On 3/11/21 9:59 AM, Mike Kravetz wrote: > On 3/11/21 4:17 AM, Michal Hocko wrote: >>> Yeah per cpu preempt counting shouldn't be noticeable but I have to >>> confess I haven't benchmarked it. >> >> But all this seems moot now http://lkml.kernel.org/r/YEoA08n60+jzsnAl@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx >> > > The proper fix for free_huge_page independent of this series would > involve: > > - Make hugetlb_lock and subpool lock irq safe > - Hand off freeing to a workque if the freeing could sleep > > Today, the only time we can sleep in free_huge_page is for gigantic > pages allocated via cma. I 'think' the concern about undesirable > user visible side effects in this case is minimal as freeing/allocating > 1G pages is not something that is going to happen at a high frequency. > My thinking could be wrong? > > Of more concern, is the introduction of this series. If this feature > is enabled, then ALL free_huge_page requests must be sent to a workqueue. > Any ideas on how to address this? > Thinking about this more ... A call to free_huge_page has two distinct outcomes 1) Page is freed back to the original allocator: buddy or cma 2) Page is put on hugetlb free list We can only possibly sleep in the first case 1. In addition, freeing a page back to the original allocator involves these steps: 1) Removing page from hugetlb lists 2) Updating hugetlb counts: nr_hugepages, surplus 3) Updating page fields 4) Allocate vmemmap pages if needed as in this series 5) Calling free routine of original allocator If hugetlb_lock is irq safe, we can perform the first 3 steps under that lock without issue. We would then use a workqueue to perform the last two steps. Since we are updating hugetlb user visible data under the lock, there should be no delays. Of course, giving those pages back to the original allocator could still be delayed, and a user may notice that. Not sure if that would be acceptable? I think Muchun had a similar setup just for vmemmmap allocation in an early version of this series. This would also require changes to where accounting is done in dissolve_free_huge_page and update_and_free_page as mentioned elsewhere. P.S. We could further optimize to check for the possibility of sleeping (cma or vmemmap) and only send to workqueue in those cases. -- Mike Kravetz
