[PATCH 1/2] mm: support fastpath if NUMA is enabled with numa off

[Vlastimil Babka]

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 at samsung.com>

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) {
>