Re: [RFC v1] mm: add the preempt check into alloc_vmap_area()

Matthew Wilcox
2018-02-27T13:06:46Z

On Tue, Feb 27, 2018 at 11:22:59AM +0100, Uladzislau Rezki (Sony) wrote:
> During finding a suitable hole in the vmap_area_list
> there is an explicit rescheduling check for latency reduction.
> We do it, since there are workloads which are sensitive for
> long (more than 1 millisecond) preemption off scenario.

I understand your problem, but this is a horrid solution. If it takes
us a millisecond to find a suitable chunk of free address space, something
is terribly wrong. On a 3GHz CPU, that's 3 million clock ticks!

I think our real problem is that we have no data structure that stores
free VA space. We have the vmap_area which stores allocated space, but no
data structure to store free space.

My initial proposal would be to reuse the vmap_area structure and store
the freed ones in a second rb_tree sorted by the size (ie va_end - va_start).
When freeing, we might need to merge forwards and backwards. Allocating
would be a matter of finding an area preferably of the exact right size;
otherwise split a larger free area into a free area and an allocated area
(there's a lot of literature on how exactly to choose which larger area
to split; memory allocators are pretty well-studied).