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

Uladzislau Rezki
2018-02-28T12:41:00Z

On Tue, Feb 27, 2018 at 05:06:43AM -0800, Matthew Wilcox 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!
>
Some background. I spent some time analyzing an issue regarding audio
drops/glitches during playing hires audio on our mobile device. It is
ARM A53 with 4 CPUs on one socket. When it comes to frequency and test
case, the system is most likely idle and operation is done on ~576 MHz.

I found out that the reason was in vmalloc due to it can take time
to find a suitable chunk of memory and it is done in non-preemptible
context. As a result the other audio thread is not run on CPU in time
despite need_resched is set.

>
> 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).
>
Thank you for your comments and proposal.

--
Vlad Rezki