those two calls? AFAIK no (would have to look up the details, but there was a good reason for the order, e.g., avoid races with other users of page isolation like alloc_contig_range()). Especially, "[PATCH RFC v4 06/13] mm: Allow to offline unmovable PageOffline() pages via MEM_GOING_OFFLINE" (which is still impatiently waiting for an ACK ;) ) also works around that ordering issue in a way we discussed back then. -- Thanks, David / dhildenb
