On 6/16/21 10:37 AM, Janghyuck Kim wrote: > Architecture might support fake node when CONFIG_NUMA is enabled but any I suppose you mean the dummy node, i.e. dummy_numa_init()? Because fakenuma is something different and I think if someone defines fakenuma nodes they actually would want for the mempolicies to be honored as if there was a real NUMA setup. > node settings were supported by ACPI or device tree. In this case, > getting memory policy during memory allocation path is meaningless. > > Moreover, performance degradation was observed in the minor page fault > test, which is provided by (https://lkml.org/lkml/2006/8/29/294). > Average faults/sec of enabling NUMA with fake node was 5~6 % worse than > disabling NUMA. To reduce this performance regression, fastpath is So you have measured this overhead is all due to mempolicy evaluation? Interesting, sounds like a lot. > introduced. fastpath can skip the memory policy checking if NUMA is > enabled but it uses fake node. If architecture doesn't support fake > node, fastpath affects nothing for memory allocation path. > > Signed-off-by: Janghyuck Kim <janghyuck.kim@xxxxxxxxxxx> Sounds like an interesting direction to improve CONFIG_NUMA built kernels on single-node systems, but why restrict it only to arm64 and not make it generic for all systems with a single node? We could also probably use a static key instead of this #define. That would even make it possible to switch in case memory hotplug onlines another node, etc. > --- > mm/internal.h | 4 ++++ > mm/mempolicy.c | 3 +++ > 2 files changed, 7 insertions(+) > > diff --git a/mm/internal.h b/mm/internal.h > index 31ff935b2547..3b6c21814fbc 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -36,6 +36,10 @@ void page_writeback_init(void); > > vm_fault_t do_swap_page(struct vm_fault *vmf); > > +#ifndef numa_off_fastpath > +#define numa_off_fastpath() false > +#endif > + > void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma, > unsigned long floor, unsigned long ceiling); > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index e32360e90274..21156671d941 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2152,6 +2152,9 @@ struct page *alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma, > int preferred_nid; > nodemask_t *nmask; > > + if (numa_off_fastpath()) > + return __alloc_pages_nodemask(gfp, order, 0, NULL); > + > pol = get_vma_policy(vma, addr); > > if (pol->mode == MPOL_INTERLEAVE) { >
