Mel Gorman <mgorman@xxxxxxx> wrote: > On Thu, Jul 27, 2017 at 04:04:20PM +0900, Minchan Kim wrote: >>> There is one issue I forgot: pte_accessible() on x86 regards >>> mm_tlb_flush_pending() as an indication for NUMA migration. But now the code >>> does not make too much sense: >>> >>> if ((pte_flags(a) & _PAGE_PROTNONE) && >>> mm_tlb_flush_pending(mm)) >>> >>> Either we remove the _PAGE_PROTNONE check or we need to use the atomic field >>> to count separately pending flushes due to migration and due to other >>> reasons. The first option is safer, but Mel objected to it, because of the >>> performance implications. The second one requires some thought on how to >>> build a single counter for multiple reasons and avoid a potential overflow. >>> >>> Thoughts? >> >> I'm really new for the autoNUMA so not sure I understand your concern >> If your concern is that increasing places where add up pending count, >> autoNUMA performance might be hurt. Right? >> If so, above _PAGE_PROTNONE check will filter out most of cases? >> Maybe, Mel could answer. > > I'm not sure what I'm being asked. In the case above, the TLB flush pending > is only relevant against autonuma-related races so only those PTEs are > checked to limit overhead. It could be checked on every PTE but it's > adding more compiler barriers or more atomic reads which do not appear > necessary. If the check is removed, a comment should be added explaining > why every PTE has to be checked. I considered breaking tlb_flush_pending to two: tlb_flush_pending_numa and tlb_flush_pending_other (they can share one atomic64_t field). This way, pte_accessible() would only consider “tlb_flush_pending_numa", and the changes that Minchan proposed would not increase the number unnecessary TLB flushes. However, considering the complexity of the TLB flushes scheme, and the fact I am not fully convinced all of these TLB flushes are indeed unnecessary, I will put it aside. Nadav -- 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
