On Mon, Feb 26, 2024 at 07:29:04PM -0500, Kent Overstreet wrote: > On Mon, Feb 26, 2024 at 04:05:37PM -0800, Paul E. McKenney wrote: > > On Mon, Feb 26, 2024 at 06:29:43PM -0500, Kent Overstreet wrote: > > > Well, we won't want it getting hammered on continuously - we should be > > > able to tune reclaim so that doesn't happen. > > > > > > I think getting numbers on the amount of memory stranded waiting for RCU > > > is probably first order of business - minor tweak to kfree_rcu() et all > > > for that; there's APIs they can query to maintain that counter. > > > > We can easily tell you the number of blocks of memory waiting to be freed. > > But RCU does not know their size. Yes, we could ferret this on each > > call to kmem_free_rcu(), but that might not be great for performance. > > We could traverse the lists at runtime, but such traversal must be done > > with interrupts disabled, which is also not great. > > > > > then, we can add a heuristic threshhold somewhere, something like > > > > > > if (rcu_stranded * multiplier > reclaimable_memory) > > > kick_rcu() > > > > If it is a heuristic anyway, it sounds best to base the heuristic on > > the number of objects rather than their aggregate size. > > I don't think that'll really work given that object size can very from < > 100 bytes all the way up to 2MB hugepages. The shrinker API works that > way and I positively hate it; it's really helpful for introspection and > debugability later to give good human understandable units to this > stuff. You might well be right, but let's please try it before adding overhead to kfree_rcu() and friends. I bet it will prove to be good and sufficient. > And __ksize() is pretty cheap, and I think there might be room in struct > slab to stick the object size there instead of getting it from the slab > cache - and folio_size() is cheaper still. On __ksize(): * This should only be used internally to query the true size of allocations. * It is not meant to be a way to discover the usable size of an allocation * after the fact. Instead, use kmalloc_size_roundup(). Except that kmalloc_size_roundup() doesn't look like it is meant for this use case. On __ksize() being used only internally, I would not be at all averse to kfree_rcu() and friends moving to mm. The idea is for kfree_rcu() to invoke __ksize() when given slab memory and folio_size() when given vmalloc() memory? Thanx, Paul
