kmemtrace provides tracing for slab allocator functions, such as
kmalloc, kfree, kmem_cache_alloc, kmem_cache_free, etc. Collected
data is then fed to the userspace application in order to analyse
allocation hotspots, internal fragmentation and so on, making it
possible to see how well an allocator performs, as well as debug
and profile kernel code.

This requires an userspace application to use. See Documentation/trace/kmemtrace.txt for more information.

Saying Y will make the kernel somewhat larger and slower. However,
if you disable kmemtrace at run-time or boot-time, the performance
impact is minimal (depending on the arch the kernel is built for).

If unsure, say N.

source code:

kernel/trace/kmemtrace.c