Hi, On Wed, Jun 16, 2021 at 07:10:06PM +0200, Vlastimil Babka wrote: > 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. > You are correct. I mean dummy node, which shows "Faking a node at ..." message at boot time. So I called it fake node. > > 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://protect2.fireeye.com/v1/url?k=32536af8-6dc85232-3252e1b7-0cc47a31bee8-e52eadd28e1e9a6e&q=1&e=39db7dd8-7f21-41a4-b4a9-9ad395d36e23&u=https%3A%2F%2Flkml.org%2Flkml%2F2006%2F8%2F29%2F294). > > 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. > It's early to conclude, but mempolicy evaluation seems to account for a large portion of the total overhead. Since this patch, performance regression has decreased from 5-6% to 2-3%. It is still unclear whether the remainder is within the margin of error of the measurement results or is affected by other NUMA-related codes. > > 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. > I'm participating in arm64 project now, so I'm not sure if other architectures will accept this way. So I tried not to touch other architecture. Of course, it can be changed in the generic way if agree. > > --- > > 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) { > > > >
