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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
