LSFMMBPF proposal [MM]: Eliminate vmap/vmalloc lock contention

[Uladzislau Rezki <urezki@gmail.com>]

Hello, LSF.

Title: Introduce a per-cpu-vmap-cache to eliminate a vmap lock contention

Description:
 Currently the vmap code is not scaled to number of CPU cores in a system
 because a global vmap space is protected by a single spinlock. Such approach
 has a clear bottleneck if many CPUs simultaneously access to one resource.

 In this talk i would like to describe a drawback, show some data related
 to contentions and places where those occur in a code. Apart of that i
 would like to share ideas how to eliminate it providing a few approaches
 and compare them.

Requirements:
 * It should be a per-cpu approach;
 * Search of freed ptrs should not interfere with other freeing(as much as we can);
 *   - offload allocated areas(buzy ones) per-cpu;
 * Cache ready sized objects or merge them into one big per-cpu-space(split on demand);
 * Lazily-freed areas either drained per-cpu individually or by one CPU for all;
 * Prefetch a fixed size in front and allocate per-cpu 

Goals:
 * Implement a per-cpu way of allocation to eliminate a contention.

Thanks!

--
Uladzislau Rezki