On Fri, Feb 14, 2025 at 05:27:36PM +0100, Vlastimil Babka wrote: > - Cheaper fast paths. For allocations, instead of local double cmpxchg, > after Patch 5 it's preempt_disable() and no atomic operations. Same for > freeing, which is normally a local double cmpxchg only for a short > term allocations (so the same slab is still active on the same cpu when > freeing the object) and a more costly locked double cmpxchg otherwise. > The downside is the lack of NUMA locality guarantees for the allocated > objects.
Is that really cheaper than a local non locked double cmpxchg? Especially if you now have to use pushf/popf... > - kfree_rcu() batching and recycling. kfree_rcu() will put objects to a > separate percpu sheaf and only submit the whole sheaf to call_rcu() > when full. After the grace period, the sheaf can be used for > allocations, which is more efficient than freeing and reallocating > individual slab objects (even with the batching done by kfree_rcu() > implementation itself). In case only some cpus are allowed to handle rcu > callbacks, the sheaf can still be made available to other cpus on the > same node via the shared barn. The maple_node cache uses kfree_rcu() and > thus can benefit from this. Have you looked at fs/bcachefs/rcu_pending.c?
