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

Andrew Morton
2018-03-02T23:34:56Z

On Tue, 27 Feb 2018 05:06:43 -0800 Matthew Wilcox <willy@infradead.org> wrote:

> 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!

Yup.

> 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.

I wonder if we can reuse free_vmap_cache as a quick fix: if
need_resched(), point free_vmap_cache at the current rb_node, drop the
lock, cond_resched, goto retry?